Advanced techniques for clay mineral analysis: invited contributions from the symposium held at the 7th International Clay Conference, September 6-12, 1981, Bologna and Pavia, Italy

DEVELOPMENTS IN SEDIMENTOLOGY 34 ADVANCED TECHNIQUES FOR CLAY MINERAL ANALYSIS Invited contributions from the Symposium...

Author: J. J. Fripiat

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DEVELOPMENTS IN SEDIMENTOLOGY 34

ADVANCED TECHNIQUES FOR CLAY MINERAL ANALYSIS Invited contributions from the Symposium held at the 7th International Clay Conference, September 6-12,1981, Bologna and Pavia, Italy

edited by

JmJm FRlPlAT Centre National de la Recherche Scientifique, Centre de Recherche sur les Solides 6 Organisation Cristalline Imparfaite, Orl6ans Cedex (France)

ELSEVIER SCIENTIFIC PUBLISHING COMPANY Amsterdam - Oxford - New York 1982

ELSEVIER SCIENTIFIC PUBLISHING COMPANY Molenwerf 1 P.O. Box 21 1, 1000 AE Amsterdam, The Netherlands

Distributors for the United States and Canada:

E LSEV IER/NORTH-HOLLAND INC 52, Vanderbilt Avenue New York, N.Y. 10017

L i b r a r y 01 Congress Cataloging i n Publication Data

Main entry under title: Advanced techniques for clay mineral analysis. (Developments in sedimentology ; v. 34) Bibliography: p. Includes index. 1. Clay minerals--Analysis--Congresses. I. Fripiat, J. J. 11. International Clay Conference (7th : 1981 : Bologna and Pavia, Italy) 111. Series. ~~389.62 .A38 553.6'1'028 81-9881 ISBN O-d4-420@-9 (U.S.) M C W

0 Elsevier Scientific Publishing Company, 1982 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying. recording or otherwise, without the prior written permission of the publisher, Elsevier Scientific Publishing Company, P.O. Box 330, 1000 AH Amsterdam, The Netherlands Printed in The Netherlands

1

PREFACE One o f t h e i m p o r t a n t aims o f an i n t e r n a t i o n a l congress devoted t o n a t u r a l o r s y n t h e t i c m a t e r i a l s i s t o i n f o r m t h e researcher

about t h e p o t e n t i a l use o f new

p h y s i c a l t e c h n i q u e s employed f o r c h a r a c t e r i z i n g t h e s e m a t e r i a l s . T h i s s p e c i f i c t a s k i s becoming more and more u r g e n t because t h e number o f p h y s i c a l t e c h n i q u e s and t h e i r new a p p l i c a t i o n s a r e i n c r e a s i n g v e r y r a p i d l y . The o r g a n i z i n g committee o f t h e 7 t h I n t e r n a t i o n a l C l a y Conference has t h o u g h t t h a t a s p e c i a l symposium on Advanced Methods i n C l a y M i n e r a l s Research s h o u l d be o f g r e a t i n t e r e s t f o r many p a r t i c i p a n t s and I have been asked t o t a k e t h e r e s p o n s i b i l i t y f o r i t s organization. The f i r s t d i f f i c u l t y o f t h a t t a s k was t o s e l e c t t h e methods t o be reviewed. There a r e t e c h n i q u e s which have been known f o r many y e a r s but,because o f t h e r e c e n t i n s t r u m e n t a t i o n p r o g r e s s , t h e r e a r e new r e s u l t s which a r e w o r t h w h i l e t o summarize. There a r e a l s o t e c h n i q u e s w h i c h a r e n o t o f g e n e r a l use i n t h e f i e l d o f c l a y r e s e a r c h b u t which can p r o v i d e i m p o r t a n t supplementary i n f o r m a t i o n . F i n a l l y , t h e r e a r e r e c e n t t e c h n i q u e s which have n o t y e t developped o r demonstrated t h e i r f u l l c a p a c i t y . A r e v i e w o f p r e l i m i n a r y r e s u l t s o b t a i n e d by t h e s e new t e c h n i q u e s can s t i m u l a t e o t h e r s c i e n t i s t s t o use them more b r o a d l y . Thus t h e c h o i c e i s n o t easy and, i n a d d i t i o n , t h e space and t i m e a l l o t t e d f o r t h a t t y p e o f r e v i e w a r e n e c e s s a r i l y l i m i t e d . N i n e t e c h n i q u e s have been s e l e c t e d f o r one o r s e v e r a l o f t h e reasons e x p l a i n e d above; t h e y c o n s t i t u t e t h e n i n e chapt e r s o f t h i s monograph. The a u t h o r s have been asked t o a v o i d as much as p o s s i b l e t h e o r e t i c a l d i s c u s s i o n s and t o c o n c e n t r a t e - t h e i r p r e s e n t a t i o n on e x p e r i m e n t a l r e s u l t s and t h e i r p h y s i c a l meaning. There was a good reason t o o p e r a t e i n t h a t way. Indeed i n 1979, a t Urbana ( I l l i n o i s ) , J.W. S t u c k i and W.L. Banwart o r g a n i z e d a Nato School devoted t o Advanced Chemical Methods f o r S o i l and C l a y M i n e r a l Research (Nato advanced s t u d y i n s t i t u t e s : S e r i e s C: Mathematical and P h y s i c a l Sciences, D. R e i d e l P u b l i s h . Co,

1980). The o b j e c t i v e o f t h a t school was t o t e a c h a l i m i t e d number o f p a r t i c i p a n t s t h e b a s i c p r i n c i p l e s o f Mossbauer spectroscopy, N e u t r o n S c a t t e r i n g , X-Ray Photoe l e c t r o n Spectroscopy, N u c l e a r Magnetic Resonance, E l e c t r o n S p i n Resonance and Photo A c o u s t i c Spectroscopy, and t o show t h e way t o a p p l y t h e i r b a s i c p r i n c i p l e s t o t h e s t u d y o f c l a y m i n e r a l s . Thus a r e c e n t book c o n t a i n i n g t h e t h e o r i e s d e a l i n g w i t h s e v e r a l c h a p t e r s o f t h e p r e s e n t monograph i s a v a i l a b l e . The p r e s e n t monograph has

as

goal t o r e a c h a l a r g e r community o f s c i e n -

t i s t s and t o d i s s e m i n a t e i n f o r m a t i o n a b o u t a p p l i c a t i o n s o f p h y s i c a l t e c h n i q u e s on

2 a more general

basis.

I t i s why modern developments o f Thermal Methods Ana-

l y s i s and o f E l e c t r o n Microscopy have been i n c l u d e d t o g e t h e r w i t h c h a p t e r s deal i n g w i t h MGssbauer Spectroscopy, N u c l e a r Magnetic Resonance, E l e c t r o n S p i n Resonance, Neutron S c a t t e r i n g and X-Ray P h o t o e l e c t r o n Spectroscopy. I t appeared a l s o d e s i r a b l e t o have a c h a p t e r on UV and V i s i b l e Spectroscopy f o r w h i c h , t o my b e s t knowledge, no r e v i e w on t h e i r a p p l i c a t i o n t o t h e s t u d y o f c l a y m i n e r a l s

e x i s t s . The same i s t r u e f o r f a r i n f r a r e d s p e c t r o s c o p y .

A l l c r i t i c i s m s c o n c e r n i n g t h e c h o i c e o f t h e t o p i c s have t o be adressed t o t h e e d i t o r . The a u t h o r s o f i n d i v i d u a l chapte,rs s h o u l d be g i v e n a l l c r e d i t f o r making t h e r e a d e r aware o f t h e new e x c i t i n g developments i n t h e p h y s i c a l t e c h n i q u e s used i n c l a y m i n e r a l s r e s e a r c h .

J . FRIPIAT.

5

Chapter 1

THERMOANALYTICAL METHODS IN CLAY STUDIES Robert C. MACKENZIE The Macaulay Institute for Soi Research, Craigiebuckl er , Aberdeen , Scot1and , UK. 1.1 INTRODUCTION There is nothing new in the thermal study of clays. Indeed, as early as about 315 BC, Theophrastus refers to the effect of "fire" (i.e. heat) on talc (as steatite) and on palygorskite (as "mountain wood") (Eichholz, 1965) and development in the use of heat as a discriminator can be traced from that time on (Mackenzie, 1981a). Even evolved gas analysis, which would be considered by some as a relatively recent technique for clays, has its roots in the eighteenth century, when the Rev. Stephen Hales (1727) found that "a cubick inch of fresh u n t r i e d earth" (his italics) yielded "43 cubick inches of air" on heating and Josiah Wedgwood (1782) detected only carbon dioxide on firing china clay in a closed system, the evolved water having condensed and gone unnoticed. The first thermoanalytical study of clays was performed in 1887, when Henri Le Chatelier recorded what were essentially heating-rate curves for halloysite, allophane, kaolinite, pyrophyllite and montmoril lonite, over the approximate temperature range 20-llOO°C, in an attempt to use their behaviour on heating as a classificational criterion. His results suggest that the samples used were remarkably authentic - surely a tribute to the mineralogists o f the time who had none of the modern methods of diagnosis available to them. Despite the differences observed, 1 ittle advance, apart from the pub1 ication of some so-called "dehydration curves" (Samoilov, 1909) and some heating curves (e.g. Mellor and Holdcroft, 1911; Ashley, 1911; Brown and Montgomery, 1912), occurred until Wallach in 1913 first applied differential thermal analysis (DTA) to clays. Even this, however, seems to have elicited little response and, although the OTA studies o f Satoh (1918, 1921) aroused more attention, it was not until the early 19405, subsequent to the detailed studies of Norton (1939) and Hendricks and Alexander (1939), that DTA blossomed forth as an investigational technique. The reason is simple: at that period clays excited much interest as the general structure of the clay minerals had been establisned and the species collected into groups, with the reLult that methods of identification and estimation additional or complementary to X-ray diffraction were being sought. Unfortunately, the indiscriminate application of DTA to problems that it could not possible solve,and even the use of unsuitable equipment and technique, led to the method being discarded by some as useless in clay mineralogy. However, by no means all clay mineralogists were

6 so d i s i l l u s i o n e d and much p a i n s t a k i n g work over t h e years (by e.g.

Ralph E. G r i m ,

Paul F. Kerr, Toshio Sudo and o t h e r s ) g r a d u a l l y demonstrated t h a t DTA d i d have a place i n c l a y m i n e r a l o g i c a l studies.

A t t h i s p o i n t t h e reader m i g h t w e l l ask why t h e r m o a n a l y t i c a l s t u d i e s (discussed i n t h e paragraph above) should be separated from p u r e l y thermal s t u d i e s ( r e f e r r e d t o i n the f i r s t paragraph).

The reason i s t h a t thermal methods have t o s a t i s f y

c e r t a i n c r i t e r i a before they can be termed thermoanalytical.

These c r i t e r i a ,

some o f t h e thermoanalytical techniques c u r r e n t l y a v a i l a b l e and t h e i r a p p l i c a t i o n and/or a p p l i c a b i l i t y i n c l a y i n v e s t i g a t i o n s a r e t h e s u b j e c t o f the remainder o f t h i s paper.

1.2

THERMAL ANALYSIS Over t h e past f i f t e e n years much a t t e n t i o n has been devoted t o nomenclature,

d e f i n i t i o n and c l a s s i f i c a t i o n o f thermoanalytical techniques w i t h t h e r e s u l t t h a t the methods included can now be c l e a r l y recognized and named. According t o t h e I n t e r n a t i o n a l Confederation f o r Thermal A n a l y s i s (ICTA), thermal a n a l y s i s covers (Lombardi,

1980):

"A group o f techniques i n which a p h y s i c a l p r o p e r t y o f a substance and/or i t s r e a c t i o n products i s measured as a f u n c t i o n o f temperature, w h i l s t t h e substance i s subjected t o a c o n t r o l l e d temperature programme". The t h r e e c r i t e r i a t h a t d i s t i n g u i s h a t h e r m o a n a l y t i c a l method are, t h e r e f o r e , t h a t a p h y s i c a l p r o p e r t y i s measured as a f u n c t i o n of t e m p e r a t u r e under a c o n t r o l l e d t e m p e r a t u r e programme.

Thus, a s i n g l e isothermal d e t e r m i n a t i o n i s n o t

thermoanalytical b u t assessment o f the r e s u l t s o f a s e r i e s o f isothermal determinations a t d i f f e r e n t temperatures as a f u n c t i o n o f temperature i s . S i m i l a r l y , non-thermal methods, such as X-ray d i f f r a c t i o n , performed under a c o n t r o l l e d temperature programme become t h e r m o a n a l y t i c a l determinations.

I n the

account t h a t f o l l o w s , however, o n l y those methods normally i n c l u d e d i n thermal a n a l y s i s w i l l be considered:

i t should be observed t h a t c l a s s i c a l c a l o r i m e t r y i s

excluded, d e s p i t e i t s c l o s e r e l a t i o n s h i p t o some thermoanalytical methods.

1.2.1

A v a i l a b l e t h e r m o a n a l y t i c a l techniques.

A general c l a s s i f i c a t i o n o f methods c u r r e n t l y recognized as t h e r m o a n a l y t i c a l i s given i n Table 1.1 along w i t h t h e p h y s i c a l p r o p e r t y on which they depend and,

'for common methods where i t i s g e n e r a l l y i n use, t h e acceptable a b b r e v i a t i o n (Lombardi, 1980).

Most o f t h e techniques can be d e f i n e d i n e x a c t l y t h e same way

as thermal a n a l y s i s , t h e p h y s i c a l p r o p e r t y i t s e l f

-

"mass" f o r thermogravimetry,

"an e l e c t r i c a l c h a r a c t e r i s t i c " f o r "thermoelectrometry': "a physical p r o p e r t y " i n t h e d e f i n i t i o n . wording i s necessary.

etc.

-

r e p l a c i n g t h e words

I n some instances, however, more p r e c i s e

For example, s i x methods a r e l i s t e d as being dependent on

change i n mass, b u t o n l y two a r e so dependent d i r e c t l y : i s o b a r i c mass-change

7

TABLE 1.1 Classification of thermoanalytical techniques Physical property

Derived techniques

Mass

Isobaric mass-change determination Thermogravimetry Evolved gas detection Evolved gas analysis Emanation thermal analysis Thermoparticulate analysis Heating-curve determination* Differential thermal analysis Differential scanning calorimetry? Thermodilatometry Thermomechanical measurement+ Thermosonimetry5 Thermoacoustimetryg Thermoptometry Thermoel ectrometry Thermomagnetometry

Temperature

En tha 1 py Dimensions Mechanical characteristics Acoustic characteristics Optical characteristics Electrical characteristics Magnetic characteristics

Abbreviation TG EGD EGA

DTA

osc

*

I n t h e c o o l i n g mode t h i s becomes Cooling-curve determination.

t

Two t y p e s , Power-compensation DSC and Heat-flux DSC, can be d i s t i n g u i s h e d .

?

T e s t s under o s c i l l a t o r y l o a d come under t h e heading Dynamic thermomechanical

measurement.

5 Thermosonimetry r e f e r s t o sound e m i t t e d by t h e sample whereas Thermoacoustimetry i n v o l v e s measurement o f changes i n t h e c h a r a c t e r i s t i c s o f imposed a c o u s t i c waves p a s s i n g through t h e sample.

determination, which covers equilibrium techniques, such as the once common "dehydration curves" under a constant partial pressure of water vapour, and thermogravimetry- (TG), which uses a dynamic temperature programme. Evolved gas detection (EGO) and evolved gas analysis (EGA) are secondary techniques whereby materials evolved during heating are detected or analysed, respectively, and the remaining two, emanation thermal analysis and thermoparticulate analysis, are tertiary techniques, being special instances of EGA related to radioactive emanation and particulate matter, respectively. A common method that is not listed i n Table 1.1 is derivative thermogravimetry (DTG), the reason being that derivative curves can be calculated for most measurements and it would be invidious to include only one. Attention should also be drawn to the distinction between derivative and differential, the former applying to the mathematical process and the latter being used only as the adjectival form of "difference" (Lombardi, 1980). Thus, in "differential thermal analysis" (and "differential scanning calorimetry") the "difference in temperature between" (and "the difference in energy inputs into"), "a substance and a reference material is measured". Moreover, heating

a curves

-

i.e.

derivatives (T) T

curves f o r sample temperature against tlme

- "heating-rate

curves", where

dT/dt

- give r i s e t o

two

i s p l o t t e d against temperature

o r time ( t ) , and "inverse heating-rate curves" where

dt/dT

i s p l o t t e d against

o r t: both these have been e x t e n s i v e l y used i n the past. The i n f o r m a t i o n given above, together w i t h t h a t i n Table 1.1, i s probably

adequate t o a l l o w appreciation o f t h e enormous s t r i d e s t h a t have been made over the past decade o r so i n o b t a i n i n g i n t e r n a t i o n a l agreement on a general nomenc l a t u r e and c l a s s i f i c a t i o n system f o r thermoanalytical techniques.

This e f f o r t ,

however, has covered n o t o n l y nomenclature o f methods b u t a l s o t h a t o f components o f equipment, o f aspects o f experimental" technique, o f c r i t i c a l p o i n t s on curves

and o f symbols (Lombardi, 1980) and has been f o r t u n a t e enough t o r e c e i v e the backing o f n a t i o n a l and i n t e r n a t i o n a l standards i n s t i t u t i o n s , such as AFNOR, ASTM and ISO, as w e l l as o f major i n t e r n a t i o n a l bodies such as IUPAC (1974, 1980). Moreover, t h e recommendations i n English have been converted i n t o forms acceptable i n many other language-speaking areas (Lombardi, 1980; Mackenzie, 1981b), since d i r e c t t r a n s l a t i o n i s n o t always p o s s i b l e because o f already accepted conventions i n other languages. 1.2.2 Simultaneous techniques I t i s o f t e n convenient t o make two o r more measurements on one sample a t the same time, leading t o "simultaneous determinations" such as DTA-EGA, TG-EGA, DTA-TG-DTG,

etc.

This has advantages and disadvantages, and one has t o study n o t

only t h e bases o f the techniques themselves b u t a l s o t h e nature o f t h e samples involved before deciding on t h e i r use. For example, EGA i s most p r o f i t a b l y employed i n conjunction w i t h DTA o r TG, as one can then r e l a t e the evolved v o l a t i l e s t o s p e c i f i c changes i n t h e sample; s i m i l a r l y , by comparing simultaneous DTA and DTG curves one can r e a d i l y r e l a t e r e a c t i o n s i n v o l v i n g mass change w i t h

s p e c i f i c enthalpy changes.

And, o f course, t h e r e i s a considerable saving i n

both time and m a t e r i a l .

The major disadvantage i s t h a t optimum c o n d i t i o n s f o r one technique may n o t necessarily be those f o r another. However, t h i s can be f o r example, i n minimized by c a r e f u l s e l e c t i o n of experimental c o n d i t i o n s

-

simultaneous DTA-TG, by using a small sample and/or employing a slow heating r a t e . 1.2.3

Standardization o f techniques

Since thermoanalytical r e s u l t s can vary w i t h experimental technique, t h e Standardization Committee o f ICTA have published a code o f p r a c t i c e l i s t i n g t h e i n f o r m a t i o n t h a t should be supplied w i t h every curve published: they have a l s o been instrumental i n p r o v i d i n g m a t e r i a l s f o r temperature c a l i b r a t i o n o f apparatus (Lombardi, 1980). These aspects should be thoroughly studied by anyone considering a p p l i c a t i o n o f thermal analysis.

9 1.3

APPLICATIONS TO CLAYS Emphasis in this article I ' s , quite deliberately, on the applications or potential applications of the various techniques now available to clays. It is, therefore, impossible to deal adequately with Instrumentation, experimental technique, or even with some basic principles, although all these are critical in determining the quality of thermoanalytical results. To overcome this deficiency the reader is referred to the books of Daniels (1973) and Wendlandt (1974) and to the excellent reviews that have appeared biennially in A n a l y t i c a l C h e m i s t r y Fundamental R e v i e w s (e.g. Murphy, 1978) for a considerable period. In the account that follows, clay mineralogical applications take priority, but due consideration must also be given to the wider field of applications to clays and clay rocks of technological or industrial importance and to accessory minerals, since the presence or absence of these may well determine the suitability of a clay for a particular application. With this wide field in mind, it will be appreciated that the references given are illustrative only: an exhaustive study of all published work would be inordinately long. 1.3.1

IIsobaric) mass-change determination In isobaric mass-change determination the sample is heated at each selected temperature until there is no further mass change and the e q u i l i b r i u m mass is plotted against the temperature: the partial pressure o f the evolved volatile (e.g. water or carbon dioxide) is maintained constant throughout the determination An excellent description of the technique has been given by Weiser and Milligan (1939).

In the past this technique, although not perhaps in an isobaric mode, was widely applied to clays in the derivation of so-called "dehydration curves". An excellent collection of these was given by Nutting (1943) and the technique was still employed for characterization purposes in the 1950s (see e.g., Mackenzie, 1957a). It is rather time-consuming and with the advent of thermogravimetry seems to have fallen into disuse. However, families of isothermal mass-change curves, particularly in their isobaric mode, can probably yield more reliable information on the kinetics of reactions than the TG curves so commonly in use (see below). 1.3.2

Thermogravirnetry (TG) Although DTA has been the most widely used technique in clay mineralogy, the use o f TG and DTG has grown markedly, particularly since the introduction of the Derivatograph, which provided simultaneous DTA-TG-DTG curves, and its commercial production in Hungary in the mid-1950s. This instrument, which is by far the most widely used, and apparently the only one comnercially produced in eastern Europe, has been upgraded several times and now has various optional additional

i 0 attachments f o r thermodilatometry, EGA, etc.

( P a u l i k and Paulik, 1978).

I t s value

i n c l a y mineralogy i s r e a d i l y assessed from t h e simultaneous curves f o r a l a r g e number o f c l a y s and c l a y m i n e r a l s published by Langier-Kuzniarowa (1967).

Out-

s i d e e a s t e r n Europe, thermogravimetry was s t i m u l a t e d by t h e commercial p r o d u c t i o n o f t h e r o b u s t Stanton thermobalance i n 1954 and simultaneous techniques by t h e i n t r o d u c t i o n o f the M e t t l e r Thermoanalyzer (Wiedemann, 1964).

A wide range o f

thermobalances and simultaneous DTA-TG instruments s u i t a b l e f o r c l a y s t u d i e s can now be purchased (e.g.

Dunn, 1980). t h e number o f the l a t t e r tending t o increase

as a v a i l a b l e s e n s i t i v i t y has increased.

O f the various types of balance system

used (Keattch and Dollimore, 1975), t h e n u l l - p o i n t e l e c t r o b a l a n c e now seems t h e most common.

When simultaneous equipment i s n o t used and a comparison i s made

between DTA and DTG curves, g r e a t care must be taken t o ensure t h a t a l l experimental v a r i a b l e s are i d e n t i c a l f o r b o t h determinations

-

f o r example, use o f a d i f f e r e n t

h e a t i n g r a t e can d i s p l a c e peaks and even a l t e r peak shape a p p r e c i a b l y ( A l i e t t i , B r i g a t t i and Poppi, 1979). The main uses o f TG add DTG (which must be considered t o g e t h e r ) i n c l a y mineralogy have been i n determining t h e reasons f o r DTA peaks, assessing t h e range over which r e a c t i o n s occur and o b t a i n i n g q u a n t i t a t i v e i n f o r m a t i o n .

The methods

a r e n o t p a r t i c u l a r l y s u i t a b l e f o r i d e n t i f i c a t i o n studies, although t h e occurrence o f one o r two peaks on a DTG curve, and the r e l a t i v e s i z e s o f t h e two peaks when they appear, can apparently be employed i n c h a r a c t e r i z i n g s e r p e n t i n e m i n e r a l s (Morandi and F e l i c e , 1979) and t h e disappearance o f the hygroscopic m o i s t u r e peak a f t e r K - s a t u r a t i o n can be used t o d i s t i n g u i s h h y d r o b i o t i t e from m o n t m o r i l l o n i t e

i n some s o i l s (Ryzhova, 1980).

These must be regarded as r a t h e r i s o l a t e d instances

and t h e main use o f the techniques has undoubtedly been t o o b t a i n q u a n t i t a t i v e i n f o r m a t i o n on evolved v o l a t i l e s , e t c .

I n such a p p l i c a t i o n s , however, g r e a t care

must be exercised, as t h e mass change, during, f o r example, a d e h y d r o x y l a t i o n r e a c t i o n , could be s e r i o u s l y affected iff e r r o u s i r o n i n t h e l a t t i c e were simultaneously o x i d i z e d t o f e r r i c .

For t h i s reason too, q u a n t i t a t i v e determina-

t i o n o f m i n e r a l s by DTG (Smalley and Xidakis, 1979) should be undertaken o n l y when s u f f i c i e n t c o n f i r m a t o r y evidence t h a t n o t h i n g l i k e l y t o i n t e r f e r e w i t h t h e DTG peak area i s present and when comparison can be made w i t h a mineral t h a t i s

i d e n t i c a l w i t h t h a t i n the c l a y .

I t i s noteworthy i n t h i s respect t h a t even t h e

s a t u r a t i n g c a t i o n o f m o n t m o r i l l o n i t e a f f e c t s t h e c h a r a c t e r and temperature o f the DTG dehydroxylation peak (Schomburg and S t b r r ,

1978a).

TG has proved

valuable i n e l u c i d a t i n g t h e nature of DTA peaks f o r p a l y g o r s k i t e and s e p i o l i t e (Fernandez Alvarez, 1978) and f o r m o n t m o r i l l o n i t e ( I l i u t a , Drimus and Preda, 1978) and OTG i n r e v e a l i n g m u l t i p l e r e a c t i o n s n o t obvious on t h e TG curve (e.g. Rautureau and Fornes, 1978).

Mifsud,

Changes i n t h e temperature range and magnitude o f

t h e step on t h e TG, o r peak on t h e DTG, curve can p r o v i d e v a l u a b l e c o n f i r m a t o r y evidence f o r a p a r t i c u l a r phenomenon, such as t h e occurrence o f NH,+

i n some

11

Japanese dioctahedral micas (Higashi, 1978): the presence o f t h i s i o n not o n l y moves t h e dehydroxylation r e a c t i o n t o a lower temperature b u t a l s o increases t h e mass l o s s because o f the e v o l u t i o n o f NH,. TG and DTG also y i e l d useful i n f o r m a t i o n on accessory minerals.

Thus, t h e

dehydroxylation o r decarbonation o f accessory hydroxide o r carbonate minerals i s u s u a l l y obvious on TG and DTG curves (e.g. Iwasa, 1978) and, provided c a r e f u l c a l i b r a t i o n i s performed i n advance, even s a l t s , such as sodium carbonate and sodium c h l o r i d e , can be q u a n t i t a t i v e l y determined i n s a l i n e c l a y s (Asomoza et ai., 1978). TG has been e x t e n s i v e l y i n v e s t i g a t e d as a means f o r studying t h e k i n e t i c s o f r e a c t i o n s because ( a ) determinations are l e s s time-consuming than isothermal i n v e s t i g a t i o n s , (b) w i t h isothermal methods, some r e a c t i o n occurs before the temperature o f i n t e r e s t i s reached and ( c ) the whole temperature range i s covered w i t h o u t any missing regions (see Sharp, 1972).

While these comments are c o r r e c t ,

actual l i m i t a t i o n s on the d e r i v a t i o n o f k i n e t i c parameters are formidable, n o t o n l y because o f the occurrence of temperature gradients w i t h i n t h e sample but a l s o because o f other more fundamental aspects d e a l t w i t h below.

Most methods

f o r i n t e r p r e t i n g TG curves are based on t h e simple r a t e equation da/dt

k(1

- a)"

(where a i s the f r a c t i o n decomposed i n time t , n the order o f r e a c t i o n and

k

the

r a t e constant) combined w i t h t h e Arrhenius equation (2)

k = Aexp (-E/RT)

(where A i s t h e pre-exponential f a c t o r , E the a c t i v a t i o n energy and R the gas constant), t h e temperature

(where

T~

T

being defined by

i s the i n i t i a l temperature and 8 , = d T / d t , the heating r a t e ) : note t h a t

the heating r a t e i s assumed t o be c o n s t a i t .

The two usual procedures are then

e i t h e r t o f o l l o w t h e d i f f e r e n t i a t i o n method o f Freeman and C a r r o l l (1958) o r the i n t e g r a t i o n method o f Coats and Redfern (1963).

The l a t t e r i s generally regarded

as y i e l d i n g most r e l i a b l e r e s u l t s - see, f o r example, t h e recent study o f Boy and BMhme (1979) and compare t h e i r r e s u l t s f o r t h e dehydroxylation o f k a o l i n i t e w i t h those determined by a v a r i e t y o f experimental methods (Sharp, 1972). I n a l l these studies approximations a r e involved and, although the d i f f i c u l t i e s inherent i n dealing w i t h a dynamic system (see Sharp, 1972) are imnense, other more fundamental aspects must a l s o be remembered.

One need o n l y consider t h e

1 2

equations above.

The f i r s t two a r e gas-phase equations and t h e r e i s no guarantee

they would apply t o a s o l i d : moreover, one could j u s t i f i a b l y query t h e physical s i g n i f i c a n c e o f the derived "order o f r e a c t i o n " and " a c t i v a t i o n energy" when so applied. Indeed, recent EGA studies have shown t h a t decomposition r e a c t i o n s can, n o t unexpectedly, change mechanism during the r e a c t i o n (Garn, Kawalec and Chang, 1978; P r i c e e t al., 1980).

Moreover, many s o l i d - s t a t e decompositions i n powder

systems a r e d i f f u s i o n c o n t r o l l e d ( s e n s u l a t o , covering both i n t e r - and i n t r a p a r t i c l e d i f f u s i o n ) , o r f o l l o w some o t h e r law, so t h a t t h e "order o f r e a c t i o n " i s r a t h e r meaningless (Sharp, 1972).

Consequently, Garn (1979) has q u i t e r i g h t l y

suggested t h a t t h e term " a c t i v a t i o n energy" should be replaced by "temperature c o e f f i c i e n t o f reaction".

While t h e use o f small samples and/or o f a constant

temperature regime over the decomposition i n t e r v a l (Rouquerol, 1970; P a u l i k and Paulik, 1972) would reduce temperature gradients t h a t i n t e r f e r e w i t h i n t e r p r e t a t i o n , these v a r i a n t s do n o t o b v i a t e t h e more fundamental o b j e c t i o n s and much study i s s t i l l r e q u i r e d d e s p i t e t h e l a r g e numbers o f papers t o be found i n t h e J o u r n a l of Thermal A n a l y s i s , Thermochimica A c t a and Thermal A n a l y s i s A b s t r a c t s

over the past decade.

It should be noted, however, t h a t the basic o b j e c t i o n s

r a i s e d above do n o t necessarily mean t h a t t h e numerical vaiues obtained f o r c e r t a i n k i n e t i c parameters have no p r a c t i c a l value. 1.3.2.1

Evolv-d gas d e t e c t i o n (EGD)

I n EGD one determines whether o r n o t gas e v o l u t i o n i s associated w i t h a thermal effect.

I t i s customary, therefore, t o use i t i n conjunction w i t h DTA (mass loss

being adequate i n d i c a t i o n o f gas e v o l u t i o n i n TG) and t h e s i m p l e s t method i s t o i n s e r t a thermal c o n d u c t i v i t y c e l l i n the c a r r i e r gas stream coming from t h e equipment (Ingraham, 1967), although several o t h e r methods are a l s o a v a i l a b l e (Daniels, 1973). An EGD technique t h a t has proved very useful i n studying polymer degradation on heating i s thermal v o l a t i l i z a t i o n a n a l y s i s (McNeill, 1977).

I n t h i s t h e sample

i s heated i n a high vacuum chamber connected t o a vacuum pump though a t r a p cooled i n l i q u i d nitrogen, the pressure between t h e sample and the t r a p being measured by a P i r a n i gauge.

Whenever t h e sample decomposes, the pressure increases and

the decomposition o f the polymer can thus be followed.

The method i n t h i s form

does n o t g i v e any i n d i c a t i o n o f t h e v o l a t i l e products formed b u t t h e equipment has been modified so t h a t the condensed v o l a t i l e s i n t h e c o l d t r a p b o i l o f f as t h e t r a p i s allowed t o heat up.

Since t h e b o i l i n g p o i n t permits each component

t o be i d e n t i f i e d (McNeill, 1980), t h e method i s upgraded t o EGA

- always

assuming

t h e r e i s no i n t e r a c t i o n between condensed v o l a t i l e s .

A method f o r simultaneous DTA and EGD by measuring the i n t e n s i t y o f a l a s e r beam t r a v e r s i n g t h e sample c e l l above the sample has r e c e n t l y been described i n N e t s u s o k u t e i (1980).

Although a p p l i e d t o d e r i v e the phase diagram o f a known

13 b i n a r y l i q u i d system, t h e method i s c l e a r l y non-specific and could probably be adapted t o DTA-EGD o f clays o r clay-organic complexes: i t s advantage l i e s i n t h e f a c t t h a t i t gives a very sharp i n f l e c t i o n immediately vapour e v o l u t i o n commences even w i t h very slow heating rates. 1.3.2.2

Evolved g a s a n a l y s i s (EGA)

EGA i s much more useful, and indeed

usual, than EGD, since i t enables

determination o f t h e i d e n t i t y and/or amount o f the evolved v o l a t i l e material.

It

i s normally employed along w i t h DTA, TG o r DTA-TG-DTG, thus enabling q u a n t i t a t i v e e v a l u a t i o n q f the e f f e c t s on t h e curves i n terms o f s p e c i f i c v o l a t i l e s . Basic i n f o r m a t i o n on various methods o f EGA, such as mass spectrometry, gas chromatography, i n f r a - r e d absorption and s e l e c t i v e sorption, w i l l be found i n the books o f Lodding (1967), Mackenzie (1970-72), Daniels (1973) and Wendlandt (1974), i n t h e Proceedings o f the various I n t e r n a t i o n a l Conferences on Thermal Analysis (Redfern, 1965; Schwenker and Garn, 1968; Wiedemann, 1972, 1980; B U Z ~ S , 1975; Chihara, 1977), i n t h e Proceedings o f the F i r s t European Symposium on Thermal Analysis (Dollimore, 1976) and i n various s c i e n t i f i c j o u r n a l s . B r i e f l y , t h e methods of EGA f a l l i n t o two classes: ( a ) those f o r which p r i o r knowledge of t h e nature o f the v o l a t i l i z e d m a t e r i a l i s unnecessary and ( b ) those f o r which such knowledge i s essential. While the former, which i n c l u d e mass spectrometry and gas chromatography, are by f a r the most generally useful, t h e l a t t e r have a d e f i n i t e place i n studies, such as those on clays, where a l i m i t e d number of v o l a t i l e m a t e r i a l s are t o be expected. Both have indeed been a p p l i e d i n c l a y studies: f o r example, Mhller-Vonmoos and M h l l e r (1975) have demonstrated how mass spectrometry combined w i t h DTA can reveal the presence o f organic carbon, p y r i t e and various carbonate minerals i n a clay, whereas Morgan (1977) has used DTA and separate detectors t o determine when and how much water, carbon d i o x i d e and- sulphur d i o x i d e (from o x i d a t i o n o f p y r i t e ) a r e evolved from clays, shales and s c h i s t s and P a u l i k and P a u l i k (1978) have used t h e i r technique o f thermal gas t i t r i m e t r y (i.e.

sorption o f the v o l a t i l e i n a suitable solution

o r solvent f o l l o w e d by t i t r a t i o n ) along w i t h DTA-TG-DTG t o determine the amounts o f contaminating a l u n i t e and c a l c i t e i n bauxites. The optimum technique depends on circumstances

-

and, n o t infrequently,on

finance.

While a s e r i e s o f detectors s p e c i f i c f o r one v o l a t i l e o n l y can be r e a d i l y and r e l a t i v e l y cheaply purchased and attached i n s e r i e s t o a s u i t a b l e thermal analysis instrument, t h e more f l e x i b l e and universal system using mass spectrometry i s expensive.

I n s e t t i n g up equipment and assessing r e s u l t s several important aspects must be kept i n mind: for example, t h e free, volume around the sample must

be r e l a t i v e l y small t o avoid undue d i l u t i o n o f evolved m a t e r i a l w i t h c a r r i e r gas, t h e i n t e r f a c e between a mass spectrometer and t h e thermal analysis instrument must be chosen w i t h care so t h a t one v o l a t i l e i s n o t p r e f e r e n t i a l l y enriched a t

1 4 t h e expense o f another, and t h e l i k e l i h o o d of two v o l a t i l e s r e a c t i n g b e f o r e measurement must be assessed.

A v a r i a n t o f EGA, pyrolysis-gas chromatography-mass spectrometry, which was developed m a i n l y f o r t h e study o f s y n t h e t i c polymers, has proved extremely u s e f u l i n t h e i n v e s t i g a t i o n and c h a r a c t e r i z a t i o n o f s o i l organic m a t t e r (Bracewell and Robertson, 1977) and should a l s o be a p p l i c a b l e t o o r g a n i c m a t t e r i n c l a y deposits. I n t h i s method the sample i s very r a p i d l y pyrolysed i n an i n e r t atmosphere, t h e products being separated by gas chromatography and i d e n t i f i e d by mass spectrometry. Such a procedure y i e l d s f a i r l y l a r g e fragments o f t h e o r i g i n a l molecules, thus g i v i n g an i n s i g h t i n t o t h e n a t u r e o f t h e organic polymers present.

I n some

circumstances, and p a r t i c u l a r l y when i n t e r a c t i o n between products i s l i k e l y t o occur, a p r e f e r a b l e system i s pyrolysis-mass spectrometry (Bracewell and Robertson, 1980), d e s p i t e i t s h i g h e r c o s t because o f t h e more e l a b o r a t e data h a n d l i n g system required. 1.3.2.2.1 Ema n a t ion t h e r m a l a n a l y s i s . T h i s i s e s s e n t i a l l y a v a r i a n t o f EGA where t h e r a d i o a c t i v e emanation evolved d u r i n g h e a t i n g o f t h e sample i s measured. While t h i s d e f i n i t i o n would n o r m a l l y i n c l u d e o n l y radon isotopes, i n p r a c t i c e t h e method has been extended t o non-radioactive i n e r t gases, such as argon, k r y p t o n o r xenon, and t h e i r r a d i o a c t i v e isotopes: t h e r e i s thus a g r a d a t i o n i n t o normal EGA.

Materials

n o t c o n t a i n i n g i n e r t gas can be l a b e l l e d by d i f f u s i n g gas i n t o t h e s o l i d a t h i g h pressures and temperatures, by i n c l u d i n g t h e gas d u r i n g synthesis o r by bombardi n g t h e s u r f a c e o f the sample w i t h a c c e l e r a t e d ions o f i n e r t gas.

Changes i n gas

r e l e a s e r a t e on h e a t i n g can then be c o r r e l a t e d w i t h dehydration, decomposition, r e c r y s t a l l i z a t i o n , phase t r a n s i t i o n , s o l i d - s t a t e r e a c t i o n s and changes i n s u r f a c e properties.

Development o f emanation thermal a n a l y s i s i n c o n j u n c t i o n w i t h DTA i s

due mainly t o Balek and h i s c o l l a b o r a t o r s i n Czechoslovakia and i n f o r m a t i o n on theory, i n s t r u m e n t a t i o n and a p p l i c a t i o n s i s b e s t obtained from a r e c e n t exhaustive review (Balek, 1977).

Although no s e r i o u s s t u d i e s seem y e t t o have been made on

c l a y s , accessory minerals such as i r o n oxides, q u a r t z and z e o l i t e s have r e c e i v e d attention.

Several p o s s i b l e a p p l i c a t i o n s t o c l a y minerals, p a r t i c u l a r l y h o l l o w

f i b r o u s and h i g h l y disordered species, s p r i n g t o mind, b u t i t has y e t t o be e s t a b l i s h e d whether t h e i n f o r m a t i o n obtained would be s u p e r i o r t o t h a t f r o m more conventional methods. So-called "temperature-programmed d e s o r p t i o n curves" might we1 1 be regarded as r e l a t e d t o t h e above, even although t h e gases i n v o l v e d a r e n o t i n e r t .

Recent . _

work w i t h these (Criado et ai., 1980) has c l e a r l y shown t h e i r value i n s t u d y i n g t h e k i n e t i c s o f d e s o r p t i o n o f sorbed gas. 1.3.2.2.2

T h e m o p a r t i c u l a t e a n a l y s i s . Since t h e degradation o f s y n t h e t i c polymers

y i e l d s condensation n u c l e i as w e l l as molecular species, t h e r m o p a r t i c u l a t e a n a l y s i s ,

1 5

whereby t h e amount (and sometimes i d e n t i t y ) o f such n u c l e i a r e measured as a f u n c t i o n o f temperature, i s c l e a r l y c l o s e l y r e l a t e d t o EGA. P a r t i c l e s o f t h e order o f 1-100 nm i n s i z e a r e i n v o l v e d and an e a r l y review o f equipment, technique and a p p l i c a t i o n s was g i v e n by Murphy (1967).

A more r e c e n t v a r i a n t ( g i v e n various

names by i t s o r i g i n a t o r s ) i s a c l o s e r e l a t i v e o f EGD, s i n c e n e i t h e r the amount nor i d e n t i t y o f t h e p a r t i c l e s i s measured, b u t when combined w i t h mass spectrometry becomes again e s s e n t i a l l y EGA (see Smith, P h i l l i p s and Kaczmarek, 1976; Smith, Meier and P h i l l i p s , 1977). Although t h e technique has been a p p l i e d o n l y t o l a r g e organic molecules t h e r e a r e p o s s i b i l j t i e s o f a p p l i c a t i o n i n c o n d i t i o n s where chemical t r a n s p o r t occurs. Since chemical t r a n s p o r t has r e c e n t l y been observed on h e a t i n g c e r t a i n i r o n oxides

(E. Paterson, personal communication), i t i s , therefore, applications e x i s t i n the c l a y f i e l d . 1.3.3

marginally possible t h a t

Heating curve determination

Heating curves and the two d e r i v a t i v e s , h e a t i n g - r a t e curves and i n v e r s e heati n g - r a t e curves, were a t one time w i d e l y used i n c l a y s t u d i e s : indeed, t h e f i r s t thermoanalytical records f o r c l a y s were, as mentioned above, h e a t i n g - r a t e curves. However, h e a t i n g curves f e l l i n t o disuse, as t h e low s e n s i t i v i t y o f r e c o r d i n g necessary t o cover the whole temperature range o f i n t e r e s t precluded d e t e c t i o n o f small thermal e f f e c t s , and i n t e r e s t i n t h e two d e r i v a t i v e curves waned as DTA became established, s i n c e e s s e n t i a l l y t h e same i n f o r m a t i o n c o u l d be obtained more readily.

So f a r as t h e author i s aware, no heating-curve determinations on clays

a r e now performed.

1.3.4 D i f f e r e n t i a l thermal a n a l y s i s (DTA) DTA i s by f a r t h e best known and most w i d e l y used thermoanalytical technique i n c l a y studies, whether on i t s own o r simultaneously w i t h o t h e r methods such as

TG, EGA, e t c .

I t s main uses a r e i n " f i n g e r - p r i n t i n g " specimens, i n d e t e c t i n g

c e r t a i n accessory m i n e r a l s and "abnormal" species o f c l a y minerals, i n d e t e c t i n g changes i n mineralogy w i t h depth o r distance, i n q u a n t i t a t i v e (or, more f r e q u e n t l y , s e m i - q u a n t i t a t i v e ) s t u d i e s and i n determining heat s t a b i l i t y o r t h e occurrence o f solid-state reactions.

L i m i t a t i o n s imposed by i n s t r u m e n t a t i o n , technique and

n a t u r a l v a r i a t i o n s i n m i n e r a l s make accurate q u a n t i t a t i v e work d i f f i c u l t and preclude i t s use d i a g n o s t i c a l l y , except i n s p e c i a l circumstances.

A s these d e t a i l s

and p o s s i b l e methods o f m i n i m i z i n g l i m i t a t i o n s a r e so wel1,known (see Mackenzie, 1957b, 1970-72),

t h e method w i l l n o t be d e a l t w i t h here a t any g r e a t l e n g t h and

reference w i l l be made o n l y t o some r e c e n t developments. Commercial equipment design has improved enormously over t h e p a s t decade and, although t h e r e may n o t be so many instruments on t h e market now as a few years ago, those companies t h a t c u r r e n t l y manufacture equipment have introduced many

16 improvements,

While no instrument i s u n i v e r s a l and t h e instrument must be chosen

having regard t o t h e proposed a p p l i c a t i o n s , t h e general t r e n d towards t h e use o f small samples o f o n l y a few m i l l i g r a m s i s t o be welcomed, as i t minimizes e r r o r s due t o thermal g r a d i e n t s and, provided r e c o r d i n g s e n s i t i v i t y i s adequate, enables simultaneous DTA-TG t o be v a l i d l y used.

The f a c t t h a t two markedly d i f f e r e n t DTA

instruments y i e l d e s s e n t i a l l y t h e same curves f o r a d e h y d r o x y l a t i o n r e a c t i o n when sample s i z e i s l e s s than 30 mg (Broersma et ai., very small samples.

1978) confirms t h e value o f using

Many manufacturers a l s o produce several types o f specimen

holders f o r e a c h instrument ,so t h a t t h e optimum (Wilburn, 1972) can be selected, and c o n t r o l o f atmosphere around the Sample i s now u n i v e r s a l .

These advances i n

i n s t r u m e n t a t i o n enable DTA t o be much more w i d e l y and v a l i d l y used i n c l a y s t u d i e s than ever before. An advance i n methodology has been the i o t r o d u c t i o n o f stepwise h e a t i n g (Staub and Perron, 1974; Simonsen and Zaharescu, 1979), by which temperature i s increased i n small steps (0.5-10°C,

depending on t h e r e a c t i o n ) i n s t e a d o f continuously,

e q u i l i b r i u m being a t t a i n e d a t each step.

This i s e s s e n t i a l l y e q u i v a l e n t t o

o b t a i n i n g a s e r i e s o f isothermal measurements d u r i n g one d e t e r m i n a t i o n w i t h t h e r e s u l t t h a t heats o f m e l t i n g , t r a n s i t i o n , etc.,

can be determined more a c c u r a t e l y .

To t h e a u t h o r ' s knowledge, t h i s technique has n o t y e t been a p p l i e d t o clays: t h e r e i s no reason t o suppose i t i s i n a p p l i c a b l e , although any apparent advantages would have t o be c r i t i c a l l y checked.

The sample c o n t a i n e r can sometimes have an undesir-

a b l e e f f e c t ( M a r t i n V i v a l d i , G i r e l a V i l c h e z and F e n o l l Hach-Ali, 1964; D o l l i m o r e and Mason, 1981): t o prevent c o n t a i n e r e f f e c t s i n m e t a l l u r g y , electromagnetic l e v i t a t i o n of the sample has been introduced (Jorda, F l U k i g e r and MUller, 1978) b u t t h i s would u n f o r t u n a t e l y be impossible w i t h c l a y s and t h e m a t e r i a l o f the specimen holder must be c a r e f u l l y selected.

The n e c e s s i t y f o r care i n sample

p r e p a r a t i o n has been emphasized by work on q u a r t z (Moore and Rose, 1979), where small exothermic peaks observed a f t e r g r i n d i n g i n s t e e l and agate v i b r a t i o n m i l l s have been a t t r i b u t e d t o o x i d a t i o n o f contaminating i r o n and r e l e a s e o f s t o r e d energy, r e s p e c t i v e l y .

On t h e o t h e r hand, t h e doubling o f t h e d e h y d r o x y l a t i o n

endotherm o f g o e t h i t e on g r i n d i n g has been a t t r i b u t e d t o p a r t i c l e s i z e e f f e c t s (Murad, 1979).

The value o f c o n t r o l l e d atmosphere has been confirmed by s t u d i e s

on carbonate minerals, which show t h a t i n a carbon d i o x i d e atmosphere t h e m i n e r a l s present can be i d e n t i f i e d and t h e amounts estimated f o r o n l y 0.125 mg i n a 50 mg sample (Warne and M i t c h e l 1 , 1979). The main use o f DTA i n c l a y studies, r e c e n t l y as i n t h e past, has been as an a d j u n c t t o o t h e r techniques i n c h a r a c t e r i z a t i o n s t u d i e s .

Yet some i n v e s t i g a t i o n s

r e v e a l t h e value o f the technique i n i t s own r i g h t , and i t may be a p p o s i t e t o c i t e some examples.

Thus, peak temperature o f the low-temperature endotherm f o r

H-saturated allophane and t h a t o f the high-temperature exotherm f o r Na-allophane increases and decreases, r e s p e c t i v e l y , w i t h i n c r e a s i n g SiO,:Al,O,

r a t i o (Henmi,

17 1980).

Moreover, t h e exotherm o f allophane i s lowered i n temperature and

broadened on admixture w i t h i r o n o x i d e g e l s o r hydrous i r o n o x i d e minerals, whereas i t i s u n a f f e c t e d by oxide m i n e r a l s such as hematite o r maghemite (Suzuki and Satoh, 1980).

For d i c k i t e s , i n c r e a s i n g breadth o f t h e dehydroxylation

endotherm seems t o be approximately associated w i t h i n c r e a s i n g degree of disorder, although some exceptions suggest t h i s r u l e i s n o t r i g i d ( B r i n d l e y and P o r t e r , 1978).

I t i s w e l l knownthat DTA curves o f i n t e r s t r a t i f i e d m i n e r a l s a r e d i f f i c u l t

to interpret

-

presumably because d e h y d r o x y l a t i o n o f one l a y e r n e c e s s a r i l y a f f e c t s

t h a t o f neighbouring l a y e r s whether o r n o t they a r e o f the same t y p e

-

b u t i t has

r e c e n t l y been shown t h a t the shape and s i z e of t h e hygroscopic m o i s t u r e peak f o r Sr-saturated mica-montmorillonites can be used t o assess t h e p r o p o r t i o n o f expansi b l e l a y e r s present (Inoue, Minato and Utada, 1978).

Since s o l i d - s t a t e r e a c t i o n s

t h a t occur between carbonate minerals, s o l u b l e s a l t s and mica on h e a t i n g a l s o take place w i t h m i c a - m o n t m o r i l l o n i t e b u t n o t w i t h p a l y g o r s k i t e (Mashhady et al., 1980) and manganese oxides r e a c t w i t h k a o l i n i t e i n t h e s o l i d s t a t e a t e l e v a t e d temperatures (Holland and Segnit, 1976), c a r e must be taken i n i n t e r p r e t i n g curves f o r systems I n p o l l u t i o n studies, t h e area o f t h e dehydroxylation

c o n t a i n i n g such mixtures.

endotherm o f c h y s o t i l e can a p p a r e n t l y be used t o assess asbestos content (Menis, Mackey and Garn, 1978).

The e f f e c t s o f sorbed organic m a t e r i a l s on DTA curves

f o r c l a y m i n e r a l s have been examined by, i n t e r alia, Hllbner (1927) and Eltantawy (1979), w h i l e c l a y - o r g a n i c r e a c t i o n products have been i n v e s t i g a t e d by Kuroda and Kato (1979). Although DTA has been w i d e l y used i n s t u d y i n g t h e k i n e t i c s o f r e a c t i o n s , t h e r e i s no doubt t h a t t h e techniques used a r e f r a u g h t w i t h even more d i f f i c u l t i e s than mentioned above f o r TG (Sharp, 1972).

However, a considerable p r o p o r t i o n o f

thermoanalytical papers do deal w i t h t h i s s u b j e c t and a t r e a t i s e on non-equilibrium k i n e t i c s has appeared (Koch, 1977).

As mentioned above under TG, i t i s u n l i k e l y

t h a t decomposition o f s o l i d s f o l l o w s t h e same k i n e t i c s throughout t h e whole range from a = 0 t o a = 100 (where a i s t h e f r a c t i o n reacted) and thus t h e best t h a t can be expected would be t o deduce values f o r a l i m i t e d a range. 1.3.5

D i f f e r e n t i a l scanning c a l o r i m e t r y (DSC)

Two types o f DSC a r e recognized, power-compensation DSC and h e a t - f l u x DSC (Table 1.1). The r e l a t i o n s h i p s between these and DTA have r e c e n t l y been discussed i n d e t a i l (Mackenzie, 1980) and need n o t be repeated here.

It need only be

mentioned t h a t DSC was o r i g i n a l l y l i m i t e d t o a maximum temperature o f about 450°C because o f t h e i n c r e a s i n g importance above t h a t temperature o f r a d i a t i v e heat t r a n s f e r : however, r e c e n t t e c h n o l o g i c a l advances have enabled t h e temperature range t o be extended t o about 750-800°C so t h a t a l i m i t e d a p p l i c a t i o n i n c l a y mineralogy i s now possible. The main advantage o f DSC over DTA i s t h a t i t i s i n h e r e n t l y q u a n t i t a t i v e f o r

18 change i n enthalpy, heat capacity, e t c .

Consequently, i t can, f o r example, be

used t o determine n o t o n l y t h e enthalpy change o c c u r r i n g d u r i n g a r e a c t a l s o t h e t o t a l energy r e q u i r e d t o f i r e a c l a y over any s p e c i f i c temperatur, up t o about 750°C.

The method has, however, n o t been w i d e l y used i n t h e study

0,

clays, although i n v e s t i g a t i o n s on m o n t m o r i l l o n i t e (Homshaw and Chaussidon, 1979), on smectites, z e o l i t e s , h a l l o y s i t e and opal (Eger, Cruz-Cumplido and F r i p i a t , 1979) and on s y n t h e t i c g o e t h i t e (Paterson, 1980) suggest i t can y i e l d v a l u a b l e r e s u l t s , both q u a l i t a t i v e and q u a n t i t a t i v e , i n connection w i t h s o l v a t i o n problems.

I t has

a l s o been u s e f u l i n d e t e c t i n g d i f f e r e n t types o f surface hydroxyl groups on s y n t h e t i c g o e t h i t e (Paterson and S w a f f i e l d , 1980) and i n r e v e a l i n g major d i f f e r e n c e s i n s o i l and rock q u a r t z samples (Barwood and Hajek, 1979). I n view o f these i n d i c a t i o n s , i t would appear t h a t DSC i s l i k e l y t o be increasi n g l y used i n c l a y s t u d i e s over t h e n e x t few years, p a r t i c u l a r l y i n q u a n t i t a t i v e and surface i n v e s t i g a t i o n s .

An extension o f t h e c u r r e n t upper temperature l i m i t

would open even g r e a t e r prospects, b u t t h i s does n o t seem l i k e l y i n t h e near f u t u r e . 1.3.6

Thermodilatometry

I n thermodilatometry t h e volume or, more u s u a l l y , t h e l i n e a r dimensionsal change on h e a t i n g i s s t u d i e d as a f u n c t i o n o f temperature under n e g l i g i b l e load. The method i s an o l d one and indeed t h e o b s e r v a t i o n o f shrinkage o f c h i n a c l a y on f i r i n g gave Wedgwood (1782) t h e idea f o r h i s famous pyrometer, which, although g r o s s l y i n a c c u r a t e i n absolute terms, was t h e o n l y means a v a i l a b l e f o r a c c u r a t e l y comparing h i g h temperatures f o r 50 years o r more from t h e 1780s.

Thermodilatometry i s a very common method i n ceramic technology, where a knowledge o f t h e f i r i n g shrinkage o f c l a y i s e s s e n t i a l and where b l e n d i n g can be used t o minimize such shrinkage.

I t has a l s o been a p p l i e d t o c l a y minerals, b u t

t h e determination i s so s e n s i t i v e t o degree o f o r i e n t a t i o n o f p l a t y minerals, t h e nature and amounts o f accessories, etc., o f g r e a t value f o r i d e n t i f i c a t i o n ,

t h a t t h e r e s u l t s have n o t been considered

There have been r e c e n t i n d i c a t i o n s , however,

t h a t t h e method can have some l i m i t e d d i a g n o s t i c uses, s i n c e i t would appear t h a t small amounts o f d i c k i t e o c c u r r i n g i n a k a o l i n can be i d e n t i f i e d by an expansion e f f e c t a t about 650°C t h a t i s p a r t i c u l a r l y s t r o n g f o r d i c k i t e (Schomberg and Schtlrr, 1978b).

Thermodilatometry has a l s o r e c e n t l y been used t o o b t a i n informa-

t i o n on t h e mechanism o f i n i t i a t i o n o f t h e d e h y d r o x y l a t i o n and " m u l l i t i z a t i o n " r e a c t i o n s o f k a o l i n i t e (Flank, 1979) and t h e r e i s no doubt t h a t , because o f i t s s e n s i t i v i t y t o o r i e n t a t i o n e f f e c t s , i t has p o t e n t i a l a p p l i c a t i o n i n assessing the degree o f o r i e n t a t i o n o f p l a t y p a r t i c l e s i n c l a y s t r a t a . Thermodilatometers and d i f f e r e n t i a l thermodilatometers a r e r e a d i l y a v a i l a b l e commercially and d e r i v a t i v e thermodilatometric curves can be e a s i l y recorded from t h e r m o d i l a t o m e t r i c measurements.

When performed simultaneously w i t h DTA-TG,

thermodilatometry and d e r i v a t i v e thermodilatometry can y i e l d r e s u l t s t h a t h e l p t o

19

explain features on DTA and TG curves ( P a u l i k and Paulik, 1978). Such simultaneous measurements may, therefore, be worthy o f f u r t h e r study by c l a y mineralogists. 1.3.7

Thermomechanical measurements

AS noted i n Table 1.1, thermomechanical measurements can be c a r r i e d out w i t h a

n o n - o s c i l l a t o r y (i.e.

s t a t i c ) o r w i t h an o s c i l l a t o r y ( i . e . dynamic) load. The former, which are c u r r e n t l y commonly r e f e r r e d t o as "thermomechanical analysis" o r "TMA" (despite t h e e r r o r o f using "analysis" i n such a connotation) can have various modes depending on whether the s t r e s s applied t o t h e sample i s compression, tension, f l e x u r e o r t o r s i o n and on whether deformation o r p e n e t r a t i o n i s measured (Daniels, 1973). Clearly, t h i s technique i s c l o s e l y r e l a t e d t o thermodilatometry, t h e d i f f e r e n c e being i n the use o f a f i n i t e load, whether p o s i t i v e o r negative: indeed, t h e same equipment can be used f o r both measurements (see Flank, 1979) and t h e r e i s some argument as t o whether thermodilatometry should be separately recognized.

Dynamic thermomechanical measurements enable a d i f f e r e n t s e t o f

parameters, such as the. shear modulus, the mechanical damping index, the loss tangent ( t a n 6 ) , etc.,

t o be measured as a f u n c t i o n o f temperature.

I t should

be noted t h a t i n d i v i d u a l isothermal measurements a r e n o t included. Thermomechanical measurements, both s t a t i c and dynamic, f i n d extensive use i n polymer science, b u t the p o t e n t i a l f o r t h e i r use i n c l a y mineralogy appears s l i g h t although i t i s f a s c i n a t i n g t o speculate on the p o s s i b l e r e s u l t o f applying the t o r s i o n a l pendulum (so-called " t o r s i o n a l b r a i d analysis"

- see Daniels,

1973) t o

c l a y samples.

I n c i v i l engineering and ceramic technology many o f the techniques are, i n f a c t , applied, o f t e n i s o t h e r m a l l y b u t sometimes, as when t e s t i n g hightemperature r e f r a c t o r i e s , using a c o n t r o l l e d temperature programme.

I n ceramic

and r e f r a c t o r y studies, measurements may be made o f deformation under load, shear modulus, modulus of e l a s t i c i t y , crushing strength, r e f r a c t o r i n e s s under load, creep, s t r e s s r e l a x a t i o n and toughness and of t h e i r v a r i a t i o n w i t h temperature. I n c i v i l engineering, load bearing properties, p e n e t r a t i o n measurements, etc., are u s u a l l y c a r r i e d o u t a t room temperature b u t may have t o be made over a l i m i t e d temperature range. Despite t h e i r p r a c t i c a l importance, these aspects are n o t s t r i c t l y w i t h i n the province o f t h i s review o r indeed o f the f i e l d o f experience o f the author

-

and w i l l n o t be f u r t h e r considered.

-

A much more r e l e v a n t a p p l i c a t i o n i s i n the

i n v e s t i g a t i o n o f o i l shales, where thermomechanical properties, such as deformation on heating, anisotropy o f compressive strength, etc.,

a r e important i n determining

the release o f t h e o i l product i n the r e t o r t (Rajeshwar, Nottenburg and DuBow, 1979).

A t present, therefore, thermomcchanical measurements are o f use mainly

i n technology and i n d u s t r y r a t h e r than i n mineralogy.

-

20 1.3.8

Thermosonimetry and thermoacoustimetr,y

These techniques are q u i t e d i s t i n c t .

I n thermosonimetry one measures t h e

sound emitted by a sample on heating, whereas i n thermoacoustimetry one measures t h e changes i n t h e c h a r a c t e r i s t i c s o f imposed sound waves caused by passage through t h e sample. As w i t h t h e two types o f thermomechanical measurement, therefore, t h e type o f i n f o r m a t i o n gained i s d i f f e r e n t . Sound emission from, o r d e c r e p i t a t i o n of, rocks was studied by Smith and Peach i n 1949 and associated by Smith (1957) w i t h t h e i r p h y l l o s i l i c a t e components.

This

l e d t o a survey o f micas (Hutchison, 1966) t h a t detected two temperature ranges o f sound emission

-

a t 305-340°C associated w i t h trapped water and a t 600-1000°C

associated w i t h p a r t i a l dehydroxylation.

The apparatus used i n these studies was

very simple and l i t t l e f u r t h e r ensued u n t i l L$nvik i n 1972 (see L l n v i k , 1974) devised a much more r e f i n e d sound-measurement arrangement using a s p e c i a l l y designed wave-guide system: sound emission was recorded e i t h e r as r a t e o r as amplitude against temperature.

Although frequency analysis o f t h e emitted sound i s possible,

i t i s r a t h e r more d i f f i c u l t t o perform.

The method has more r e c e n t l y been taken

up by Clark (1978) and co-workers i n the UK, who have developed a concurrent ( n o t simultaneous, as two samples a r e used) thermosonimetry-DTA apparatus (Clark and Garlick, 1979).

A l e s s elaborate, b u t a l s o l e s s v e r s a t i l e , system has been

described by Poulou and Baudracco-Gritti

(1978) and a S o v i e t instrument, whereby

pressure changes i n h i g h vacuum are measured as t h e sample decrepitates, i s comnercially a v a i l a b l e (Pawlikowski, 1979).

The l a s t i s s p e c i f i c a l l y designed

f o r measurement o f release o f m a t e r i a l from i n c l u s i o n s and i t i s doubtful whether i t would p i c k up o t h e r causes o f sound emission such as s t r a i n release, microcrack

propagation, etc. LBnvik (1974, 1978) has a p p l i e d the technique to, i n t e r alia, q u a r t z i t e s and boehmite and has noted very s t r o n g sound emission associated w i t h phase t r a n s i t i o n s and w i t h decomposition reactions.

C l a r k and G a r l i c k (1979), w i t h t h e i r combined

equipment, have come t o the conclusion t h a t maximum sound emission occurs a t t h e

-

i.e. sound extrapolated onset o f t h e DTA peak associated w i t h such processes emission i s v i r t u a l l y complete a t t h e p o i n t a t which measurable enthalpy changes occur

- and have r e l e a t e d t h e emissionof

sound e s s e n t i a l l y t o t h e release o f s t r a i n :

once the s t r a i n i s released, through t h e commencement o f a phase change o r a decomposition reaction, sound emission ceases.

A s i m i l a r mechanism would presumably

apply t o microcrack propagation b u t other r e l a t i o n s h i p s c o u l d w e l l h o l d f o r other phenomena such as t h e release o f inclusions. The r a p i d development i n t h i s f i e l d over t h e l a s t few years suggests t h a t t h e time i s now r i p e t o apply i t t o c l a y minerals: i n i t s most advanced form i t might w e l l h e l p t o e l u c i d a t e t h e mechanisms o f some r e a c t i o n s s t i l l i l l - u n d e r s t o o d . Thermoacoustimetry has so f a r been a p p l i e d mainly t o polymers and, l i k e a l l measurements i n v o l v i n g o s c i l l a t i o n o r waves, whether sound o r electromagnetic,

21

gives i n f o r m a t i o n on t h e shear modulus, t h e modulus o f e l a s t i c i t y , the l o s s tangent and such l i k e .

An e x c e l l e n t d e s c r i p t i o n has been given by Perepechko (1975).

I t s a p p l i c a t i o n t o clays a t t h e moment i s uncertain: i t s most l i k e l y f i e l d would be i n r e l a t i o n t o f i r e d c l a y products but, l i k e dynamic thermomechanical measurements and a l t e r n a t i n g e l e c t r i c a l measurements, i t has already found a p p l i c a t i o n i n o i l - s h a l e technology (Mraz, Rajeshwar and DuBow, 1980). 1.3.9 Therrnoptornetry Thermoptometry i s a wide term covering a whole range o f techniques, such as

thermophotometry (measurement o f t o t a l 1 i g h t ) , thermospectrometry (measurement of l i g h t o f a s p e c i f i c wavelength), thermorefractometry (measurement o f r e f r a c t i v e index) and thermomicroscopy f o r e i t h e r emitted o r r e f l e c t e d l i g h t .

Thermo-

luminescence i s a special case o f thermophotometry where emitted l i g h t only i s measured a t temperatures below r e d heat. Thermoluminescence i s probably t h e most widely used l o f thesetechniques, having been widely a p p l i e d t o l u n a r and m e t e o r i t e samples (Nambi, Bhasin and Bapat, 1978) as w e l l as t o c h a r a c t e r i z a t i o n o f marbles and limestones (Afordakos, Alexopoulous and M i l i o t i s , 1974; Nambi and Mitra, 1978; Chistyakova, 1979). i s a l s o used f o r age determination i n geology

-

It

f o r example, t o date the baking

o f a sediment (Huxtable, A i t k e n and Bonhommet, 1978)

-

and i n a,rchaeology

-

to

date ceramics and other a r t i f a c t s (Cairns, 1976). I t s r e l a t i o n s h i p t o DTA, DTG and other techniques has been discussed by Chen (1976). Basic equipment f o r simultaneous thermoluminescence-DTA has been described by David (1972) and the more s e n s i t i v e and elaborate equipment used i n achaeological and chemical studies by Cairns (1976) and Wendlandt (1980), r e s p e c t i v e l y . David (1972) reproduces a glow curve f o r "a c l a y " (which from i t s DTA curve i s a g i b b s i t i c bauxite) and Cairns (1976) discusses i n d e t a i l t h e o r i g i n o f thermoluminesce i n c l a y s and i n ceramic and other materials. Thermomicroscopy has been widely applied i n studies on glasses, r e f r a c t o r i e s and ceramics, and neat instruments f o r simultaneous DTA, o r DSC, and thermomicroscopy have been described and successfully used ( M i l l e r and Sommer, 1966; Sommer and Jochens, 1971; Kunihisa, 1979). Several standard hot-stage microscopes w i t h temperature c o n t r o l are a l s o commercially a v a i l a b l e b u t clays as such, presumably because o f t h e i r small p a r t i c l e size, have been r a t h e r neglected. Equipment f o r use w i t h some o f the o t h e r techniques r e f e r r e d t o above w i l l be found ( b u t n o t necessarily under t h e same names) i n standard t e x t s (e.g. Wendlandt and Hecht, 1966; Wendlandt, 1974).

It seems t o t h e author t h a t some o f these

methods have as y e t untapped p o t e n t i a l i-n studying, f o r example, changes i n t h e c o l o u r o f smectites on s a t u r a t i o n w i t h non-chromophoric ions and moderate heating.

2 2

Therrnoelectrometry Like thermoptometry, thermoelectrometry i s essentially a portmanteau term covering measurement of any e l e c t r i c a l property as a function of temperature. Thus i t covers variations in resistance, conductance, inductance and capacitance, f o r both d.c. and a.c., as well as variations in d i e l e c t r i c constants, thermoe l e c t r i c i t y , thermally stimulated current, etc. The value of d.c. conductance (or resistance) in showing up decomposition reactions, solid-state transitions, etc., was f i r s t appreciated by Berg and Burmistrova (1960), who constructed an instrument f o r simul taneous d.c. conductance measurement and DTA. Developments in :equipment and applications up t o 1974 have been discussed by Wendlandt (1974): more recently, a.c. conductance measurements have been favoured (Wendlandt, 1979). The terms "thermally stimulated conductivity" and "thermally stimulated current" have come into very widespread use, particularly in relation t o semi-conductor research: unfortunately, the f a c t t h a t both a r e given the abbreviation "TSC" has led t o widespread confusion in indexing - even in C h e m i c a l A b s t r a c t s . Freund and co-workers, in t h e i r fundamental study of proton tunneling i n hydroxides, have recently used d.c. conductance and thermally stimulated depolarization measurements t o provide evidence of the occurrence o f proton conductivity and transitory HOH species in gibbsite, brucite and portlandite before the comnencement of dehydroxylation (Freund and Wengeler, 1980; Wengeler, Martens and Freund, 1980). In these studies, they observed t h a t the d.c. conductance depends on the nature of the electrodes used and that thermopotential measurements are sensitive t o electron acceptors or donors sorbed on p a r t i c l e surfaces. D.c. conductance has also been used t o investigate water sorbed on goethite (Kaneko and Inouye, 1979). Similar applications are likely with clays. In o i l shales, the d.c. resistance decreases exponentially w i t h increasing temperature - an observation attributed t o carbonate ions being the transporting species o r t o breakdown of hydrocarbon units i n the trapped kerogen (Rajeshwar, Nottenburg and DuBow, 1979). Such systems are extremely complicated and i t i s d i f f i c u l t t o separate the effects due t o the clay matrix from those due to the kerogen. Attempts have been made t o solve the problem using a.c. measurements involving conductance i.e. t h a t involve d i e l e c t r i c constant, d i e l e c t r i c loss factor and loss tangent and equipment for simultaneous and concurrent a.c. capacitance measurement and DTA has been described (Rajeshwar, Nottenburg and DUBOW, 1978; Nottenburg e t ai., 1979). The l a t t e r i s considered the more r e l i a b l e as the optimum sample s i z e f o r DTA and for capacitance measurement i s quite different. I t seems t o the author that examination of non-oil-bearing shales of similar mineralogical composition could a s s i s t in resolving some of the d i f f i c u l t i e s of interpretation encountered, b u t there may be practical d i f f i c u l t i e s related t o p a r t i c l e orientation, pore space, etc. 1.3.10

23

Thermally stimulated c u r r e n t s have been r e l a t e d t o various types o f l a t t i c e defects i n i o n i c c r y s t a l s and a review has been given by Radhakrishna and Haridoss (1978).

These c u r r e n t s can a l s o be r e l a t e d t o thermoluminescence e f f e c t s ( F i e l d s

and Moran, 1974)

-

as, indeed, can thermally stimulated conductance (Chen, 1976;

Braeunlich, K e l l y and F i l l i a r d , 1979)

-

b u t no a p p l i c a t i o n t o clays can be traced.

I t may be o f i n t e r e s t t o some t o note t h a t values o f t h e thermoelectric power

( i n uV/"C against lead) have been measured by Lee (1973) f o r a considerable number o f minerals, i n c l u d i n g some that, l i k e hematite, magnetite and ilmenite, can occur as accessories i n clays: i n d i c a t i o n s are given o f t h e spread o f values f o r the samples t e z t e d as w e l l as mean values. I n conclusion, i t would appear t h a t some thermoelectrometric techniques may have considerable p o t e n t i a l i n c l a y studies

-

e s p e c i a l l y i n r e l a t i o n t o water-

loss reactions and l a t t i c e d e f e c t i n v e s t i g a t i o n s

- q u i t e a p a r t from t h e i r p o t e n t i a l

value i n technological assessments. Thermomagnetometry

1.3.11

The most common magnetic c h a r a c t e r i s t i c t o be measured as a f u n c t i o n o f temperature i s magnetic s u s c e p t i b i l i t y , w i t h consequent d e r i v a t i o n o f t h e Curie point soils.

-

a parameter t h a t has proved extremely useful i n some studies on clays and Thus, on t h e basis t h a t t h e very l a r g e v a r i a t i o n i n the Curie p o i n t o f

i l m e n i t e (below 77 K t o 841 K) can be c o r r e l a t e d w i t h i t s o r i g i n , i t has been claimed t h a t samples w i t h a Curie p o i n t i n t h e r e g i o n 103-223 K are l i k e l y t o be associated w i t h k i m b e r l i t e deposits (Garanin, Kudryavtseva and Soshkina, 1979). Moreover, f o r red-yellow d e s e r t i c s o i l s , i t has been possible t o i n t e r p r e t Curie p o i n t s i n terms o f the presence o f hematite and maghemite (Timofeev and Smirnov, 1980): i n t h i s study, non-coincidence of the thermomagnetic curve on a second heating has been i n t e r p r e t e d as i n d i c a t i n g t h e presence o f f e r r i h y d r i t e

-

although

formation o f i r o n oxides from some other minerals present could p o s s i b l y a l s o contribute.

Be t h a t as i t may, i t would seem t h a t thermomagnetometry may w e l l

have s i g n i f i c a n t a p p l i c a t i o n s i n c l a y mineralogy, e s p e c i a l l y i n the i n v e s t i g a t i o n o f c e r t a i n accessory minerals. 1.4

CONCLUSIONS

From the above account, i t i s evident t h a t , although TG and DTA a r e by f a r the most widely used thermoanalytical techniques i n c l a y mineralogy a t present, many o t h e r methods t h a t can e l u c i d a t e s p e c i f i c features o r behaviour a r e available. Thus, EGA and DSC a r e p r e s e n t l y r e c e i v i n g greater a t t e n t i o n and thermosonimetry along w i t h some thermoptometric, thermoelectrometric and thermomagnetic methods deserve consideration i n t h e f u t u r e .

Methods using o s c i l l a t i o n s o r waves t h a t

j i v e i n f o r m a t i o n on t h e shear modulus, the modulus o f e l a s t i c i t y , etc.,

are

r e l e v a n t e s s e n t i a l l y t o technology and industry, although t h e i r possible uses i n

24 more fundamental studies should not be neglected. All-in-all, however, thermoanalytical methods are not in themselves necessarily good in mineral identification: theirforte is in yielding information on characteristics or detail that it would be difficult, if not impossible, to obtain by any other method. REFERENCES Afordakos, G. , Alexopoulos, K. and Miliotis, D., 1974. Using artificial thermoluminescence to reassemble statues from fragments. Nature, Lond., 250: 47-48. Alietti, A., Brigatti, M.F. and Poppi, L . , 1979. An illite/montmorillonite interlayer mineral in porphyritic rock 'Alpe di Siusi' (BZ-Italy): The Chemistry of I/M interlayer minerals. Clay Miner., 14: 39-46. Ashley, H.E., 1911. The decomposition of clays and the utilization of smelter and other smoke in preparing sulphate from clays. J. ind. Engng Chem., 3: 91-95. Asomoza, P.M., Razo, M.L., Chaidez, T.L. and Casillas, S.R., 1978. Quantitative evaluation of Na,CO, and NaCl content of the clays of the ex-lake of Texcoco (Valley of Mexico) by means of thermogravimetry. J. therm. Analysis, 13: 327-339, Balek, V., 1977. Emanation thermal analysis. Thermochim. Acta, 22: 1-156. Barwood, H.L. and Hajek, B.F., 1979. Differential thermal characteristics of soil and reference quartz. J. Soil Sci. SOC. Am., 43: 626-627. Berg, L.G. and Burmistrova, N.P., 1960. [Thermographic analysis of salts with simultaneous determination of temperature effects and electrical conductivity.1 Zh. neorg. Khim. 5: 676-683. Boy, S. and BMhme, K. , 1979. GegenUberstellung verschiedener LBsungsverfahren zur Ermittlunq kinetischer Parameter anhand von Experimentalwerten. Thermochim. Acta, 28: 2491258. Bracewell, J.M. and Robertson, G.W., 1977. Pyrolysis studies on humus in freely drained Scottish soils. In C.E.R. Jones and C.A. Cramers [Editors). Analytical Pyrolysis: Proc. 3rd Int. Symp. Analyt. Pyrol. , Amsterdam. ' Elsevier, Amsterdam, pp. 167-178. Bracewell, J.M. and Robertson, G.W. , 1980. Pyrolysis-mass spectrometry studies of humification in a peat and a peaty podzol. J. analyt. appl. Pyrol., 2: 53-62. Braeunlich, P., Kelly, P. and Filliard, J.P., 1979. Thermally stimulated luminescence and conductivity. Top. appl. Phys. ,.37: 35-92. Brindley, G.W. and Porter, A.R.D., 1978. Occurrence of dickite in Jamaica ordered and disordered varieties. Am. Miner., 63: 554-562. Broersma, A., de Bruyn, P.L., Jens, J.W. and Stol, R.J., 1978. Simultaneous DTA and DTG measurements on aluminium oxide monohydroxides. J. therm. Analysis, 13: 341-355. Brown, G.H. and Montgomery, E.T., 1912. The dehydration of clays. Trans. Am. Ceram. SOC., 14: 709-721. Buzds, I. (Editor), 1975. Thermal Analysis: Proc. 4th ICTA, Budapest. Heyden, London, 3 vols. Cairns, T. , 1976. Archaeological dating by thermoluminescence. Analyt. Chem. , 48: 266A-280A. Chen, R., 1976. Methods for kinetic analysis o f thermally stimulated processes. 3. Mater Sci., 11: 1521-1541. Chihara, H. (editor), 1977. Thermal Analysis: Proc. 5th ICTA, Kyoto. Heyden, London, 573 pp. Chistyakova, A.A., 1979. [Investigation of the thermoluminescence of limestones and limes.] Zap. vses. Miner. Obshch., 108: 91-99. Clark, G.M., 1978. Instrumentation for thermosonimetry. Thermochim. Acta, 27: 19-25. Clark, G.M. and Garlick, R., 1979. Thermosonimetry of the NBS-ICTA temperature standards. Thermochim. Acta, 34: 365-375. Coats, A.W. and Redfern, J.P. , 1963. Thermogravimetric analysis. Analyst, Lond. , 88: 906-924.

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....

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26 Ingraham,,T.R. , 1967. Thermal c o n d u c t i v i t y d e t e c t o r s and t h e i r a p p l i c a t i o n t o some thermodynamic and k i n e t i c measurements. I n W. Lodding ( E d i t o r ) . Gas E f f l u e n t A n a l y s i s . Marcel Dekker, New York, pp. 25-69. Inoue, A., Minato, H. and Utada, M., 1978. M i n e r a l o g i c a l p r o p e r t i e s and occurrence o f i l l i t e / m o n t m o r i l l o n i t e mixed l a y e r m i n e r a l s formed f r o m Miocene v o l c a n i c g l a s s i n Waga-Omoho d i s t r i c t . Clay Sci., 5: 123-136. IUPAC, 1974. Recomnendations f o r nomenclature o f thermal a n a l y s i s . Pure appl. Chem., 37: 439-444. IUPAC, 1980. Nomenclature f o r thermal a n a l y s i s I 1 and 111. Pure appl. Chem., 52: 2385-2391. Iwasa, Y., 1978. Occurrence o f l e p i d o c r o c i t e i n a y e l l o w o r g a n i c c o l o r e d s o i l i n humid s u b - t r o p i c a l region. Nendo Kagaku, 18: 135-144. Jorda, J.L., FlUkiger, R. and MUller, J., 1978. Thermal a n a l y s i s o f l e v i t a t i o n heated samples: A p p l i c a t i o n t o t h e niobium-germanium system. J. Mater. Sci., 13: 2471-2476. Kaneko, K. and Inouye, K., 1979. A d s o r p t i o n o f water on i r o n h y d r o x i d e as s t u d i e d by e l e c t r i c a l c o n d u c t i v i t y measurements. B u l l . chem. SOC. Japan, 52: 315-320. Keattch, C.J. and Dollimore, D., 1975. I n t r o d u c t i o n t o Thermogravimetry, Second E d i t i o n . Heyden, London, 164 pp. Koch, E., 1977. Non-Isothermal Reaction A n a l y s i s . Academic Press, London, 607 pp 1979. An apparatus f o r t h e r m o a n a l y t i c a l microscopy and i t s Kunihisa, K.S., a p p l i c a t i o n s . Thermochim. Acta, 31: 1-11. Kuroda, K. and Kato, C., 1979. Synthesis o f t h e t r i m e t h y l s i l y l a t i o n d e r i v a t i v e o f h a l l o y s i t e . Clays Clay Miner., 27: 53-56. 1967. Mass Spectrometric i d e n t i f i c a t i o n of gaseous Langer, H.G. and Gohlke, R.S., products from thermal a n a l y s i s . I n W. Lodding ( E d i t o r ) . Gas E f f l u e n t A n a l y s i s . Marcel Dekker, New York, pp. 71-100. Langier-Kuzniarowa, A., 1967. Termogramy Mineralov I l a s t y c h . Wydawn. Geologiczne, Warsaw, 316 pp. Le C h a t e l i e r , H., 1887. De l ' a c t i o n de l a c h a l e u r s u r des a r g i l e s . B u l l . SOC. fr. Miner. C r i s t a l l o g r . , 10: 204-211. Lee, Chia-Chou, 1973. Measurements o f t h e r m o e l e c t r i c i t y o f m i n e r a l s . S c i e n t i a Geol. S i n i c a , 4: 317-326. Lodding, W. ( E d i t o r ) , 1967. Gas E f f l u e n t Analysis. Marcel Dekker, New York, 220 pp. Lombardi, G., 1980. For B e t t e r Thermal Analysis, Second E d i t i o n , I s t i t u t o d i M i n e r a l o g i a d e l l ' U n i v e r s i t a d i Roma and ICTA, Rome, 46 pp. Ldnvik, K., 1974. Thermosonimetry: A method o f m a t e r i a l c h a r a c t e r i z a t i o n . I n I. Buzds ( E d i t o r ) . Thermal A n a l y s i s : Proc. 4 t h ICTA, Budapest. Heyden, London, 3: 1089-1105. Ldnvik, K., 1978. An experimental i n v e s t i g a t i o n o f t h e thermal decomposition o f b r u c i t e by thermasonimetry. Thermochim. Acta, 27: 27-44. and Berezowski, R.M., 1980. Thermal and MLIssbauer s t u d i e s o f MacKenzie, K.J.D. i r o n - c o n t a i n i n g hydrous s i l i c a t e s . 11. H i s i n g e r i t e . Thermochim. Acta, 41: 335-355. ... - - - Mackenzie, R.C. , 1957a. Saponite from A l l t Ribhein, F i s k a v a i g Bay, Skye. Mineralog. Mas. 31: 672-680. Mackenzie, R.C. ( E d i t o r ) , 1957b. The D i f f e r e n t i a l Thermal I n v e s t i g a t i o n o f Clays. M i n e r a l o g i c a l Society, London, 456 pp. Mackenzie, R.C. ( E d i t o r ) , 1970-72. D i f f e r e n t i a l Thermal A n a l y s i s . Academic Press, London, 2 v o l s . 1980. D i f f e r e n t i a l thermal a n a l y s i s and d i f f e r e n t i a l scanning Mackenzie, R.C., c a l o r i m e t r y : S i m i l a r i t i e s and d i f f e r e n c e s . A n a l y t . Proc. , Lond., 17: 217-220. Mackenzie, R.C. , 1981a. De c a l o r e : Prelude t o thermal a n a l y s i s . Thermochim. Acta, i n press. Mackenzie, R.C., 1981b. Nomenclature i n thermal analysis, P a r t V . Symbols. J. therm. Analysis, i n press. 1977. I n s t r u m e n t a l methods f o r t h e study o f thermal degradation o f McNeill, I.C., polymers. I n N. Grassie ( E d i t o r ) . Developments i n Polymer Degradation. A p p l i e d Science, London, pp. 43-66.

-

2 7

McNeill, I.C., 1980. The use o f subambient thermal v o l a t i l i z a t i o n a n a l y s i s t o study v o l a t i l e products o f polymer degradation. I n H.G. Wiedemann ( E d i t o r ) . Thermal Analysis: Proc. 6 t h ICTA, Bayreuth. B i r k h l u s e r Verlag, Basel, 1: 319-324. M a r t i n V i v a l d i , J.L., G i r e l a Vilchez, F. and F e n o l l Hach-Ali, P., 1964. The thermal decomposition o f NH,-montmorillonites. P a r t 11. Clay Miner. B u l l . , 5: 401-406. Reda, M., Wilson, M.J. and Mackenzie, R.C., 1980. Clay and s i l t Mashhady, A.S., mineralogy o f some s o i l s f r o m Qasim, Saudi Arabia. J. S o i l Sci., 31: 101-115. M e l l o r , J.W. and H o l d c r o f t , A.D., 1911. The chemical c o n s t i t u t i o n o f t h e k a o l i n i t e molecule. Trans. Eng. Ceram. SOC., 10: 94-120. Menis, O., Mackey, J.A. and Garn, P.D., 1978. Measurement o f peak area o f c h y s o t i l e i n 640°C r e g i o n t o assess asbestos i n p o l l u t i o n s t u d i e s . Proc. 4 t h J o i n t Conf. Sensing Environm. P o l l u t . , New Orleans, Am. Chem. Sot., Washington, pp. 899-908. Mifsud, A., Rautureau, M. and Fornes, V., 1978. Etude de l ' e a u dans l a p a l y g o r s k i t e a l ' a i d e des analyses thermiques. Clay Miner., 13: 367-374. M i l l e r , R.P. and Sommer, G., 1966. A h o t stage microscope i n c o r p o r a t i n g a d i f f e r e n t i a l thermal a n a l y s i s u n i t . J. s c i e n t . Instrum. , 43: 293-297. Moore, G.S.M. and Rose, H.E. , 1978. Thermal e f f e c t s of contamination, adsorbed J. therm. Analysis, 15: 37-45. water and annealing on t h e DTA o f powdered q u a r t z . Morandi, N. and F e l i c e , G., 1979. Serpentine m i n e r a l s from v e i n s i n s e r p e n t i n i t e rocks. Mineralog. Mag., 43: 135-140. Morgan, D.J., 1977. Simultaneous DTA-EGA o f m i n e r a l s and n a t u r a l m i n e r a l mixtures. J. therm. Analysis, 12: 245-263. Mraz, T., Rajeshwar, K. and DUBOW, J., 1980. An automated technique f o r thermoacoustimetry o f s o l i d s . Thermochim. Acta, 38: 211-223. MUller-Vonmoos, M. and MUller, R., 1975. A p p l i c a t i o n o f DTA-TG-MS i n t h e i n v e s t i g a t i o n o f c l a y s . I n I. Buzds ( E d i t o r ) , Thermal Analysis: Proc. 4 t h ICTA, Budapest. Heyden, London, 2: 521-530. Murad, E. , 1979. Mbssbauer s p e c t r a o f g o e t h i t e : evidence f o r s t r u c t u r a l i m p e r f e c t i o n s . Mineralog. Mag., 43: 355-361. Murphy, C.B., 1967. T h e r m o p a r t i c u l a t e a n a l y s i s . I n W. Lodding ( E d i t o r ) . Gas E f f l u e n t A n a l y s i s . Marcel Dekker, New York, pp. 195-209. Murphy, C.B., 1978. Thermal a n a l y s i s . A n a l y t . Chem., 50: 143R-153R. Nambi , K.S.V. and M i t r a , S., 1978. Thermoluminescence i n v e s t i g a t i o n s o f o l d carbonate sedimentary rocks. Neues Jb. Miner. Abh., 133: 210-226. Nambi, K.S.V., Bhasin, B.D. and Bapat, V.N., 1978. Thermoluminescence c h a r a c t e r i s t i c s o f USGS standard b a s a l t i c r o c k BCR-1. Thermochim. Acta, 25: 126-129. [ Netsusokutei , 19801. Q u e s t i o n and Answer. Netsusokutei 7: 100-101. Norton, F.H., 1939. C r i t i c a l study o f t h e d i f f e r e n t i a l thermal method f o r t h e i d e n t i f i c a t i o n o f t h e c l a y minerals. J. Am. Ceram. SOC., 22: 54-63. Nottenburg, R., Freeman, M., Rajeshwar, K. and DUBOW, J . , 1979. Concurrent d i e l e c t r i c a n a l y s i s - d i f f e r e n t i a l thermal a n a l y s i s . A n a l y t . Chem., 51: 1149-1155. N u t t i n g , P.G., 1943. Some standard thermal d e h y d r a t i o n curves. P r o f . Pap. US Geol. Surv., No. 197-E, 197-217. Paterson, E., 1980. Use of thermal methods i n t h e study of surface phenomena. Analyt. Proc., Lond., 17: 234-236. Paterson, E . and S w a f f i e l d , R., 1980. I n f l u e n c e o f adsorbed i o n s i n t h e d e h y d r o x y l a t i o n o f s y n t h e t i c g o e t h i t e . 3. therm. A n a l y s i s , 18: 161-167. Paulik, F. and P a u l i k , J., 1972. Quasi-isothermal thermogravimetry. I n H.G. Wiedemann ( E d i t o r ) . Thermal Analysis: Proc. 3 r d ICTA, Davos. B i r k h l u s e r Verlag, Basel, 1: 161-167. Paulik, F. and Paulik, J., 1978. Simultaneous techniques i n thermal a n a l y s i s . Analyst, Lond., 103: 417-437. Pawlikowski, M., 1979. Wyniki wstepnych badan d e k r e p i t o m e t r y c z w c h galeny i d o l o m i t u z k o p a l n i "Boleslaw". Inform. Zjazdu Polsk. Towarzyst. Miner. , Krakow-Wiel iczka-Trzebionka. Wydawn. Geologiczne, Warsaw, p . 26. Perepechko, I., 1975. A c o u s t i c Methods. of I n v e s t i g a t i n g Polymers (Translated by G. L e i b ) . M i r Publishers, MOSCOW, 312 pp. Poulou, R. and Baudracco-Gritti, C., 1978. Un e n r e g i s t r e u r d e c r e p i t o m e t r i q u e a f i l t r a g e e l e c t r o n i q u e . B u l l . Miner., 101: 402-405.

Price, D., Dollimore, D., Fatemi, N.S. and Whitehead, R., 1980. Mass s p e c t r o m e t r i c d e t e r m i n a t i o n of k i n e t i c parameters f o r s o l i d s t a t e decomposition r e a c t i o n s . Thermochim. Acta, 42: 323-332. Radhakrishna, S. and Haridoss, S., 1978. Thermo c u r r e n t s i n i o n i c c r y s t a l s . C r y s t . L a t t i c e Defects, 7: 191-207. Rajeshwar, K., Nottenburg, R.N. and DUBOW, J.B., 1978. Simultaneous d i e l e c t r i c a n a l y s i s - d i f f e r e n t i a l thermal a n a l y s i s o f s o l i d m a t e r i a l s . Thermochim. Acta, 26: 1-17. Rajeshwar, K., Nottenburg, R. and DUBOW, J., 1979. Thermophysical p r o p e r t i e s of o i l shales. J. Mater. Sci., 14: 2025-2052. Redfern, J.P. ( E d i t o r ) , 1965. Thermal A n a l y s i s 1965: Proc. 1 s t ICTA, Aberdeen. Macmillan, London, 293 pp. Rouquerol, J., 1970. L ' a n a l y s e thermique a v i t e s s e de decomposition constante. J. therm. Analysis, 2: 123-140. Ryzhova, L.V., 1980. [Use o f X-ray d j f f r a c t o m e t r i c and thermogravimetric methods i n t h e i n v e s t i g a t i o n o f ion-exchange r e a c t i o n s i n s o i l s . 1 Pochvovedenie, No. 1, 143-151. Samoilov, Ya.V., 1909. [On t h e water of k a o l i n i t e . 1 I z v . imp. Akad. Nauk. S. Petersb., 3: 1137-1152. Satoh, S., 1918. On t h e endo- and exothermal change o f t h e k a o l i n i t e i n Japan. Kogyo-Kwagaku-Zasshi, 21 : 631-648. Satoh, S . , 1921. A study o f t h e h e a t i n g and c o o l i n g curves o f Japanese k a o l i n i t e . J. Am. Ceram. SOC., 4: 182-194. Schomburg, J. and S t M r r , M., 1978a. Kombinierte thermische Analyse a n D r e i s c h i c h t tonmineralen. Thermochim. Acta, 25: 313-324. Schomburg, J. and S t M r r , M., 1978b. Nachweis D i c k i t g e h a l t e i n Kaolinen m i t t e l s D i l a t o m e t r i e . Chem. Erde, 37: 107-108. Schwenker, R.F. and Garn, P.D. ( E d i t o r s ) , 1969. Thermal A n a l y s i s . Academic Press, New York, 2 v o l s . Sharp, J.H., 1972, Reaction k i n e t i c s . I n R.C. Mackenzie ( E d i t o r ) . D i f f e r e n t i a l Thermal Analysis. Academic Press, London, 2: 47-77. Simonsen, K.A. and Zaharescu, M., 1979. A new d i f f e r e n t i a l thermal a n a l y s i s method. J. therm. A n a l y s i s , 15: 25-35. Smalley, I.J. and Xidakis, G.S., 1979. Thermogravimetry of a n expansive c l a y s o i l from Adelaide: Approximate m i n e r a l o g i c a l a n a l y s i s u s i n g standard m o n t m o r i l l o n i t e s . Clay Sci., 5: 189-193. Smith, F.G., 1957. D e c r e p i t a t i o n c h a r a c t e r i s t i c s o f igneous rocks. Can. Miner., 6: 78-86. 1949. Apparatus f o r t h e r e c o r d i n g o f d e c r e p i t a t i o n Smith, F.G. and Peach, P.A., i n m i n e r a l s . Econ. Geol., 44: 449-454. Smith, J.D.B., Meier, J.F. and P h i l l i p s , D.C., 1977. T h e r m o p a r t i c u l a t i n g o r g a n i c compounds. J. therm. Analysis, 11: 423-430. Smith, J.D.B., P h i l l i p s , D.C. and Kaczmarek, T.D., 1976. T h e r m o p a r t i c u l a t i o n analyses o f malonic a c i d compounds. A n a l y t . Chem., 48: 89-95. 1971. Techniques of microthermal a n a l y s i s based on Sommer, G. and Jochens, P.R., a thermocouple as h e a t i n g source and specimen h o l d e r , Miner. S c i . Engng, 3: 3-16. Staub, H. and Perron, W., 1974. New Method o f p u r i t y d e t e r m i n a t i o n by means of c o l o r i m e t r i c d i f f e r e n t i a l thermal a n a l y s i s . A n a l y t . Chem., 46: 128-130. Suzuki, S. and Satoh, R., 1980. Effect o f i r o n on t h e exothermic peak temperature of allophane from a M i s o t s u c h i weathered pumice bed. S o i l S c i . P1. Nutr., 26: 441-445. Timofeev, B.V. and Smirnov, Yu.A., 1980. [ C h a r a c t e r i s t i c s o f i r o n accumulation and t h e n a t u r e o f i t s compounds i n s o i l s o f t h e h i g h p l a t e a u x o f A l g i e r s . ] Pochvovedenie, No. 11, 131-136. Wallach, R., 1913. L'analyse thermique des a r g i l e s . C.r. hebd. Seanc. Acad. Sci., P a r i s , 157: 48-50. Warne, S.St.J. and M i t c h e l l , B.D., 1979. V a r i a b l e atmosphere DTA i n i d e n t i f i c a t i o n and d e t e r m i n a t i o n o f anhydrous carbonate m i n e r a l s i n s o i l s . J. S o i l Sci., 30: 111-116. Wedgwood, J., 1782. An a t t e m p t t o make a thermometer f o r measuring t h e h i g h e r degrees o f h e a t from a r e d h e a t t o t h e s t r o n g e s t t h a t vessels made o f c l a y can support. P h i l . Trans. R. SOC., 72: 305-326.

29 Weiser, H.B. and M i l l i g a n , W.O., 1939. The c o n s t i t u t i o n o f t h e c o l l o i d a l systems o f t h e hydrous oxides. Chem. Rev., 25: 1-30. Wendlandt, W.W., 1974. Thermal Methods o f Analysis, Second E d i t i o n . Wiley, New York, 505 pp. 1979. Concurrent TG-DTA measurements using a Cahn RG balance. Wendlandt, W.W., Thermochim. Acta, 30: 361-363. Wendlandt, W.W., 1980. An apparatus f o r thermophotometry. Thermochim. Acta, 35: 255-257. Wendlandt, W.W. and Hecht, H.G., 1966. Reflectance Spectroscopy. Interscience, New York, 298 pp. Wengeler, H., Martens, R. and Freund, F., 1980. Proton c o n d u c t i v i t y o f simple i o n i c hydroxides. P a r t 11. I n s i t u formation o f water molecules p r i o r t o dehydration. Ber. Bunsenges. phys. Chem., 84: 873-880. Wiedemann, H.G., 1964. U n i v e r s e l l e s Messgerlt fur g r a v i m e t r i s c h e Untersuchungen u n t e r v e r l n d e r l i c h e n Bedingungen. Chemie-Ingr-Technik, 11: 1105-1114. Wiedemann, H.G. ( E d i t o r ) , 1972. Thermal Analysis: Proc. 3 r d ICTA, Davos, B i r k h h s e r Verlag, Basel, 3 vols. Wiedemann, H.G. ( E d i t o r ) , 1980. Thermal Analysis: Proc. 6 t h ICTA, Bayreuth. B i r k h l u s e r Verlag, Basel, 2 vols. Wilburn, F.W., 1972,. Glass-making r e a c t i o n s i n three-component systems t h a t i n c l u d e s i l i c a . Ph.D. Thesis. U n i v e r s i t y o f S a l f o r d , UK.

31 CHAPTER 2 H I G H RESOLUTION ELECTRON-MICROSCOPY APPLIED TO CLAY MINERALS Jean P i e r r e EBERHART L a b o r a t o i r e de C r i s t a l l o g r a p h i e , M i n e r a l o g i e e t P e t r o g r a p h i e U n i v e r s i t e L o u i s P a s t e u r , S t r a s b o u r g , France. 2.1

INTRODUCTION Clay m i n e r a l s a r e f i n e g r a i n e d m a t e r i a l s ; t h e average s i z e o f t h e i r l a m e l l a r

p a r t i c l e s l i e s i n t h e m i c r o m e t e r range, u s u a l l y a t t h e r e s o l u t i o n l i m i t of l i g h t microscopes ., U n t i l t h e h e v e l o p m e n t o f e l e c t r o n microscopy, a t o m i c - s c a l e i n f o r m a t i o n s have been p r o v i d e d by X-ray d i f f r a c t i o n , w h i c h has l e d t o t h e knowledge o f t h e b a s i c c l a y miner a l s s t r u c t u r e s . However X-rays can g i v e o n l y an averaged i n f o r m a t i o n i s s u e d from a g r e a t number o f u n i t - c e l l s , a s s u m i n g

t h a t they are s t a t i s t i c a l l y i d e n t i c a l .

T h e o r e t i c a l l y , owing t o t h e s m a l l e l e c t r o n wavelength, t h e e l e c t r o n - m i c r o s c o p e should be a b l e t o p r o v i d e d i r e c t s t r u c t u r a l i n f o r m a t i o n s on i n d i v i d u a l c l a y c r y s t a l s w i t h an a t o m i c r e s o l u t i o n . I n f a c t , f o r many y e a r s , E.M.

r e s e a r c h on c l a y

m i n e r a l s has been l i m i t e d t o m i c r o m o r p h o l o g i c a n a l y s i s w i t h a r e s o l u t i o n o f about 10-20

4,

w e l l below t h e t h e o r e t i c a l .one. I d e n t i f i c a t i o n o f m i n e r a l s has been ena-

b l e d by t h e r e c o g n i t i o n o f c a r a c t e r i s t i c a l shape f e a t u r e s ; c a t a l o g u e s o f m i c r o graphs have been p u b l i s h e d t o t h a t end. The main reasons o f t h a t l i m i t a t i o n have been t h e l o w i n s t r u m e n t a l r e s o l u t i o n and t h e l a c k o f knowledge o f t h e imaging process. Even nowadays a g r e a t d e a l o f clay-E.M.

r e s e a r c h i s s t i l l devoted t o mor-

phol ogy . I n t h e s e v e n t i e s appeared a new g e n e r a t i o n o f E.M. w i t h an e f f e c t i v e r e s o l u t i o n o f 2-5

w,

c l o s e t o t h e t h e o r e t i c a l l i m i t imposed by s p h e r i c a l a b e r r a t i o n . I t opened

t h e way t o t h e s o - c a l l e d h i g h - r e s o l u t i o n e l e c t r o n - m i c r o s c o p y (H.R.E.M.)

and t o

atomic r e s o l u t i o n . Numerous micrographs o f m i n e r a l s , showing a c o r r e l a t i o n w i t h s t r u c t u r e p r o j e c t i o n , were soon p u b l i s h e d (e.g. Buseck and I i j i m a , 1974 on s i l i cates). Furthermore a b e t t e r knowledge o f e l e c t r o n - i m a g i n g and t h e use o f computer p r o c e s s i n g o f t h e image c o n t r a s t have l e d i n r e c e n t y e a r s t o new developments. The p r e s e n t c o n t r i b u t i o n w i l l be m a i n l y d e v o t e d t o h i g h - r e s o l u t i o n aspects i n t r a n s m i s s i o n microscopy. A f t e r an e l e m e n t a r y a b s t r a c t on image f o r m a t i o n and p r o c e s s i n g , t h e s p e c i f i c problems o f c l a y m i n e r a l s w i l l be o u t l i n e d . 2.2

PRINCIPLES OF ELECTRON MICROSCOPE I M A G I N G . A m o s t l y d e s c r i p t i f o u t l i n e o f t h e t h e o r y w i l l be g i v e n i n t h i s c h a p t e r . I t

aims t o be comprehensive t o e v e r y e a r t h s c i e n t i s t who has some knowledge o f t h e X-ray d i f f r a c t i o n t h e o r y . F o r f u r t h e r d e t a i l s and a complete t r e a t m e n t , t h e r e a d e r can c o n s u l t r e c e n t s p e c i a l i z e d books. Some r e f e r e n c e s a r e g i v e n a t t h e end.

32 Approximations and c o n v e n t i o n s .

2.2.1

-

-

(a) I m p l i c i t approximations : S c a t t e r i n g i s supposed t o t a k e p l a c e i n k i n e m a t i c a l c o n d i t i o n s . The waves and t h e Ewald-sphere a r e supposed t o be p l a n e w i t h i n t h e a n g u l a r

aperture. ( b ) Conventions : Wave-functions i n 2-D d i r e c t space ( o b j e c t and image p l a n e ) a r e n o t e d by

-

-t

s m a l l l e t t e r s , e.g. f ( x y ) = f ( r ) . - The c o r r e s p o n d e n t d a t a i n 2-D r e c i p r o c a l space (back f o c a l p l a n e ) a r e n o t e d -+ by c a p i t a l l e t t e r s , e.g. F ( h k ) = F ( R ) . The F o u r i e r t r a n s f o r m s a r e n o t e d by t h e symbol plr. T h e i r f o r m u l a t i o n has been

-

s i m p l i f i e d by o m i s s i o n o f c e r t a i n f a c t o r s w h i c h a r e n o t e s s e n t i a l f o r imaging comprehension. -+ -+ Wave v e c t o r s a r e n o t e d ko ( i n c i d e n t ) and k ( s c a t t e r e d ) , w i t h k

-

ko = 1 / X .

4.2.2

Wave t r a n s m i s s i o n t h r o u g h t h e microscope. I n an i d e a l i z e d imaging p r o c e s s , t h e c o h e r e n t e l e c t r o n - w a v e s t r a n s m i t t e d and

s c a t t e r e d f r o m a p o i n t o f t h e o b j e c t a r e focused by t h e o b j e c t i v e and b r o u g h t t o g e t h e r i n t h e c o r r e s p o n d i n g image-plane p o i n t w i t h t h e r e l a t i v e phases. The d i f f e r e n c e s i n a m p l i t u d e and phase f r o m one p o i n t t o a n o t h e r g i v e r i s e t o t h e image c o n t r a s t . T h i s process may be decomposed f o r t h e a n a l y s i s i n s e v e r a l s t e p s ( s e e F i g . 1): (a)

O b j e c t s t r u c t u r e a s "seen" by t h e e l e c t r o n s . O b j e c t w a v e - f u n c t i o n i . e . a m p l i t u d e and phase d i s t r i b u t i o n a t t h e e x i t

(b) face o f t h e o b j e c t .

( c ) Wave t r a n s m i s s i o n t h r o u g h t h e microscope. F i l t e r i n g by l e n s c a r a c t e r i s t i c s ( a p e r t u r e , a b e r r a t i o n s and d e f o c u s ) ; r e c o m b i n a t i o n t o t h e image-plane wave-function. (d)

Observed image. I n t e n s i t y d i s t r i b u t i o n i n t h e r e c o r d e d m i c r o g r a p h .

The aim i s t o c o r r e l a t e ( d ) t o ( a ) . I n p r a c t i c e t h e imaging p r o c e s s i n v o l v e s o n l y t h e o b j e c t i f l e n s ; t h e c o n t r i b u t i o n o f t h e o t h e r l e n s e s i s n e g l i g i b l e i n terms o f r e s o l u t i o n and c o n t r a s t . (a) Object. Viewed f r o m t h e e l e c t r o n - i n t e r a c t i o n , t r i b u t i o n v(x,y,z).

t h e o b j e c t a c t s as a p o t e n t i a l d i s -

I n f a c t , as t h e image i s two-dimensional,

a c t s l i k e a 2-0 p o t e n t i d ptojection

the object i t s e l f

v ( x y ) = v(P) on t h e e x i t p l a n e .

A c t y b X u U i ~ ~object e c a n be expanded i n t o a F o u r i e r s e r i e s as f o l l o w s v(:)

= E E v(;) h k

exp.(-Zai

li) = S(6)

33

OBJECT

F i g . 2.1.

1

FOCAL PLANE

PLANE IMAGE

P r i n c i p l e o f . imaging.

+ The p o t e n t i a l p r o j e c t i o n v ( r ) can be c o n s i d e r e d as a sum o f sine-waves w i t h spa+ + t i a l f r e q u e n c y R and a m p l i t u d e V ( R ) . R corresponds t o r e c i p r o c a l l a t t i c e p o i n t s

w i t h i n t e g e r c o o r d i n a t e s h,k. (b)

O b j e c t wave.

D u r i n g i t s t r a v e l t h r o u g h t h e o b j e c t , t h e n c i d e n t wave undergoes an a b d o h p t h n d k i d t e66ec.t; t h e second i s t h e most i m p o r t a n t i n t h i n c r y s -

eddect and a p h a c

+

t a l l i n e o b j e c t s w i t h o u t heavy atoms. A t a PO n t r ( x y ) in t h e e x i t p l a n e t h e -t e f f e c t can be r e p r e s e n t e d by a Rhanbpahency ~ u n c t i a na ( r ) {l-s(G)}exp i$(G) + + where s ( r ) i s t h e a b s o r p t i o n o r a m p l i t u d e component, @ ( r ) t h e phase component. o(;)

=

I f t h e emergent w a v e - f u n c t i o n f(;)

i s r e f e r e d t o an u n i t y i n c i d e n t a m p l i t u d e +

-

f

w h i c h i s modulated by t h e above t r a n s p a r e n c y f u n c t i o n , i t w r i t e s s i m p l y f ( r ) = o ( r ) . I n a s o - c a l l e d weakey-dcattehing a b j e c t t h e t r a n s p a r e n c y components a r e assumed +. t o be s m a l l ( s < < l , $ < < l )so , t h a t f ( r ) can be approximated t o + + + + f(r) a(r)= 1 s(r) + i$(r) (1) + The phase 4 k i d . t $ ( r ) f o r a g i v e n i n c i d e n t e l e c t r o n - e n e r g y Eo and wavelength h + i s d i r e c t l y r e l a t e d t o t h e o b j e c t p o t e n t i a l p r o j e c t i o n v ( r ) by

-

3 4 c ) Wave t r a n s m i s s i o n t h r o u g h t h e o b j e c t i v e . A c c o r d i n g t o Abbe's t h e o r y o f a pendect L e u , a l l t h e beams i s s u e d f r o m a -+

e x i t - p l a n e come t o f o c u s i n a p o i n t '; o f t h e image-plane + + (Gaussian p l a n e ) so t h a t r ' ( x ' , y ' ) = -m r ( x , y ) , where m i s t h e m a g n i f i c a t i o n o f t h e l e n s . To s i m p l i f y t h e f o r m u l a t i o n , t h e image w i l l be r e f e r e d t o t h e o b j e c t point r i n the object

s c a l e ; t h e image wave f u n c t i o n t h e n becomes i d e n t i c a l t o t h e o b j e c t w a v e - f u n c t i o n and can be w r i t t e n fa(;)

= f(7.). F o r a b e t t e r u n d e r s t a n d i n g o f t h e imaging p r o c e s s i n a r e a l l e n s , i t i s u s e f u l

t o c o n s i d e r t h e t r a n s m i s s i o n i n two s t e p s : di66huc,7%tion i n t h e back f o c a l p l a n e and .imaging i n t h e Gaussian p l a n e . Back f o c a l p l a n e o f a p e r f e c t o b j e c t i v e - l e n s . -+ The beams s c a t t e r e d w i t h an a n g l e 28 ( d i r e c t i o n k) a r e focused a t a p o i n t + R(hk) i n t h e f o c a l p l a n e . The observed d i f f r a c t i o n p a t t e r n , l o c a t e d i n t h a t p l a n e , i s o f t h e F r a u n h o f f e r t y p e . The Ewald-sphere w i l l be a s s i m i l a t e d t o t h e back f o c a l - p l a n e a t a d i s t a n c e f o f t h e l e n s ; i t f o l l o w s (see F i g . 4) + - + + R (k ko) X f , R 2f8

-

+ The s c a l e f a c t o r h f w i l l be o m i t t e d h e n c e f o r t h . The s c a t t e r e d a m p l i t u d e F(R) a t + a p o i n t R(hk) i n t h e b a c k - f o c a l p l a n e becomes now + + ++ 'L+ F(R) = 11 f ( r ) exp 2a i r R d r 2 = f ( r ) (3) + The f o c a l - p l a n e w a v e - f u n c t i o n a t R i s t h u s t h e F o u r i e r - t r a n s f o r m o f t h e o b j e c t

w a v e - f u n c t i o n . A c c o r d i n g t o E q . ( l ) , i t can be w r i t t e n f o r a w e a k - s c a t t e r i n g o b j e c t as + + 'L 'L = 6 ( ~ ) S(R) t i Q(R) where S = s F(;) and O = $ (4)

-

A

The d i f f r a c t i o n p a t t e r n F(R) i n t h e back f o c a l p l a n e r e p r e s e n t s t h e s p a t i a l frequency-spectrum o f t h e o b j e c t - f u n c t i o n : The 6 - f u n c t i o n r e p r e s e n t s a sharp c e n t r a l peak c o r r e s p o n d i n g t o u n s c a t t e r e d

-

e l e c t r o n s (6 = o f o r R # 0). The second and t h i r d terms r e p r e s e n t t h e s c a t t e r e d a m p l i t u d e . F o r a c r y a l i t

-

I s m a i n l y c o n c e n t r a t e d i n sharp peaks a t t h e r e c i p r o c a l l a t t i c e p o i n t s ( w i t h i n t e g e r hk c o o r d i n a t e s ) , each r e l a t e d t o a l a t t i c e p l a n e p e r i o d i c i t y = f X/R(hko). Around t h e d i s c r e t e peaks o c c u r s a d i f f u s e s c a t t e r i n g due d ( hko) + t o t h e c r y s t a l morphology ( f o r a s i n g l e u n i t - c e l l , F(R) would r e p r e s e n t t h e structure factor). Back f o c a l - p l a n e o f a r e a l o b j e c t i v e - l e n s .

I n f a c t an e l e c t r o n i c l e n s , l i k e a l i g h t r e f r a c t i o n l e n s , i n t r o d u c e s s e v e r a l l i m i t a t i o n s t o t h e wave p r o p a g a t i o n , and a c t s l i k e a f i l t e r .

-

Aperture l i m i t a t i o n .

The presence of a c i r c u l a r diaphragm o f r a d i u s Ro i n

35

t h e b a c k - f o c a l p l a n e can be r e p r e s e n t e d by t h e a p e r t u r e - f u n c t i o n A ( R ) ,

A = 1 for

R

<

Ro;

A = o

for

where

R > Ro

- S p h e r i c a l a b e r r a t i o n ( s e e F i g . 2 . 2 ) . , Defocus due t o s p h e r i c a l a b e r r a t i o n grows as t h e 3rd power o f t h e s c a t t e r i n a n g l e 28; f o r an a n g u l a r a p e r t u r e a i t l e a d s t o a smearing-disc o f r a d i u s Cs a ( r e f e r e d t o t h e o b j e c t p l a n e ) ; C s i s t h e spher i c a l a b e r r a t i o n c o n s t a n t . A t a d i s t a n c e R i n t h e f o c a l p l a n e i t produces a phase s h i f t y,

3

-

A d e v i a t i o n A f r o m t h e i d e a l f o c u s b r i n g s a phase s h i f t yA a t a distance R from the o p t i c a l a x i s Defocus. =

yA (R)

- -ax ( 2 8 ) 2 =nX R 2

m

OBJECTIVE

F i g . 2.2.

-

c,a3

GAUSSIAN PLANE

S p h e r i c a l a b e r r a t i o n and o p t i m a l defocus.

A s t i g m a t i s m and c h r o m a t i c a b e r r a t i o n w i l l be n e g l e c t e d ; t h e f i r s t c a n be c o r -

r e c t e d ; t h e o m i s s i o n o f t h e second i s c o n d i t i o n n e d by t h e w e a k - s c a t t e r i n g o b j e c t hypot h e s i s

.

The t o t a l phase s h i f t comes t h e r e f o r e Y

=y,

+ yA =

aX

(1 C 2 s

A2 R4

-

AR2)

I t can be assumed t h a t t h e whole spread a c t i o n due t o a p e r t u r e , a b e r r a t i o n

and f o c u s i s l o c a t e d i n t h e b a c k - f o c a l p l a n e where t h e aperture-diaphragm a c t u a l l y l i e s . T h i s a c t i o n a t a d i s t a n c e R ca? be r e p r e s e n t e d by t h e s o - c a l l e d + T(R) m u l t i p l y i n g t h e i d e a l d i f f r a c t i o n w a v e - f u n c t i o n F ( R ) ,

&anddc~ d u n d o n

t h e l e n s a c t i n g l i k e a space-frequency f i l t e r . The a c t u a l d i f f r a c t i o n f u n c t i o n becomes

36 + + -f F1(R) = F(R)T(R) = F(R) A(R) exp { - i y ( R ) } (6) + For a weakly-scattering o b j e c t , F ' ( R ) can be developed as f o l l o w s , according t o Eq.(4) and ( 6 )

- SAcosY

Is'

F'(R)

t

QAsinY)

+

i { S A s i n y t @A COSY)

(7)

d i t h i n t h e small angular range corresponding t o an a x i a l aperture i n c l u d i n g +

6'(R) ( b r i g h t f i e l d image), the imaginary p a r t o f F ' ( R ) may be neglected, l e a ding t o F'(Z)

5

6'

-

SA cosy

+

OAAsiny

(8)

Image plane.

+ The amplitude a t a p o i n t r i n t h e image plane i s the summation o f t h e ami n t h e f o c a l plane : p l i t u d e s scattered a t a l l p o i n t s

;

The image wave-function i s the convolution o f t h e o b j e c t - f u n c t i o n w i t h a smearing+ f u n c t i o n t(r) ?(R). For t h e weakly-scattering approximation, f ' ( r ) can be w r i t t e n as f o l l o w s , according t o Eq. (8) +

' L +

f ' ( r ) = F'(R) = 1

-

'L

(SAcosy)

+

%

(OAsiny)

(d) Image i n t e n s i t y d i s t r i b u t i o n

-+ What i s a c t u a l l y observed o r recorded i s an i n t e n s i t y d i s t r i b u t i o n j ( r )

j(F)

=

=

If(;)

t(r)I2

According t o Eq. ( l o ) , the b r i g h t - f i e l d image i n t e n s i t y o f a weakly-scattering o b j e c t can be approximated t o %

j(;)

= l-Z(SAcosy)

=

+

%

2(4Asiny)

The image c o n t r a s t c i s the d i f f e r e n c e between non scattered and scattered i n t e n s ity + + c(r) = j(r)

-

%

%

1 = -2(SAcos~) + 2(@AsinY)

(11)

Spatial-frequency spectrum o f t h e image c o n t r a s t +

ry-,

C(R) = c ( r ) =

-2SAcosy

+

2OAsiny

The Fourier-transform o r spatial-frequency spectrum o f t h e image-contrast i n weakly-scattering c o n d i t i o n s i s a 1 i n e a r combination o f t h e spatial-frequency spectra o f t h e amplitude and phase transparency f u n c t i o n s s and @, modulated by the amp1 i t u d e and phase c o n t r a s t f u n c t i o n s (-2cosy) and ( 2 s i n y ) r e s p e c t i v e l y . This r e s u l t c a l l e d the &fieah h u g e a p p o x i m a t i o n i s important f o r image processing,as w i l l be mentioned l a t e r . For a given microscope, t h e values of -b s i n y and cosy a t a p o i n t R o f the f o c a l plane depend on defocus A,as can be

3 7

seen from Eq. ( 5 ) . As those values determine t h e respective weight of t h e two contrast components, t h e image c o n t r a s t w i l l be very sensible t o defocus. Two l i m i t instances can be o u t l i n e d , according t o Eq. (12). I f s i n y = o f o r a g r e a t p a r t o f t h e d i f f r a c t i o n p a t t e r n i n s i d e the aperture,

-

the absorption o r amp1 i t u d e c o n t r a s t i s enhanced + c(r) = 2s (13) The image c o n t r a s t i s then i n t e r p r e t a b l e i n terms o f t h e o b j e c t absorption

-

-

I f siny =

2

1 f o r a g r e a t p a r t o f t h e d i f f r a c t i o n p a t t e r n i n s i d e the aperture,

the phase c o n t r a s t p r e v a i l s .(see F i g . 3) -+

c ( r ) = 24 =

27T -

v();

EO

The image c o n t r a s t i s then d i r e c t l y i n t e r p r e t a b l e i n terms o f t h e o b j e c t potent i a l p r o j e c t i o n , e.g. the s t r u c t u r e . It must be pointed o u t t h a t f o r a d a r k - f i e l d image the above approximation

does n o t hold any longer. The imaging then i n v o l v e s o n l y a d i f f r a c t e d beam + + R(hk) and t h e r e f o r e the imaginary p a r t i n F’(R) cannot be neglected any more. The foregoing treatment was based on coherent plane waves. L i m i t e d coherence due t o energy- and angular d i s p e r s i o n can be taken i n account by an envelope f u n c t i o n mu1 t i p l y i n g t h e t r a n s f e r f u n c t i o n , t h e r e f o r e reducing the c o n t r a s t a t high resol u t i o n and important d e f o c a l i s a t i o n . CRYSTAL I M A G I N G .

2.3

2.3.1 D i r e c t l y i n t e r p r e t a b l e images. The r e s o l u t i o n d o f a t h i n o b j e c t i s l i m i t e d by t h e o b j e c t i v e angular aperture u and spherical a b e r r a t i o n c o e f f i c i e n t C,. Aperture d i f f r a c t i o n leads t o a smearing d i s k w i t h diameter 0.6 A/a Spherical a b e r r a t i o n leads t o a smearing d i s k w i t h diameter 2 Csu 3.

-

-

.

When a increases, t h e d i f f r a c t i o n term decreases, t h e a b e r r a t i o n term increases; the best compromise according t o Rayleigh i s achieved when t h e t w o terms are equal, thus leading t o what can be c a l l e d t h e RayLeigh h e 6 o W o n do and aperture a, f o r the Gaussian focus 0.8 (A Cs) 1’4 w i t h a. 0 6X 1/4 do

=[c)

5

Defocusing reduces t h e a b e r r a t i o n smearing (see F i g . 2 ) . thus a l l o w i n g a l a r g e r dedocud) aperture. Scherzer (1949) has shown t h a t f o r an optimum defocus ( S c h M z ~ Az 1.2( C,X) 1/2

-

t h e r e s o l u t i o n improves t o dIo

z

0.6(X Cs) 14’

with

a l o e

4

?ao

T h i s r e s o l u t i o n can be c a l l e d t h e Schehzeh de60cu-6 hedoLuLion. I t can be seen from F i g . 2.3 t h a t i t corresponds a p p r o x i m a t e l y t o t h e f i r s t z e r o o f t h e phasec o n t r a s t t r a n s f e r - f u n c t i o n and t h a t t h e siny-component i s c l o s e t o u n i t y w i t h i n a good p a r t o f t h e a p e r t u r e , p r o v i d i n g an a l m o s t p h a s e - o b j e c t imaging a c c o r d i n g

.

t o Eq (14). I t 6 0 U o w ~t h a t in 6uch opehating c o n d X i o n ~ , t h e image can be dihectLy intmpmted i n tenms 06 t h e ~&uctwre.The Scherzer d e f o c u s r e s o l u t i o n can t h e r e f o r e be c a l l e d t h e b ~ & , ' L e h u o U o n .

I sin 7

.

100 k e V

F i q . 2 . 3 . O v e r a l l a s p e c t o f t h e phase c o n t r a s t t r a n s f e r f u n c t i o n i n t h e f o c a l p l a n e f o r Scherzer d e f o c u s , p l o t t e d a g a i n s t s p a t i a l p e r i o d i c i t i e s , f o r 100 and 1000 keV. I t i s easy t o understand t h a t i n t h o s e c o n d i t i o n s t h e l i g h t c o n t r a s t f r o m

small phase s h i f t corresponds t o a l o w p o t e n t i a l p r o j e c t i o n (e.g.

structural

" t u n n e l s " ) , whereas t h e d a r k c o n t r a s t f r o m l a r g e phase s h i f t corresponds t o a h i g h p o t e n t i a l p r o j e c t i o n (e.g. a t o m i c rows). F o r a 100 kV microscope w i t h Cs = 0.16 cm t h e f o l l o w i n g v a l u e s a r e achieved: Gaussian f o c u s

: A

Scherzer defocus : A =

0

- 900

: do

= 4.4.

: d I o = 3.3.

;;

w i t h a0 = 5.3.10m3 r d w i t h a'0 = 6.7.10m3rd

F o r a L g h uoMage m i a o ~ c o p et h e s t r u c t u r e r e s o l u t i o n i s a g r e a t deal b e t t e r . F o r a 600 kV E.M.

0

w i t h Cs = 0.33 cm, i t g e t s down t o a b o u t 2 A ( s e e F i g . 3 ) .

F u r t h e r m o r e s c a t t e r i n g and energy l o s s e s a r e reduced, so t h a t t h e t h i c k n e s s can be i n c r e a s e d w i t h i n t h e weak-object a p p r o x i m a t i o n . These q u a l i t i e s e x p l a i n t h e growing i n t e r e s t f o r h i g h - v o l t a g e h i g h - r e s o l u t i o n E.M. J o u f f r e y e t a l . 1979).

( C o s s l e t and Smith, 1979;

3 9

S i l i c a t e s t r u c t u r e - i m a g e s have been performed u s i n g f o r e g o i n g o p e r a t i n g c o n d i t i o n s , e.g.

by Buseck e t a l . (1974), Thomas e t a1.(1979).

The c r y s t a l o r i e n t a t i o n s

which a r e most l i k e l y t o p r o v i d e u s e f u l and c a r a c t e r i s t i c s t r u c t u r e - p r o j e c t i o n s a r e t h o s e f o r w h i c h t h e beam i s p a r a l l e l t o an i m p o r t a n t z o n e - a x i s ( a s f o r example
a p p r o x i m a t i o n should f a i l even f o r

f a i r l y t h i n c r y s t a l s . An i n t e r p r e t a t i o n i n terms o f c h a r g e - d e n s i t y p r o j e c t i o n has been proposed f o r strong-phase o b j e c t s (Lynch e t a1.1975). I t h o l d s o n l y f o r n e g l i g i b l e s p h e r i c a b e r r a t i o n ( s i n y = o ) , i . e . f o r a s m a l l a p e r t u r e and a l i m i t e d r e s o l u t i o n . C a u t i o u s i n t e r p r e t a t i o n must p r e v a i l i n those, most f r e q u e n t cases. However t h e use o f computer m a t c h i n g ( s e e 3.3.c)

has proven t h a t i n many cases

" w h i t e - d o t t e d " images o b t a i n e d i n such main o r i e n t a t i o n s c o u l d be d i r e c t l y i n t e r p r e t e d i n terms o f s t r u c t u r a l channels between atom rows o f s i l i c a t e c r y s t a l s 0

up t o 100 A t h i c k (e.g. F i g . 5 ) . 2.3.2

Two-beams i n t e r f e r e n c e imaoes.

L e t t h e c r y s t a l l i n e o b j e c t have (hkO) l a t t i c e p l a n e s para1 1e l t o t h e o p t i c a l a x i s w i t h space p e r i o d i t i e s d ( h k 0 ) such t h a t t h e Bragg d i f f r a c t i o n a n g l e 8 i s smaller than the aperture

e z

A 2d(hkl)

< a

+

-

d(hk1) > A 2a

Both t h e d i r e c t 000 beam and t h e d i f f r a c t e d hkO beam c a n t h e n pass t h r o u g h t h e a p e r t u r e diaphragm when o p e r a t i n g w i t h s y m n e t r i c d i r e c t - b e a m d e f l e c t i o n (see F i g . 2.4) and i n t e r f e r e t o f o r m image p l a n e - f r i n g e s w i t h s p a c i n g m d ( h k l ) . T h a t corresponds t o l i m i t t h e F o u r i e r s e r i e s i n Eq. ( 9 ) t o t h e c o n s t a n t t e r m and one 0 s i n e term. W i t h 100 keV, a 3.5 A s p a c i n g c a n t h u s be r e s o l v e d . F u r t h e r m o r e , f o r a g i v e n a p e r t u r e g r e a t e r t h a n a O s t h e s p h e r i c a l a b e r r a t i o n c a n be e x a c t l y compensated b y a c o r r e s p o n d i n g d e f o c u s , t h a n k s t o t h e 2-beam symmetry. L a t t i c e

F i g . 2.4. Symmetri c a l 2-beam o p e r a t i o n .

40

r e s o l u t i o n down t o 1

and even l e s s can be achieved; t h e o n l y l i m i t a t i o n s are

Instrumental s t a b i l i t y ( e l e c t r o n i c a l and mechanical) and chromatic a b e r r a t i o n . That r e s o l u t i o n can t h e r e f o r e be c a l l e d the i m m e n t d te.bo.&u%n. The p o s i t i o n o f t h e f r i n g e s depends on the operating parameters (plane o r i e n t a t i o n , phase s h i f t s , e t c ...) and cannot be d i r e c t l y c o r r e l a t e d t o t h e o b j e c t s t r u c t u r e planes. However such observations can g i v e valuable informations on e x i s t i n g c r y s t a l l a t t i c e - d e f e c t s and disorder; f o r example w e l l c r y s t a l l i z e d mica gives good hkO l a t t i c e - i m a g e s whereras smectites almost never g i v e any. Indirect interpretation o f imaqs. Apertures which are s i g n i f i c a n t l y l a r g e r than

2.3.3

t r a n s m i t a g r e a t number o f

d i f f r a c t e d beams hk0 together w i t h t h e d i r e c t beam, leading t o a n image F o u r i e r series w i t h numerous sine-terms i n c l u d i n g s p a t i a l p e r i o d i c i t i e s d(hk0) smaller than d l 0 . D e t a i l s as f i n e as t h e instrumental r e s o l u t i o n can then be observed i n t h e image. However d i r e c t i n t e r p r e t a t i o n o f those d e t a i l s i n terms of s t r u c t u r e - p r o j e c t i o n i s no longer possible. Thanks t o t h e knowledge o f the imaging theory and t o the a v a i l a b l e means o f computing, an i n d i r e c t image-object c o r r e l a t i o n can now be achieved u s i n g image processing. Several methods have been proposed and applied i n recent years. Only a rough o u t l i n e w i l l be given; r e f e rences w i l l be found a t t h e end. (a)

Image improvement.

A noisy background o f t e n a f f e c t s h i g h - r e s o l u t i o n images. I t can be removed

by e l i m l n a t i n g high s p a t i a l frequencies i n processed image-contrast F o u r i e r transforms (low-pass f i l t e r ) , provided t h a t t h e noise g r a i n s are smaller than the i n t e r e s t i n g s t r u c t u r a l d e t a i l s . The p e r i o d i c i t y o f b a d l y c r y s t a l l i s e d objects can be enhanced i n the image by Fourier-transform o f d i s c r e t e d i f f r a c t i o n spots (equivalent t o o p t i c transforms). U n t i l

now these techniques have mainly been used f o r b i o l o g i c a l o b j e c t s . However they could be u s e f u l f o r c e r t a i n c l a y studies. Thus C o l l i e x e t a1 (1980) have enhanced t h e imaging o f f a u l t e d areas i n l a y e r - s i l i c a t e s by o p t i c a l transforms. (b) Image processing based on image f i l t e r i n g . I n the micrograph o f a weakly-scattering specimen i t i s t h e o r e t i c a l l y possible t o recover S and @, hence t h e o b j e c t - f u n c t i o n , from t h e l i n e a r c o n t r a s t transform + C(R) -(Eq.12). The simplest case concerns a phase-object where @ = C/siny. Several images are recorded w i t h d i f f e r e n t defocus (through-focal series); t h e F o u r i e r transforms are performed i n c l u d i n g an a d d i t i o n a l noise-term. An optimal estimate o f 0 can be obtained by a least-mean-square method. Image processing needs t h e knowledge o f t h e microscope t r a n s f e r - f u n c t i o n values. They are u s u a l l y obtained from the o p t i c a l Fourier-transform o f a t h i n amorphous carbon o r s i l i c o n f i l m image. Least square f i t t i n g o f t h e experimental and t h e t h e o r e t i c a l r a d i a l i n t e n s i t y d i s t r i b u t i o n s leads t o t h e r e q u i r e d values o f y.

41

Several l i n e a r f i l t e r i n g attempts have been published. Processing methods f o r more general non l i n e a r cases by i t e r a t i o n have been e s t a b l i s h e d . A 3-dimensional r e c o n s t r u c t i o n has been proposed on t h e b a s i s o f t h e microscope's d i f f r a c t o m e t e r f e a t u r e s ( r e v i e w i n Hawkes , ( 3 ) ) . ( c ) Matching o f computed and observed images. U n t i l now t h e most successful way o f image i n t e r p r e t a t i o n has been a t r i a l and-error method i n which an i n i t i a l s t r u c t u r e - h y p o t h e s i s (e.g. based on X-Ray data) i s r e f i n e d u n t i l t h e t h e o r i c a l t r a n s m i t t e d wave-image matches w i t h t h e observed image. The most used object-wave c a l c u l a t i o n i s t h e s o - c a l l e d muetis&e

method

(Cowley and Moodie, 1957; Goodman and Moodie, 1974). The c r y s t a l i s assumed t o be a s t a c k i n g o f i n d i v i d u a l t h i n s l i c e s s c a t t z r i n g one a f t e r another t h e i n c i d e n t wave. The t h e o r e t i c a l d i f f r a c t e d amplitude F(R) i n t h e back-focal plane i s obt a i n e d by an i t e r a t i o n formula f o r t h e N s c a t t e r e d waves which c o n t r i b u t e t o t h e image and i s then m u l t i p l i e d by t h e t r a n s f e r - f u n c t i o n . The squared F o u r i e r - t r a n s form r e s t o r e s t h e t h e o r e t i c a l image i n t e n s i t y - d i s t r i b u t i o n which can now be compared w i t h t h e observed image. Computation programmes have been c a r r i e d o u t on s i l i c a t e m i n e r a l s , p a r t i c u l a r l y by O'Keefe and Buseck (1978). An example i s shown i n F i g . 2.5.

Fig.2.5. 2ML-mica viewed along ,w i t h p r o j e c t e d s t a c k i n g sequences: a) S t r u c t u r e model p r o j e c t i o n b) Computed charge-density p r o j e c t i o n 0 c) M u l t i s l i c e - p r o essed image f o r Eo = 100 keV, C s = 0.18 cm,A = 1200 A, t h i c k n e s s 100 d ) Observed " w h i t e - d o t t e d " s t r u c t u r e image. The d o t s correspond t o s t r u c t u r e channels i n t h e K-layer. ( x 8,880,000) (Amouric e t a1 1981a).

F\

-

42 The same t h e o r y can t a k e i n t o a c c o u n t t h e e f f e c t o f c r y s t a l d e f e c t s . I t a p p l i e s t o t h e g e n e r a l case, w i t h o u t r e s t r i c t i v e h y p o t h e s i s on t h e o b j e c t . I t s o n l y l i m i t i s determined by t h e c o m p u t a t i o n means. However t h e q u e s t i o n o f t h e uniqueness o f t h e s o l u t i o n must n o t be o v e r l o o k e d . I n t h e m u l t i s l i c e method each s l i c e s t r u c t u r e i s approximated t o i t s p r o j e c t i o n . That a p p r o x i m a t i o n f a i l s w i t h l a r g e u n i t - c e l l s o r d i s o r d e r a l o n g t h e i n c i d e n t b e a m - d i r e c t i o n . A somewhat d i f f e r e n t t r e a t m e n t has been proposed by Van Dyck (1980), based on a d i r e c t - s p a c e quantum-mechanical c a l c u l a t i o n i n w h i c h t h e f i r s t - o r d e r m u l t i s l i c e expansion i s t r a n s f o r m e d i n t o a second-order expansion. It should t h e r e f o r e be more a c c u r a t e , ,'should a p p l y t o t h i c k e r s l i c e s and s h o u l d

be a b l e t o account f o r d i s o r d e r e d s t r u c t u r e s . F u r t h e r m o r e t h e c a l c u l a t i o n t i m e 2 becomes p r o p o r t i o n a l t o N ( i n s t e a d o f N w i t h t h e f o r e g o i n g t e c h n i q u e ) , w h i c h s h o u l d enable i t t o be performed on s m a l l cwnputers. 2.4

CLAY SPECIMENS I N H.R.E.M.

2.4.1

Structure

U n t i l now H.R.E.M.

has been m a i n l y a p p l i e d t o w e l l c r y s t a l l i z e d m a t e r i a l s ,

s e l e c t e d and s y n t h e s i z e d i n v i e w o f an easy s t r u c t u r e imaging, i . e . w i t h s i m p l e 0

s t r u c t u r e s , l a r g e atom spacings (4-5 A ) , and good electron-beam r e s i s t a n c e . The c l a y m i n e r a l o g i s t must a c c e p t t h e m a t e r i a l as he f i n d s i t , w i t h a r a t h e r 0

complex s t r u c t u r e and main a t o m i c d i s t a n c e s down t o 2.6 A, w i t h d i s o r d e r and d e f e c t s acquired d u r i n g growth o r a l t e r a t i o n , with a h i g h s e n s i t i v i t y t o e l e c t r o n bombardment and a l o w e l e c t r o n - c o n d u c t i v i t y .

The b e s t c r y s t a l 1 i z e d c l a y m i n e r a l ,

mica, i s t h e r e f o r e t h e one w h i c h has been t h e most f r e q u e n t l y s t u d i e d by means o f H.R.E.M. 2.4.2. (a)

Specimen o r i e n t a t i o n Normal o r i e n t a t i o n

Due t o t h e i r l a y e r e d s t r u c t u r e , c l a y m i n e r a l s a r e prone t o a p r e f e r r e d l a m e l l a r o r i e n t a t i o n when d e p o s i t e d on a m i c r o g r i d . Thus t h e c l a s s i c a l p r e p a r a t i o n mode which makes use o f g r i n d i n g , suspension, g r a n u l o m e t r i c a l s e p a r a t i o n and sediment a t i o n b r i n g s most o f t h e (001) l a m e l l a s p e r p e n d i c u l a r t o t h e beam and l e a d s t o t h e imaging o f t h e < O O b zone p l a n e s . A x i a l i l l u m i n a t i o n w i t h Scherzer defocus i n c l u d i n g t h e i n n e r hexagon (020 and 110 t y p e r e f l e c t i o n s c o r r e s p o n d i n g t o a 0

4.5 A

s e p a r a t i o n ) p r o v i d e s a hexagonal l a t t i c e image w h i c h i n c e r t a i n c o n d i t i o n s

i s r e l a t e d t o a s i n g l e - l a y e r s t r u c t u r e - p r o j e c t i o n as shown by J e f f e r s o n and Thomas (1974). However such l a t t i c e - f r i n g e images a r e a l m o s t t h e same f o r a l l c l a y miner a l s and can h a r d l y g i v e any i n t e r e s t i n g s t r u c t u r a l i n f o r m a t i o n . T h e i r occurence i n d i c a t e s t h a t t h e s t a c k i n g i s n o t a t u r b o s t r a t i c one ( s e e F i g . 6 ) . H i g h e r r e s o l u t i o n imaging combined w i t h computer m a t c h i n g seems n o t t o have been a t t e m p t e d so f a r i n t h a t o r i e n t a t i o n .

43

Fig. 2.6.Crossed l a t t i c e images due t o r e f l e c t i o n s on (020) and (110). a M o n t m o r i l l o n i t e (Yoshida, 1976) ( x 6,900,000) b{ Mica muscovite (Ehret, Lab. de Mineralogie, Strasbourg). ( x 7,670,000)

44

( b ) Layer-imaging o r i e n t a t i o n The most i n t e r e s t i n g s t r u c t u r a l f e a t u r e o f c l a y m i n e r a l s i s t h e l a y e r - s t a c k i n g . I t d e t e r m i n e s a p a r t o f t h e i r p r o p e r t i e s and i s r e l a t e d t o t h e i r o r i g i n . I n o r d e r t o v i s u a l i z e t h e l a y e r s , t h e y must be b r o u g h t t o become p a r a l l e l t o t h e beam. F o r s e p a r a t e c l a y p a r t i c l e s t h o s e c o n d i t i o n s can b e s t be achieved b y t h i n sect i o n n i n g normal t o t h e i r b a s a l p l a n e s u s i n g an u l t r a m i c r o t o m e w i t h diamant k n i f e . Whereas v e r y common f o r b i o l o g i c a l m a t e r i a l , t h a t method i s d i f f i c u l t t o a p p l y t o r a t h e r b r i t t l e m a t e r i a l s l i k e c l a y c r y s t a l s . A g r e a t a t t e n t i o n must be payed t o a p e r f e c t and o r i e n t e d embedding i n an e p o x i c o r m e t h a c r y l i c r e s i n w h i c h s h o u l d have a hardness c l o s e t o t h a t o f t h e m i n e r a l . V a r i o u s t e c h n i q u e s have been d e s c r i b e d , f i t t e d t o t h e r e s p e c t i v e m a t e r i a l (e.g.

E b e r h a r t and T r i k i , 1972;

Lee e t a1 , 1975a; Tchoubar e t a1 , 1973). A one-dimensional s t r u c t u r e - p r o j e c t i o n i s observed i n a g e n e r a l a z i m u t h ( w i t h o n l y 001 beams). 2-D s t r u c t u r e - p r o j e c t i o n s

demand a h i g h l y a c c u r a t e a z i m u t h a l o r i e n t a t i o n (e.g. <110> o r p a r a l l e l t o t h e beam f o r mica, see F i g . 2.5. The average t h i c k n e s s o f t h e m i c r o t o m e - s e c t i o n s i s about 500

A , too

l a r g e f o r s t r u c t u r a l imaging. T h e r e f o r e t h e o b s e r v a t i o n s

have t o be achieved on t h e edges o f wedge-shaped ' c r y s t a l l i t e s . A c c u r a t e o r i e n t a t i o n a l o n g dense rows can t h e o r e t i c a l l y be performed by u s i n g a t i l t i n g s t a g e and c h e c k i n g by means o f t h e d i f f r a c t i o n p a t t e r n . However, owing t o t h e beams e n s i t i v i t y , t h i s method i s t o o slow t o be p r o p e r l y used f o r c l a y m i n e r a l s . The s i m p l e s t way seems t o r e c o r d images o f a g r e a t number o f p a r t i c l e s h a v i n g random azimuth; t h e p r o b a b i l i t y o f f i n d i n g d o t t e d s t r u c t u r e - i m a g e s becomes t h u s f a i r l y h i g h . S t r u c t u r e - i m a g e s o b t a i n e d i n such c o n d i t i o n s and combined w i t h computer s i m u l a t i o n have b r o u g h t t h e most v a l u a b l e r e s u l t s u n t i l now i n H.R.E.M.

clay

study.

E . M . specimens o f i n r o c k s embedded c l a y s a r e r e a d i l y p r e p a r e d f r o m a c c u r a t e l y o r i e n t e d p e t r o g r a p h i c t h i n s e c t i o n s w h i c h have been l o c a l l y t h i n n e d by a r g o n i o n - m i l l i n g under g r a z i n g i n c i d e n c e . (e.g. Amouric e t a l , 1981a). 2.4.3

Beam s e n s i t i v i t y

Electron-beam induced d e h y d r a t i o n and d e h y d r o x y l a t i o n l e a d s soon t o s t r u c t u r a l damage and t h e r e f o r e t o an image b l u r r i n g , i n c o m p a t i b l e w i t h h i g h r e s o l u t i o n . T h i s comnands q u i c k o p e r a t i o n , g e n e r a l l y e x c l u d i n g c a r e f u l o r i e n t a t i o n and t h r o u g h - f o c u s s e r i e s ( s e e f o r e g o i n g p a r a ) . P r o c e s s i n g methods a r e t h e r e f o r e spec i a l l y u s e f u l t o e x t r a c t t h e maximum amount o f i n f o r m a t i o n o u t o f a s i n g l e m i c r o g r a p h. Well adapted i m a g e - a m p l i f i e r s (e.g. equiped w i t h h i g h - r e s o l u t i o n micro-channel p l a t e s ) c o u l d be v e r y u s e f u l i n a l l o w i n g l o w e r e l e c t r o n i n t e n s i t i e s .

45

2.5

APPLICATIONS OF H.R.E.M.

TO CLAY MINERALOGY

The main us e o f H.R.E.M. i n c l a y m i n e r a l o g y c o n s i s t s i n i n v e s t i g a t i n g l o c a l s t r u c t u r e f e a t u r e s d e p a r t i n g f r o m t h e p e r f e c t c r y s t a l s t r u c t u r e as shown by X-ray d i f f r a c t i o n . Viewing a l o n g t h e (001) l a y e r s can g i v e an i n s i g h t i n t o t h e most f r e q u e n t d e f e c t s t r u c t u r e s i n v o l v i n g t h e s t a c k i n g sequence o f t h e 1 ayers, t h e knowledge o f w h i c h i s o f paramount i m p o r t ance f o r t h e comprehension o f growt h and w eat h erin g mechanisms and f o r t h e i n f e r e n c e o f surrounding c o n d i t i o n s . The answer t o t h e d i f f i c u l t q u e s t i o n o f a l t e r a t i o n e i t h e r t h r o u g h repeat ed d i s s o l u t i o n and neoro rma t i o n , o r t h r o u g h s o l i d - s t a t e t r a n s f o r m a t i o n should t o be found by means o f t h a t t e c h n i q u e . Numerous works on E.M. r e l a t e d t o c l a y m i n e r a l o g y have been p u b l i s h e d i n t h e l a s t t e n y e a r s . Most o f them a r e based on l a t t i c e - f r i n g e images o r on one-dimens i o n a l s t r u c t u r e p r o j e c t i o n s . Very few have achieved a r e a l 2-D s t r u c t u r e imaging. Some examples o f t y p i c a l a p p l i c a t i o n s w i l l be o u t l i n e d h e r e a f t e r . On a g i v e n t o p i c , o n l y t h e most r e c e n t s t u d i e s w i l l be quoted. M e r e l y morphological lowr e s o l u t i o n works on c l a y s w i l l n o t be mentioned. Examples o f mic r o m o r p h o l o g i c a l s t u d i e s by means o f E.M.

a r e g i v e n by t h e now

w e l l known s e r p e n t i n e polymorphism, i.e. t h e t u b u l a r s t r u c t u r e o f c h r y s o t i l e (e.g. Yada, 1972), t h e c o r r u g a t i o n o f a n t i g o r i t e and t h e p l a t y morphology o f l i z a r d i t e (e.g. Thomas e t a l , 1979 and i n ( 7 ) ) . I n t h e same way t h e c h a n n e l - s t r u c t u r e o f s e p i o l i t e has been shown by Rautureau e t a1 (1974, 1977). S o l i d - s t a t e t r a n s f o r m a t i o n s o f c l a y m i n e r a l s i s one o f t h e i m p o r t a n t t o p i c s 0 approached by E.M. A g r a d a t i o n o f a l l o p h a n e t o f i b r o u s 10 A - h a l l o y s i t e has been p o i n t e d o u t b y Sudo and Yotsumoto (1977). Ordered and unordered i n t e r s t r a t i f i c a t i o n can be s t u d i e d on an i n d i v i d u a l l a y e r s c a l e by l a t t i c e - f r i n g e imaging (e.g. Lee e t a l , 1975b). - I n t e r l a y e r s o r p t i o n o f o r g a n i c compounds has been v i s u a l i z e d as shown i n F i g . 2.7. Lay er-by -la y er imaging o f t h e c r y s t a l g r o w t h o f c l a y m i n e r a l s i s p r o b a b l y one of t h e most i n t e r e s t i n g f i e l d o f a p p l i c a t i o n o f E.M., e s p e c i a l l y when combined w i t h t h i n sectioning. The f i r s t s t a ges of s y n t h e t i c a l c h r y s o t i l e g r o w t h have been shown by Yada and I i s c h i (1977) i n d i f f e r e n t pH and hydrothermal c o n d i t i o n s . Layer f o r m a t i o n and s t a c k i n g f o l l o w e d b y c u r l i n g t o t u b u l a r f i b e r s c o u l d be d i r e c t l y observed on lattice-fringe

images w i t h i n l e s s t h a n an hour, whereas X-ray d i f f r a c t i o n c o u l d

n o t d i s c l o s e any t r a c e o f c h r y s o t i l e c r y s t a l l i z a t i o n b e f o r e one o r several days. The p o s s i b i l i t y o f d i r e c t imaging o f s t a c k i n g f a u l t s i n s e l e c t e d l a y e r e d mat e r i a l s has been shown f i r s t by J e f f e r s o n and Thomas (1974). Real sub-atomic

46

f

Fig. 2.7. I n t e r l a y e r s o r p t i o n o f lauryl-ammonium ions on i n t e r s t r t i f i e d micasmectite as shown by a l a t t i c e image. Unexpanded l a y e r s have 10 spacing; expanded l a y e r s have 24 1 spacing. (Yoshida, 1973). ( x 1,300,000) s t r u c t u r e r e s o l u t i o n has been achieved r e c e n t l y , mostly on mica, associated w i t h computer simulation. I n d i v i d u a l l a y e r stacking can be d i r e c t l y viewed i n mica c r y s t a l s by means o f " b r i g h t - d o t t e d " s t r u c t u r e images (see F i g . 5); the observed stacking sequences can then be c o r r e l a t e d t o the growth c o n d i t i o n s by comparing s y n t h e t i c a l c r y s t a l growth I n we1 1 defined c o n d i t i o n s and n a t u r a l c r y s t a l growth. Various ordered and disordered stacking sequences have been observed i n t h a t way i n s y n t h e t i c a l and n a t u r a l m a t e r i a l s by Amouric e t a1 (1978, 1981) and by I i j i m a and Buseck (1978). Mica i n t e r g r o w t h i n pyroxene has even been pointed o u t by Buseck and Veblen (1978). Nucleation o f s y n t h e t i c a l mica i n hydrothermal c o n d i t i o n s i s being studied by Amouric (1981) who has n o t i c e d on s t r u c t u r e 0 images t h a t few-layered nucleous are o f t e n made up o f an i n i t i a l 30 A l a y e r sur0 rounded symmetrically by 10 A l a y e r s . Disordered stacking i n c h l o r i t e has a l s o been observed (see F i g . 8 ) . I t must be pointed o u t t h a t i n kinematical d i f f r a c t i o n c o n d i t i o n s ( i . e . on very t h i n c r y s t a l edges) o n l y t h e basic p e r i o d i c i t y o f the layers can be observed, owing t o the absence o f the s u p e r l a t t i c e r e f l e c t i o n s which are then forbidden by the space group (e.g. the 001 r e f l e x i o n s w i t h 1 odd i n 2M micas). Thus the polytype sequence can o n l y appear i n t h i c k e r areas where the mu1 t i p l e s c a t t e r i n g p r e v a i l s and t h e space-group e x t i n c t i o n breaks down. Recent microscopes a1 low the combined study o f h i g h - r e s o l u t i o n images and energy-dispersive X-ray microanalysis. Using t h a t method,a c o r r e l a t i o n o f major and t r a c e element content w i t h c r y s t a l d e f e c t s and stacking sequences has been pointed o u t by Buseck and Veblen (1978).

0

F i g . 2 . 8 . Chlorite structure image with 14 A layer spacing and random stacking. (Amouric et a1 , 1981b). ( x 6,140,000)

48

2.6

CONCLUSION

F u t u r e p r o g r e s s i n H.R.E.M. c a n be expected i n s e v e r a l d i r e c t i o n s . F u r t h e r r e f i n e m e n t o f image p r o c e s s i n g c o u l d l e a d t o a b e t t e r mat ching w i t h o b s e r v a t i o n i n g en e r a l cases, e s p e c i a l l y w i t h d i s o r d e r e d s t r u c t u r e s f r e q u e n t l y encountered i n c l a y s . I n - l i n e p r o c e s s i n g c o u l d make e a s i e r t h e d i r e c t i n t e r p r e t a t i o n o f s t r u c t u r e images i n p e c u l i a r cases (e.g. L a b e r r i g u e e t a l , 1980). 0

X-ray m i c r o a n a l y s i s t o g e t h e r w i t h m i c r o d i f f r a c t i o n on areas smal l e r t han 100 A i n diame t e r, ac hie v e d w i t h f i e l d e m i s s i o n cathodes and combined w i t h H.R.E.M. , c o u l d make i t p o s s i b l e t o i n v e s t i g a t e t h e r e l a t i o n s between l o c a l s t r u c t u r e , c o m p o s i t i o n and r e a c t i o n mechanisms, t h u s l e a d i n g t o u s e f u l c o r r e l a t i o n s w i t h geochemical c o n d i t i o n s o f c l a y m i n e r a l s growth. However a f a i r s i g n a l - t o - n o i s e r a t i o f r o m such a s m a l l number o f atoms needs a h i g h e l e c t r o n i n t e n s i t y incompat i b l e w i t h t h e electron-beam s e n s i t i v i t y o f c l a y s . E l e c t r o n - e n e r g y - l o s s s p e c t r o m e t r y c o u l d b r i n g an answer t o h i g h - r e s o l u t i o n a n a l y s i s ( J o u f f r e y , 1977). A w e l l designed h i g h - r e s o l u t i o n e l e c t r o n - i m a g e amp1 i f i c a t i o n c o u l d a1 l o w t o r e c o r d s t r u c t u r e images i n o p t i m a l o p e r a t i n g c o n d i t i o n s w i t h o u t beam-damaging t h e clays mineral s. H igh-v o lt a ge E.M. c o u l d a f f o r d a much b e t t e r s t r u c t u r e - r e s o l u t i o n t o g e t h e r w i t h reduced energy-spread and o b j e c t - h e a t i n g and w i t h a t h i c k e r specimen. However s e v e r a l equipment d i f f i c u l t i e s must be overcome t o r e a c h t h e t h e o r e t i c a l r e s o l u t i o n a t 1 MeV o r h i g h e r . The s c a nning-t ra n s m i s s i o n - e l e c t r o n - m i c r o s c o p e i s n o t hampered by e i t h e r spher i c a l o r c h r o m a t i c a l a b e r r a t i o n , i s l e s s specimen damaging and b e t t e r s u i t e d f o r i n - l i n e pro c es s i n g . I t c o u l d t h e r e f o r e c h a l l e n g e t h e t r a d i t i o n a l t r a n s m i s s i o n microscope. However, above a l l i t must be k e p t i n mind t h a t t h e use o f r e a l h i g h - r e s o l u t i o n elec t ron-mic ro s c opy t o i n v e s t i g a t e c l a y - m i n e r a l s t r u c t u r e s on a n e a r l y at omic s c a l e needs a t h oro u g h knowledge o f t h e imaging process. Observed s t r u c t u r e images must be compared w i t h computed images r a t h e r t h a n l o o k e d a t as e x c i t i n g p i c t u r e s . F or c l a y m i n e r a l o g i s t s , w h o agree t o t a k i n g i n account t hose i m p e r a t i v e s , H.R.E.M. may w e l l r e v e a l a most p o w e r f u l t o o l f o r e x p l o r i n g l o c a l s t r u c t u r e - f e a t u r e s on an atomic s c a l e , as i t can be expected f r o m t h e f i r s t s e r i o u s i n v e s t i g a t i o n s summarized i n paraqraph 2.5. On t h e o t h e r hand,a merely d e s c r i p t i f i n t e r p r e t a t i o n can l e a d t o g r o s s m i s i n t e r p r e t a t i o n s . ACKNOWLEDGEMENTS The a u t h o r expresses h i s g r a t i t u d e t o a l l t h e e l e c t r o n - m i c r o s c o p i s t s who have k i n d l y s e n t him documents and h e l p e d him w i t h t h e i r advices.

49

REFERENCES Books and genera l r e v i e w s (1) E b erh art , J.P. , 1976. Methodes p h y s i q u e s d ' e t u d e des mineraux e t m a t e r i a u x s o l i d e s . Doin, P a r i s , 507 pp. (2) Gard, J.A. ( E d i t o r ) , 1971. The e l e c t r o n - o p t i c a l i n v e s t i g a t i o n o f c l a y s . Min era l .Soc. London , 381 pp. (3) Hawkes, P.W. ( E d i t o r ) , 1980. Computer p r o c e s s i n g o f e l e c t r o n microscope images. S p r i n g e r , B e r l i n , 296 pp. (4) He idenre ic h, R.D. , 1964. Fundamentals o f t r a n s m i s s i o n e l e c t r o n microscopy. Wiley, New-York, 405 pp. (5) K i h l b o r g , L. ( E d i t o r ) , 1979. D i r e c t imaging o f atoms i n c r y s t a l s and molecu les . Nobel Symposium 47. The Royal Swedish Academy o f Sciences, 295 pp. (6) Saxton, W.O. , 1978. Computer t e c h n i q u e s f o r image p r o c e s s i n g i n e l e c t r o n microscopy. Academic Press, London, 304 pp. (7) Sudo T., Shimoda S., Yotsumoto H., A i t a S., 1981. E l e c t r o n micrographs o f c l a y m i n e r a l s . Kodansha-Elsevier, 203 pp. (8) Wenk, H.R. ( E d i t o r ) , 1976. E l e c t r o n m i c r o s copy i n mineralogy. S p r i n g e r , B e r l i n , 564 pp.

Papers Amouric, M. , 1981. P r i v a t e communication. Amouric, M., Baronnet, A,, F i n c k , C., 1978. P o lyt ypisme e t desordre dans l e s micas d i o c t a e d r i q u e s s y n t h e t i q u e s . Etude p a r i m a g e r i e de reseau. Mat.Res. B u l l . , U.S.A. , 13: 627-634. Amouric, M., M e r c u r i o t , G., Baronnet, A., 1981a. On computed and observed H.R.T.E.M. images o f p e r f e c t mica p o l y t y p e s . Bull. Minera1. . t o be p u b l i s h e d . Amouric, M. , Baronnet, A. , C a t t i , M. , 1981b. P r i v a t e communication. Buseck, P.R., I i j i m a , S. , 1974. H i g h r e s o l u t i o n e l e c t r o n microscopy o f s i l i c a t e s . Amer.Mineral., 59: 1-21. Buseck, P.R., Veblen, D.R., 1978. Trace elements, c r y s t a l d e f e c t s and h i g h reso1u t i o n e l e c t r o n microscopy. Geochim .Cosmochim .Acta, 42 : 669-678. C o l l i e x , C., G a it e , J.M., Mory, C., Rautureau, M., Tchoubar, C., 1980. 0pt ical f i l t e r i n g o f f a u l t e d a r e a s i n e l e c t r o n m i c r o graphs o f l a y e r s i l i c a t e s . J.Microsc.Spectrosc. E l e c t r o n . , 5: 33-40 Coss le t , V.E. , Smith, D.J. , 1979. H i g h r e s o l u t i o n imaging o f amorphous m a t e r i a l s and c r y s t a l l i n e d e f e c t s . I n (5), pp. 39-45. Cowley, J.M. , Moodie, A.F. , 1957. The s c a t t e r i n g o f e l e c t r o n s by atoms and c r y s t a l s . I . A new t h e o r e t i c a l approach, A c t a Cryst . , 10: 609-619. E berh art , J.P. , T r i k i , R. , 1972. D e s c r i p t i o n d ' u n e t e c h n i q u e p e r m e t t a n t d ' o b t e n i r des coupes minces d e mineraux a r g i l e u x p a r u l t r a m i c r o t o m i e . J. Microsc. (F ) ,

15: 111-120.

Goodman, P., Moodie, A.F., 1974. Numerical e v a l u a t i o n o f N-beam wove f u n c t i o n i n e l e c t r o n s c a t t e r i n g by t h e m u l t i s l i c e method. Act a Cryst.A 30: 280-290. I i j i m a , S., Buseck, P.R., 1978. Experimental s t u d y o f d i s o r d e r e d mica s t r u c t u r e s by h i g h - r e s o l u t i o n e l e c t r o n microscopy. A c t a C r y s t . , A 34: 709-719. Jeffe rs o n, D.A., Thomas, J.M. , 1974. H i g h - r e s o l u t i o n e l e c t r o n m i c r o s c o p i c s t u d i e s o f s t r u c t u r a l f a u l t s i n l a y e r e d s i l i c a t e s . J.Chem.Soc.Faraday Trans.G.B.

70: 1691-1695.

J o u f f r e y , B., 1977. I n e l a s t i c s c a t t e r i n g and e l e c t r o n spect roscopy. P r o c . E l e c t r . Microsc.Soc.Amer. , 35: 692-695. J o u f f r e y , B., Do rig n a c , 0. , Tanaka, M., 1979. Atomic l e v e l e l e c t r o n microscopy as a f u n c t i o n o f a c c e l e r a t i n g v o l t a g e . I n (5), pp. 63-73. Labe rrig ue, A., B a l o s s i e r , G . , B e o r c h i a , A., Bonhomme, P., Bonnet, N., Trayon, M., 1980. T r a i t e m e n t d i r e c t en MET. D e c o n v o l u t i o n holographique a 1 ' a i d e de 1 ' e l e c t r o t i t u s . U t i l i s a t i o n d ' u n diaphragme de phase de t y p e e l e c t r o s t a t i q u e . J.Microsc.Spectrosc.Electron., 5: 651-660.

50

Lee, S.Y., Jackson, M.L., Brown, J.L., 1975a. Micaceous v e r m i c u l i t e , g l a u c o n i t e and mixed -1 ayered k a o l i n i t e - m o n t m o r i l l o n i t e e x a m i n a t i o n by u l t r a m i c r o t o m y and h i g h - r e s o l u t i o n e l e c t r o n microscopy. Proc.Soi1 Science Soc.Amer., 39: 793-800. Lee S.Y., Jackson, M.L., Brown, J.L., 1975b. Micaceous o c c l u s i o n s i n k a o l i n i t e observed by u l t r a - m i c r o t o m y and h i g h - r e s o l u t i o n e l e c t r o n m i c r o s c o p y . Clays, c l a y m i n e r . , 23: 125-129. Lynch, D.F., Moodie, A.F., O'Keefe, M.A., 1975. The u s e o f t h e c h a r g e - d e n s i t y a p p r o x i m a t i o n i n t h e i n t e r p r e t a t i o n o f l a t t i c e images. Acta C r y s t . , A31:300-307. O'Keefe, M.A., Buseck, P.R. , I i j i m a , S, 1978. Computed c r y s t a l s t r u c t u r e images f o r h i g h - r e s o l u t i o n e l e c t r o n microscopy. N a t u r e 274: 322-324. Rautureau, M., C l i n a r d , C., Tchoubar, C., 1974. M i s e en e v i d e n c e des canaux d e l a s e p i o l i t e p a r examen d e coupes u l t r a - m i n c e s en m i c r o s c o p i e e l e c t r o n i q u e a haute r e s o l u t i o n . J.App1 .Cryst.; 7: 293-294. Rautureau, M., M i f s u d , A., 1977. Etude p a r m i c r o s c o p i e e l e c t r o n i q u e des d i f f e r e n t s e t a t s d ' h y d r a t a t i o n de l a s e p i o l i t e . C l a y Min.Bul1 12: 309-318. Scherzer, 0. , 1949. The t h e o r e t i c a l r e s o l u t i o n o f t h e e l e c t r o n microscope. J.Appl.Phys., 20 : 20-29. Sudo, T., Yotsumoto, H., 1977. The f o r m a t i o n o f h a l l o y s i t e t u b e s f r o m s p h e r u l i t i c h a l l o y s i t e . Clays and c l a y miner., 25: 155-159. Tchoubar, C., Rautureau, M., C l i n a r d , C., Ragot, J.P., 1973. Technique d ' i n c l u s i o n a p p l i q u e e a 1 ' e t u d e des s i l i c a t e s l a m e l l a i r e s e t f i b r e u x . J.Microsc.(F), 18: 147-154. Thomas, J.M., J e f f e r s o n , D.A. , M a l l i n s o n , L.G., Smith, D.J., Crawford, E . S . , 1979. The e l u c i d a t i o n o f t h e u l t r a s t r u c t u r e o f s i l i c a t e m i n e r a l s by h i g h r e s o l u t i o n e l e c t r o n m i c r o s c o p y and X-ray e m i s s i o n m i c r o a n a l y s i s . I n ( 5 ) , pp.167-179. Van Dyck, D., 1980. F a s t c o m p u t a t i o n a l procedures f o r t h e s i m u l a t i o n o f s t r u c t u r e images i n complex o r d i s o r d e r e d c r y s t a l s : a new approach. J.Microsc. G.B., 119: 141-152. Yada K. , 1972. Study o f m i c r o s t r u c t u r e o f c h r y s o t i l e asbestos by h i g h r e s o l u t i o n microscopy. A c t a C r y s t . , A 27: 659-664. Yada, K., I i s c h i , K., 1977. Growth and m i c r o s t r u c t u r e o f s y n t h e t i c c h r y s o t i l e . Amer.Minera1. , 62: 958-65. Yoshida, T., 1973. Elementary l a y e r s i n t h e i n t e r s t r a t i f i e d c l a y m i n e r a l s as r e v e a l e d by e l e c t r o n microscopy. Clays and C l a y Min., 21: 413-420. Yoshida, T. , 1976. Study o f m i c r o s t r u c t u r e o f mica and m o n t m o r i l l o n i t e by h i g h r e s o l u t i o n e l e c t r o n microscopy. C l a y S c i . , 5 : l - 7 .

.,

51 Chapter 3 NEUTRON SCATTERING TECHNIQUES FOR THE STUDY OF CLAY MINERALS.

P e t e r L. HALL University of Birmingham, Edgbaston, Birmingham B 1 5 2TT, U.K. INTRODUCTION

3.1

3 . 1 . 1 General background Among t h e advanced methods forming t h e s u b j e c t of t h i s symposium, neutron s c a t t e r i n g is perhaps one of t h e l e a s t f a m i l i a r t o e a r t h s c i e n t i s t s , Although t h e f i r s t reported a p p l i c a t i o n of t h e technique t o c l a y mineralogy d a t e s back 15 years (Naumann e t a l . , 1966), t h e t o t a l number of p u b l i c a t i o n s i n t h e f i e l d t o d a t e remains f a i r l y small. This may be a t t r i b u t e d p r i n c i p a l I y t o two f a c t o r s . F i r s t l y , t h e experimental f a c i l i t i e s a r e a v a i l a b l e a t only a few l o c a t i o n s , normally a t research r e a c t o r s o r p a r t i c l e a c c e l e r a t o r s . Secondly, t h e development of purpose-huilt neutron s c a t t e r i n g c e n t r e s having s u p e r i o r neutron f l u x e s , improved instrument design and d a t a handling f a c i l i t i e s is a r e l a t i v e l y recent occurrence. These c e n t r e s , p a r t i c u l a r l y t h e high-flux r e a c t o r s a t t h e Brookhaven and Oak Ridge Laboratories i n t h e USA and a t t h e I n s t i t u t Laue Langevin i n Grenoble, France, and a l s o t h e pulsed neutron sources a t Argonne and Los Alamos in t h e USA (and under c o n s t r u c t f o n a t t h e Rutherford Laboratory i n England), i l l u s t r a t e t h e continuing e v o l u t i o n of t h e technique. The widening range of i t s f i e l d s of a p p l i c a t i o n during t h e p a s t few years make t h i s an a p p r o p r i a t e time t o review t h e progress i n c l a y r e s e a r c h using neutron s c a t t e r i n g methods. Section 1.2 o u t l i n e s t h e b a s i c p r o p e r t i e s which make thermal neutrons a u s e f u l probe of t h e microscopic p r o p e r t i e s of condensed matter. Section 2 d i s c u s s e s b r i e f l y t h e b a s i c p r i n c i p l e s of t h e technique and t h e types of experimental measurement which may be made. Section 3 d i s c u s s e s i n g r e a t e r d e t a i l t h e g e n e r a l n a t u r e of q u a s i - e l a s t i c s c a t t e r i n g s p e c t r a and t h e i r dependence on t h e d e t a i l s of t h e molecular d i f f u s i o n processes, and reviews t h e a p p l i c a t i o n of t h e incoherent q u a s i - e l a s t i c s c a t t e r i n g method t o s t u d i e s of the motions of water a s s o c i a t e d with expanding l a t t i c e c l a y s . Section 4 then b r i e f l y d e s c r i b e s t h e progress made by coherent s c a t t e r i n g methods ( d i f f r a c t i o n and small a n g l e s c a t t e r i n g ) i n studying s t r u c t u r a l a s p e c t s of c l a y minerals and t h e i r i n t e r c a l a t i o n complexes.

No d e t a i l e d treatment of t h e theory of t h e s u b j e c t is attempted h e r e f o r reasons of space. The r e a d e r is r e f e r r e d i n s t e a d t o a number of books and review a r t i c l e s which cover t h i s ground (Turchin, 1965; Marshall and Lovesey, 1971; W i l l i s , 1973; Lovesey and Springer, 1977 and Squires 1978). P a r t i c u l a r a s p e c t s of t h e s u b j e c t a r e a l s o well-documented, e.g. d i f f r a c t i o n (Bacon, 1975); q u a s i - e l a s t i c s c a t t e r i n g (Springer, 1972, 1977) and small angle s c a t t e r i n g (Schmatz e t a l . , 1974; Kostorz, 1979). A previous review of t h e a p p l i c a t i o n of neutron s c a t t e r i n g t o c l a y research a l s o c o n t a i n s t h e general theory of t h e s u b j e c t (Ross and Hall, 1980). 3 . i .2 The use of neutrons a s a spectroscopic probe I n common with electromagnetic r a d i a t i o n and e l e c t r o n s , neutrons e x h i b i t wave p r o p e r t i e s and t h e a s s o c i a t e d d i f f r a c t i o n and i n t e r f e r e n c e phenomena. A neutron of momentum E has a d e Broglie wavelength h

h

I !

mv

A=-=-

and an energy

E =

-

P2

hk

2m

2m

- =(-I

52

where m and v are the neutron's mass and velocity, k is the magnitude of the wavevector (2n/X) and I( is Dirac's constant. 'Ihe neutron has a halfintegrerspin and an associated magnetic moment. Neutrons produced in a reactor or particles accelerator initially have relatively large energies which subsequently decrease rapidly through collisions with light nuclei in a suitable moderating material (A20, D20 or graphite). When the neutrons reach thermal equilibrium with the moderator, their e n e r w spectrum is a is Maxwellfan function whose peak flux occurs at an energy of %T, where Boltzmann's constant and T is the moderator temperature. F i g . 3 . 1 illustrates flux distributions for moderator temperatures of 20, 300 and 2000OK. For a moderator at ambient temperature, the maximum flux corresponds to an energy For hot and cold moderators of approximately 0.038 eV (wavelengthWO.15 nm). the peak flux shifts to correspon,dinglyhipher or lower energies respectively, e.g. a 200K cold source (typically liquid hydrogen or deuterium) yields a peak flux at approximately 0.003 eV (wavelength 0 . 5 nm).

5

T 300 K

0

5

10

Velocity, k m s

15

-1

+

Fig. 3.1 Neutron f Z u distributions for moderator temperatures of 20, 300 and ZUU@'K.

Both "thermal" and "cold" neutrons are commonly used in neutron scattering experiments on condensed matter. They are unique probes of structural and dynamic properties, excitations and interatomic forces for two principal reasons. Firstly, their wavelengths are comparable to interatomic separations, and secondly their energies correspond to those characteristic of vibrational modes of crystals and molecular solids and liquids. This dual comparability may be contrasted with electromagnetic radiation such as Xrays (satisfying only the first condition) and infra-red radiation (satisfying only the second). Another essential difference between neutrons and electromagnetic radiation lies in the fact that, except in the case of magnetic materials, neutrons are scattered by nuclei and not electrons. The nature of the neutron-nucleus interaction depends on the relative orientations of the neutronic and nuclear spins. As will be shown in the next section, this implies the existence of a randomly varying component of the scattered wave amplitude in addition to a uniformly varying component.

53 The l a t t e r alone is r e s p o n s i b l e f o r i n t e r f e r e n c e phenomena l e a d i n g t o coherent d i f f r a c t i o n of neutrons ( p r e c i s e l y analogous t o X-ray d i f f r a c t i o n ) . The e x i s t e n c e of two s c a t t e r e d components, among o t h e r reasons, l e a d s t o a number of d i s t i n c t experiments o r types of measurement which can he made with neutrons. These w i l l be discussed i n t h e next s e c t i o n .

3.2 3.2.1

BASIC

PRINCIPLES OF NEUTRON SCATTERING

Types of scattering experiment

A s s t a t e d above, as a consequence of t h e e x i s t e n c e of t h e s p i n magnetic moment of t h e neutron, t h e s c a t t e r i n g process is s e n s i t i v e t o t h e r e l a t i v e d i r e c t i o n s of t h e s p i n s of t h e n u c l e i and t h e neutron. I n general t h i s r e s u l t s i n a v a r i a t i o n from atom t o atom of t h e amplitude of t h e s c a t t e r e d wave, not only f o r d i f f e r e n t i s o t o p e s of t h e same element, b u t a l s o f o r Roth i s o t o p i c and p a r a l l e l and a n t i p a r a l l e l s p i n s t a t e s of a given isotope. s p i n d i f f e r e n c e s t h u s c o n t r i b u t e t o an i n c o h e r e n t l y s c a t t e r e d component, i n addition t o t h e coherently s c a t t e r e d component a r i s i n g from i s o t o p i c and s p i n uniformity ( i n g e n e r a l , zero nuclear s p i n ) . The majority of m a t e r i a l s , including those having i d e a l , s t o i c h i o m e t r i c c r y s t a l s t r u c t u r e s , t h e r e f o r e In c o n t r a s t , X-rays e x h i b i t only give r i s e t o both types of s c a t t e r i n g . coherent s c a t t e r i n g from p e r f e c t c r y s t a l s , incoherent o r d i f f u s e s c a t t e r i n g being a consequence of c r y s t a l l i n e imperfections o r s t r u c t u r a l d i s o r d e r ( a l l o y s , amorphous s o l i d s ) . It follows t h a t t h e r e l a t i v e i n t e n s i t i e s of neutron coherent and incoherent s c a t t e r i n g can be varied by a l t e r i n g t h e i s o t o p i c composition of a A technique of some importance i n neutron s c a t t e r i n g experiments, material. p a r t i c u l a r l y d i f f r a c t i o n and small-angle s c a t t e r i n g measurements ( s e e Section 4 ) , is s e l e c t i v e o r complete d e u t e r a t i o n of hydrogenous m a t e r i a l s , f o r reasons which w i l l be explained below. The two components of t h e s c a t t e r i n g provide d i s t i n c t information: only the coherent component can produce i n t e r f e r e n c e phenomena such a s Bragg The incoherent component cannot c o n t a i n such information, diffraction. although under a p p r o p r i a t e c o n d i t i o n s i t can provide information concerning v i b r a t i o n a l and o t h e r dynamic p r o p e r t i e s of m a t e r i a l s , a s discussed l a t e r . A t t h i s p o i n t , a f u r t h e r d i s t i n c t i o n between t h e various types of This d i s t i n c t i o n , independent of t h e coherent o r s c a t t e r i n g must be made. We incoherent n a t u r e of t h e s c a t t e r i n g , is on t h e b a s i s of energy t r a n s f e r . therefore distinguish (no energy t r a n s f e r ) from i n e l a s t i c s c a t t e r i n g (one o r more energy quanta exchanged between neutron and s c a t t e r i n g m a t e r i a l ) . The term "neutron s c a t t e r i n g " t h e r e f o r e covers four c a t e g o r i e s of measurement, which may be summarized as follows:( i ) Coherent, e l a s t i c s c a t t e r i n g , i.e.

neutron d i f f r a c t i o n (ND).

(ii)

Coherent, i n e l a s t i c s c a t t e r i n g , abbreviated INS.

(iii)

Incoherent, e l a s t i c s c a t t e r i n g , abbreviated ENS.

(iv)

Incoherent, i n e l a s t i c s c a t t e r i n g (IINS).

Before summarizing t h e information provided by each experiment, t h e c l a s s i f i c a t i o n must be widened s l i g h t l y . In t h e f i r s t category, i n a d d i t i o n t o Bragg d l f f r a c t i o n , m u s t be included coherent low-angle s c a t t e r i n g due t o microscopic inhomogenities such a s pores o r c o l l o i d a l p a r t i c l e s . This is t h e small-angle neutron s c a t t e r i n g (SAYS) technique. F i n a l l y , in t h e t h i r d

54 category (ENS), there must be included a closely related phenomena where the scattering is not quite elastic due to relative motions between scattered neutrons and nuclei which are mobile on the appropriate time-scale (lo-'' seconds). The small energy changes (essentially a neutron Mppler effect) contain information regarding the motions of such nuclei. The technique is called quasi-elastic neutron scattering (QENS) because the energy changes are extremely small, of the order of micro electron-volts, i.e. a small percentage of the incident energies of the neutrons. QENS provides information complementary to pulsed NMR, although on a somewhat shorter time-scale. The information yielded by neutron scattering methods may therefore be summarised as follows:ND: Crystal structure,:including the positions of hydrogen atoms, structures of liquids and amorphous materials and magnetic structures. SANS: Inhomogenities such as pore sizes, particle sizes in colloidal materials, lattice defects etc.

INS: Collective excitations,.such as phonon dispersion relations, i.e. dependence of phonon energies on direction of propagation within the crystal (Squires, 1978; Ross and Hall, 1980). ENS + QENS: Study of vibrational, reorientational and diffusive motions of molecules and groups. IINS:

the energy spectrum of excitations (e.g. phonon density of states). Similar to the information provided by infrared and Raman spectroscopy but (1) heavily weighted towards hydrogen-containing groups due to the large incoherent cross-section of the proton (see below), and (ii) independent of the selection rules governing the coupling of vibrational excitations with electromagnetic radiation. The essential principles of the more important of the above types of neutron experiment will be discussed in section 2 . 3 . The next section gives a brief definition of coherent and incoherent scattering lengths and cross-sections. 3.2.2

Coherent and incoherent scattering lengths and cross-sections

The theory of the scattering of neutrons from individual nuclei and sets of fixed nuclei has been treated in detail elsewhere (e.8. Ross and Hall, 1980). Here a simple treatment leading to the definition of the scattering lengths and cross-sections is given, following "urchin (1965). Let the scattering nucleus have spin I. The "compound nucleus" existing momentarily during the scattering event therefore has a spin of J+ I + 1 / 2 or JI 1/2 in the parallel and antiparallel cases, having respective probabilities p+ = (I + 1)/(2I + 1) and p- = I/(2I + 1). By defining appropriate quantum mechanical operators, and utilizing the Schradinger equation for the neutron-nucleus system, it can be shown that the wave function consists of two terms: (a) a plane wave representing the incident neutrons, and (b) a spherical wave representing the scattered neutrons (see Fig. 3 . 2 ) . The scattered wave intensity is defined by a scattering amplitude or scattering length b, which is related to the above quantities by the expression

-

where

-

-

r and 2 are the spin operators for the nucleus and neutron

5 5

respectively and b+ and b- are the scattering lengths corresponding to the parallel and antiparallel spin orientations. The first two terms of the above equation are spin independent, and define f o r the nucleus a coherent scattering length:

I

I + 1

bcoh =

b+ + - b21 + 1 21 + 1

The corresponding coherent scattering cross-section i s 0coh =

(3)

4nbcoh2

The third term, when averaged over all possible spin directors, gives the incoherent component. From quantum mechanics the average value of (I.2)’ is equal to 1(1+1)/4. The incoherent scattering length is then given by the expression

and the corresponding incoherent scattering cross-section by 0

inc

=

4n

(b+

-

b-1’

(21

+

1>*

I(1

+

1)

(5)

e i k‘r-r’l

Scatted

beam

k

Fig.3.2 Wave representation of neutron scattering. The incident plane uave i s scattered by t h e isotropic potential due t o the p a r t i c l e a t P’, t h e scattered wave having spherical symmetry. The relative values of ocoh and Oinc vary randomly between nuclei with increasing mass number, depending on the relative magnitudes and signs of b+ and b-. This contrasts strongly with X-ray scattering amplitudes, which increase smoothly with the number of electrons per atom and are identical for different isotopes of the same element. For most isotopes, the coherent neutron scattering cross-sections are comparable with or larger than, the incoherent cross-sections. Two notable exceptions, however, are hydrogen ( l H ) and vanadium (51V) in which spin interactions

5 6

cause b and b- to be of opposite sign and of relative mapnitudes such that ucoh,.<sinc (Ross and Hall, 1980). Thus in structural work, it is useful to replace H by D, since for the latter 0coh‘

‘inc’

Vanadium is

conventionally used as a standard incoherent scatterer for calibration purposes in incoherent scattering experiments. Values of scattering crosssections for a few common isotopes and elements are listed in Table3.1. TABLE 3.1

Neutron scattering cross sections for common isotopes and elements

Isotope or element

Spin

(x 10-28m2)

“coh

1H

lI2

2H

1 0

1 2C

14N 160 Mg 27611

1 0 5i2

5 1v

?I2

0

si

0

Fe 3.2.3

Cross Sections

0

“inc

1.76 5.6

79.7 2 .o 0 0.3 0 0.36 0.01 0.03 4.8 0.44

5.517

11.1 4.24 3.34 1.49 2.17 0.033 11.36

Basis of experimental methods

The basis of the neutron scattering experiment is to prepare an initial state, 1.e. a monochromatic and collimated beam characterised hy an energy After scattering from the sample one detects a final E and wavevector k seats characterisepby an energy E/ and wavevector k/ at a scattering angle (This definition preserves 0 as the Bragg angle). The detector may of 28. or may not perform an energy analysis of the scattered beam.

.

Neutron scattering processes can most easily be represented as vector diagrams in reciprocal (momentum) space, since momentum = 1( x wavevector. Thus coherent Bragg scattering is illustrated in F i g . 3a, where lf, and k/ k / is the scattering are the incident and scattered wavevectors and 9 = k vector or wavevector transfer, corresponding to a msent& transfer KQ. Fig.3.3b illustrates a schematic view of a simple neutron diffractometer. Bragg diffraction i s only possible if 9 coincides with some reciprocal lattice vector of length 2 ~ r l d , ~ ~This condition is equivalent to Bragg’s law in momentum space. Since the process is elastic, E’ = Eo and therefore

-

.

62

- (k” - ko2)

= 0

2m

so that k/ = ko = k say, From Fig. 3.3a it therefore follows that, for elastic scattering,

4n

-sin8

(71 A Fig.3.3~ shows the reciprocal space diagram for coherent inelastic Q = 2k s i n 8 =

57

k/

scattering. Here the d i f f e r e n c e between k and does not simply correspond t o a reciprocal l a t t i c e vectorTxcept when the wave vector of a c o l l e c t i v e e x c i t a t i o n ( e . g . a l a t t i c e phonon) i s incorporated. Here

-0

D

/’

=YM C

L

Ch.

F i g . 3 . 3 Reciprocal space s c a t t e r i n g diagrams and schematic experimental laoyouts. ( a ) and ( b ) : Coherent e l a s t i c s c a t t e r i n g and t h e neutron diffractornefier. (el and ( d ) : Coherent i n e l a s t i c s c a t t e r i n g and t h e t r i p l e

a x i s spectrometer. ( e l and (fl: Incoherent i n e l a s t i c s c a t t e r i n g and t h e t i m e - o f - f l i g h t spectrometer. C[ = collimator, M = monochromator, A = analyser c r y s t a l , D = d e t e c t o r , S = sample, Ch. = neutron chopper.]

5 8 where l'fy is t h e phonon momentum and Q =

hka

The corresponding schematic diagram (Fig.3.3d)corresponds t o t h e t r i p l e a x i s spectrometer. F i n a l l y , E'ig.3.3e r e p r e s e n t s t h e v e c t o r diagram f o r incoherent i n e l a s t i c s c a t t e r i n g , where k and k' d i f f e r i n magnitude, corresponding t o Fig. 3 . 3 f , which g i v e s t h e scT-tic o u t l i n e of t h e time-of-flight spectrometer, i n which t h e f i n a l energy E is measured by timing t h e f l i g h t of t h e pulsed In t h i s experiment i n c i d e n t beam between t h e chopper Ch. and t h e d e t e c t o r . both energy and momentum t r a n s f e r s occur, r e s p e c t i v e l y given by t h e expresions and

E s s e n t i a l l y , e l a s t i c s c a t t e r i n g experiments measure an i n t e n s i t y p r o p o r t i o n a l t o do/dQ where 0 is t h e a p p r o p r i a t e (coherent o r incoherent) c r o s s s e c t i o n . I n e l a s t i c experiments measure t h e douhle d i f f e r e n t i a l c r o s s s e c t i o n d20/dME' where d Q i s t h e s o l i d angle subtended by t h e d e t e c t o r a t t h e s a m p l e and I n the d i f f r a c t i o n dE' is t h e energy range analysed by t h e d e t e c t o r . experiment t h e s c a t t e r e d i n t e n s i t y i s p r o p o r t i o n a l t o t h e sauare of t h e ) o r S ( Q ) , which is t h e Fourier transform of s t a t i c structure factor, F(T +kQ t h e neutron s c a t t e r i n g d e n s i t y d i s t r i b u t i o n i n t h e c r y s t a l (Racon, 1975). In the case of i n e l a s t i c s c a t t e r i n g t h e double d i f f e r e n t i a l c r o s s s e c t i o n is p r o p o r t i o n a l t o a q u a n t i t y c a l l e d t h e incoherent ( o r coherent) s c a t t e r i n g These f u n c t i o n s d e s c r i b e w) o r Sc ,(q, o) r e s p e c t i v e l y . function S t h e d i s t r i & i o n of momentum and energy t r a n s f e r s among t h e s c a t t e r e d neutrons, and c o n t a i n information regarding t h e v i b r a t i o n a l o r d i f f u s i v e motions of atoms i n t h e s c a t t e r i n g m a t e r i a l (Van Hove, 1954). A b r i e f d e s c r i p t i o n of simple neutron d i f f r a c t o m e t e r s and i n e l a s t i c s c a t t e r i n g spectrometers has been given elsewhere (Ross and H a l l , 1980).

(A,

3.3 3.3.1

INCOHERENT NEUTRON SCATTERING

-

STUDY OF DYNAMICS OF ADSORBED MOLECULES

P r i n c i p l e s of t h e q u a s i - e l a s t i c s c a t t e r i n g (QENS) method.

(i) Van Hove s e l f - c o r r e l a t i o n function. In t h e QENS experiment, neutrons s c a t t e r e d incoherentlv i n t o a d e t e c t o r Dositionen a t a eiven a n e l e t o t h e i n c i d e n t beam a r e energy analysed (by r e f l e c t i o n from an a n a l y s e r c r y s t a l o r by time-of f l i g h t methods), t h e spectrum from each d e t e c t o r c o n s i s t i n g of t h e i n t e n s i t y of s c a t t e r e d neutrons versus energy ( o r time-of-flight). Apart from i n e l a s t i c processes, t h e s p e c t r a a r e u s u a l l y dominated by a l a r g e peak from e l a s t i c a l l y s c a t t e r e d neutrons. Neutrons s c a t t e r e d from a r i g i d l a t t i c e e x h i b i t an e l a s t i c peak whose width i n energy d e f i n e s t h e instrumental r e s o l u t i o n which depends on both design f a c t o r s and t h e i n c i d e n t neutron wavelength. This peak is r e f e r r e d t o a s t h e r e s o l u t i o n f u n c t i o n (Fig.3.4a). It is measured using a vanadium s h e e t a s a s t a n d a r d incoherent s c a t t e r e r . I n c o n t r a s t , when s c a t t e r i n g t a k e s p l a c e from n u c l e j which a r e d i f f u s i n g o r r e o r i e n t a t i n g on a time-scale comparable with t h e seconds) t h e e l a s t i c r e s o l u t i o n f u n c t i o n i n t e r a c t i o n time (lo-'' becomes broadened in energy because of t h e r e l a t i v e motions between t h e neutrons and t h e mobile n u c l e i (Fig.3.4b). This phenomenon is termed

5 9

q u a s i - e l a s t i c b r o a d e n i n p , and t h e dependence of t h e q u a s i - e l a s t i c l i n e widths and i n t e n s i t i e s on a n g l e ( i . e . momentum t r a n s f e r ) c o n t a i n s i n f o r m a t i o n concerning t h e s p a c e and time-dependence of t h e m o l e c u l a r motions. A f t e r c o r r e c t i o n s f o r a b s o r p t i o n , r e s o l u t i o n and v a r i o u s o t h e r i n s t r u m e n t a l e f f e c t s , d i s c u s s e d i n d e t a i l by H a l l e t a l . ( 1 9 7 9 a ) , t h e s p e c t r a p r o v i d e measurements of t h e double d i f f e r e n t i a l c r o s s - s e c t i o n d i s c u s s e d above i n section 2.3. T h i s q u a n t i t y can be r e l a t e d t o t h e i n c o h e r e n t s c a t t e r i n g function, S ( Q ,A) which d e s c r i b e s t h e d i s t r i b u t i o n of e n e r g y and momentum i n c -’ t r a n s f e r s i n t h e s c a t t e r i n g process. Van Hove (1954) showed t h a t f o r systems which can be d e s c r i b e d c l a s s i c a l l y ( t 3 s e c o n d s ) , Sinc(Q’ W ) i s r e l a t e d hy a d o u b l e (5.e. s p a c e and t i m e ) F o u r i e r t r a n s f o r m t o t h e s e l f - c o r r e l a t i o n f u n c t i o n , G - (r, t ) , d e f i n e d a s t h e p r o h a h i l i t y t h a t a n atom i n i t i a l l y a t t h e o r i g i n ” i s l a t e r found a t p o s i t i o n f a t time t. This connection i s e x p r e s s e d by t h e e q u a t i o n

I n p r i n c i p l e one could a p p l y t h e r e v e r s e t r a n s f o r m t o c a l c u l a t e G ( r t ) d i r e c t l y from t h e observed d a t a . Tn p r a c t i c e , however, i n s u f f i c i e n t EoTits w) s p a c e a r e e x p e r i m e n t a l l y a c c e s s i b l e , so t h a t i t i s i n g e n e r a l in n e c e s s a r y t o c a l c u l a t e t h e form of G ( r t ) f o r some assumed model of t h e molecular motions i n o r d e r t o o b t a i n s a - ; h e o r e t i c a l scattering function for comparison with e x p e r i m e n t .

(a,

( i i ) Behaviour of t h e c o r r e l a t i o n f u n c t i o n a t l o n g times. The n a t u r e of t h e s c a t t e r i n g a t n e a r - e l a s t i c e n e r g i e s ( W W 0) i s a s s o c i a t e d w i t h t h e hehaviour of t h e s e l f - c o r r e l a t i o n f u n c t i o n a t long times ( i n t h i s c o n t e x t , much g r e a t e r t h a n t h e i n t e r a c t i o n time ( ~ p i1c o s e c o n d ) ) . For unbounded o r macroscopic t r a n s l a t i o n a l d i f f u s i o n t h e p r o h a b i l i t y of a n atom r e t u r n i n g t o i t s o r i g i n a l p o s i t i o n decays with t i m e and become i n f i n i t e s i m a l , so t h a t G s ( ~ ,t ) tends t o zero a t t + I n c o n t r a s t , f o r r o t a t i o n a l motions ( o r s p a t i a l l y bounded t r a n s l a t i o n s ) t h e p r o b a b i l i t y of a r e t u r n t o t h e o r i g i n a l t ) does n o t v a n i s h a t l o n g times. In p o s i t i o n remains f i n i t e , and G t h e s e c a s e s t h e c o r r e l a t i o n f u z c t i o n may be d i v i d e d i n t o time-dependent and time-independent components:

=.

(z,

On s u b s t i t u t i n g t h i s e x p r e s s i o n i n t o e q u a t i o n (11) i t may be shown t h a t t h e corresponding s c a t t e r i n g f u n c t i o n a l s o c o n t a i n s two t e r m s , one c o r r e s p o n d i n g t o f i n i t e energy t r a n s f e r ( i . e . q u a s i - e l a s t i c s c a t t e r i n g ) and one, a r i s i n g In general, from t h e c o n s t a n t term i n G ( r t ) , which i s p u r e l y e l a s t i c .

s

-’

t h e r e l a t i v e a m p l i t u d e s of t h e e l a s t i c and a u a s i - e l a s t i c terms a r e f u n c t i o n , of a d i m e n s i o n l e s s v a r i a b l e Q a , where a i s a c h a r a c t e r i s t i c dimensions of t h e molecular system, e.g. t h e r a d i u s of a r o t a t i o n a l motion. (For example, f o r a water molecule undergoing C 2 r o t a t i o n s , t h e r o t a t i o n a l r a d i u s of t h e p r o t o n s i s a p p r o x i m a t e l y a = 0.758). In t h i s case t h e s c a t t e r i n g function may be r e p r e s e n t e d by t h e e x p r e s s i o n

Qua s j - e l a s t i c Elastic where A o ( Q a ) , t h e a m p l i t u d e of t h e e l a s t i c t e r m , is known a s t h e g l a s t i c . i n c o h e r e n t s t r u c t u r e f a c t o r ( E I S F L , which may h e r e g a r d e d as t h e F o u r i e r t r a n s f o r m of t h e f i n a l d i s t r i b u t i o n of d i f f u s i n g atoms. The spectrum t h u s

6 0 consists of a sharp line, represented by the delta function, plus a broadened (commonly a Lorentzian or sum of component corresponding to a function L(w), Lorentzians), of relative amplitudes A (Qa) and A 1 (Qa) respectively. The idealized situation i s illustrated in pig. 3.4~.Thg requirement that the scattering function is normalized implies that, with the correct definition of L(w), A 1 (Qa) = 1 - A. (Qa).

In practice, because of the additional broadening due to the finite instrument resolution, a separation as clear as that illustrated in Fig.3.4~ is rarely observed. In practice the situation i s more commonly like that illustrated in Fig.3.4d. ( I n fact the gradient change may be even less pronounced). Separation of elastic and quasi-elastic amplitudes then requires computational line-profile fitting methods, as discussed in detail elsewhere (e.g. Hall et al., 1979a). 'Ihe parameters extracted by such fitting methods include (i) the width of the quasi-elastic line, or the related energy AE = hAw and (if) the relative amplitudes A and A 1 . By collecting data at many scattering angles the dependence oP Aw and A on Q i s determined. A plot of Aw (or AE) versus Q2 is known as a quasicelastic broadening curve and provides information concerning the time-dependence of the diffusive process (i.e. a diffusion coefficient or rotational correlation time). A typical broadening curve is illustrated j n Fig.3.5a; t h e gradient of the curve at law Q may be related to the diffusion coefficient (see section 3 . 2 ) . A typical form for .A (Qa) is illustrated in Fig.3.5b; invariably A (0) = 1, i.e. the spectrum is purely elastic at 0 = 0. The rate of deca? of the EISF with Qa gives a measure of the parameter a, the distance scale of the bounded motion. The next section discnsses the Q-dependence of the quasi-elastic broadening and the EISF for various specific rotational and translatisnal diffusion models. F i g . 3.4 Typical incoherent

a

C

b

d

e l a s t i c arld q u a s i - e l a s t i c p r a k shapes. ( a ) E l a s t i c scntfrrq'nn onlu. ( b ) Puasielastically broadened Zinc. (el E l a s t i c and auasie l a s t i c components-idealised case. ( d l E l a s t i c and q u a s i - e l a s t i c componmts w i t h m s o l u t i o n broadening.

6 1

r

QZ

+

Qa

F i g . 3 . 5 T y p i c a l forms of:

(a) t h e quasi-elastic broadening curve, and (bl t h e eZastic incoherent s t r u c t u r e factor for spatiaZZy bounded motion.

3.3.2

Scattering function for translational and rotational diffusion models

In this section a number of models for the incoherent scattering function, leading to certain characteristic forms of the quasi-elastic spectra, are discussed. These correspond to a variety of simple diffusion models, the examples selected being those which have relevance to the possible motions of small adsoibed molecules, e.g. water in layer silicates. Only the general features of the results are given here together with references to the original papers where the theoretical details m y be found. The models which are discussed fall into three categories: (i) unbounded tranplational diffusion; (ii) spatially bounded translational diffusion; and (iii) reorientational motions. In addition, each type of motion may also be continuous (i.e. Fickian) or stochastic (i.e. rapid jumps between fixed sites). (i) Unbounded translational diffusion. In the case of continuous diffusion, Fick’s law predicts a correlation function of Gaussian shape: Gs

(fl,

t)

=

(4~Dt)-~” exp (-r2/4Dt)

(14)

whose space-time Fourier transformation (equation 11) yields a Lorentzianshaped scattering function

where D is the diffusion coefficient. Thus the Lorentzian HWHM is DQ’, and the quasi-elastic broadening curve is linear on a Q2 scale and of gradient D The model was derived by Vineyard (1958). (see curve c of Fig.3.6).

6 2 It has, however, been found by experiment t h a t t h e d i f f u s i v e processes which take place i n most m a t e r i a l s d e v i a t e from t h e p r e d i c t i o n s of t h e continuous d i f f u s i o n model above a c e r t a i n value of Q (1.e. when t h e measurement samples small d i s t a n c e s and times), and a r e more a c c u r a t e l y described by jump models, e s p e c i a l l y of rapid jumps between s i t e s a t which When t h e the d i f f u s i n g moecules remain f o r some mean residence time T. quasi-equilibrium s i t e s a r e d i s t r i b u t e d on a l a t t i c e of known geometry, r a t e equations i n c o r p o r a t i n g t h e known jump v e c t o r s may be solved t o c a l c u l a t e t h e s e l f - c o r r e l a t i o n f u n c t i o n (Springer, 1972). Alternatively random walk methods may be u t i l i z e d (Chandrasekhar, 1943). Assun.inp t h a t successive jumps a r e uncorrelated but descrihed by t h e same s p a t i a l d i s t r i b u t i o n p ( x ) , t h i s method p r e d i c t s a s c a t t e r i n g f u n c t i o n of t h e form

where F(Q) i s t h e Fourier transform of p ( r ) and r = l / ~ i.e. t h e mean A t high Q, where jump r a t e . This i s a Lorentzian of half-width r[l-F(Q)]. u s u a l l y F(Q) + 0, t h e q u a s i - e l a s t i c half-width tends t o an asymptotic The shape of t h e broadening curve depends on t h e form value .‘I of P(L). Fig.3.6 (curves a and b ) i l l u s t r a t e s c a l c u l a t e d broadening curves f o r two d i f f e r e n t forms of t h e d i s t r i b u t i o n of jump l e n g t h s ( S i n p i and Sjolander, 1960; H a l l and Ross, 1981). Both models d e v i a t e considerably from t h e p r e d i c t i o n s of t h e Vineyard model (curve c ) and tend t o asymptotic widths Au = r a t l a r g e Q.

( i i ) S p a t i a l l y r e s t r i c t e d t r a n s l a t i o n a l d i f f u s i o n . In many m a t e r i a l s t h e space a v a i l a b l e f o r d i f f u s i o n may be geometrically bounded, e.g. d i f f u s i o n For d i f f u s i o n may be confined t o a s u r f a c e o r r e s t r i c t e d t o a given volume. confined t o a plane s u r f a c e (freedom of motion in two dimensions), i t can be shown t h a t Sinc(Q, u) =

i.e.

1

n

B sin2 a

(B s i n 2 a l 2

+ w2

a Lorentzian of Hk7MI-I B s i n a, where a i s t h e angle between

0

and t h e

R = DQ2 s u r f a c e normal. For continuous two-dimensional d i f f u s i o n (Dianoux e t a l . , 1975) while f o r jumps of length a between s i t e s of mean residence time T , B = [ l - J o ( Q a ) ] / ~ where Jo i s t h e zeroth o r d e r Bessel function. (Stockmeyer, 1976). Both models p r e d i c t a n i s o t r o p i c q u a s i - e l a s t i c s c a t t e r i n g ; i f Q i s p a r a l l e l t o t h e plane Aw = 0, i.e. t h e s c a t t e r i n g i s purely e l a s t i c . As before, t h e jump motions a r e c h a r a c t e r i s e d by a maximum broadening a t high Q equal t o l / r ( o r H I T i n energy u n i t s ) . In a p o l y c r y s t a l l i n e m a t e r i a l t h e degree of anisotropy w i l l depend on t h e e x t e n t of p r e f e r r e d o r i e n t a t i o n . In a randomly o r i e n t a t e d specimen, t h e s c a t t e r i n g However, f u n c t i o n w i l l be i s o t r o p i c though not in g e n e r a l lorentzian-shaped. t h e best f i t l o r e n t z i a n i n t h i s case has w i d t h < B > = 2B/3, i.e. two-thirds of t h e in-plane component.

An a l t e r n a t i v e s i t u a t i o n occurs when t h e d i f f u s i n g molecules a r e bounded by impermeable s u r f a c e s . Here t h e a p p r o p r i a t e s o l u t i o n s derived from d i f f u s i o n equations o r random walk theory can be modified by assuming r e f l e c t i v e boundaries i n o r d e r t o o b t a i n t h e a p p r o p r i a t e form of t h e s e l f - c o r r e l a t i o n f u n c t i o n f o r restricted d i f f u s i o n . ‘Ihe following problems have been solved e x p l i c i t l y : (a) continuous d i f f u s i o n between p a r a l l e l boundaries ( H a l l and Ross, 1978); (b) random jump d i f f u s i o n between p a r a l l e l

63 boundaries ( H a l l and Ross, 1981); and ( c ) c o n t i n u o u s d i f f u s i o n w i t h i n a n impermeable s p h e r e (Volino and Dianoux, 1980). Each of t h e s e models c o n t a i n s a p u r e l y e l a s t i c component of t h e t y p e d i s c u s s e d i n s e c t i o n 3.1 which arises from t h e s p a t i a l l y r e s t r i c t e d c h a r a c t e r of t h e motion. The EISF is u n i t y a t Q = 0, and r e t a i n s a s i g n i f i c a n t magnitude up t o Q = 2n/R where R i s a c h a r a c t e r i s t i c l e n g t h , i.e. t h e i n t e r p l a n e s e p a r a t i o n or s p h e r e r a d i u s . A t h i g h e r Q ( i . e . small d i s t a n c e ) t h e measurements are n o t a f f e c t e d s i g n i f i c a n t l y by t h e e x i s t e n c e of t h e b o u n d a r i e s , and t h e models r e v e r t t o t h e behaviour of t h e analogous, unbounded d i f f u s i o n models ( H a l l and Ross, 1981). For both v e r s i o n s of t h e p a r a l l e l - b o u n d a r y d i f f u s i o n problem (i.e. t h e problem of d i f f u s i o n i n a n i n f i n i t e l y deep one-dimensional p o t e n t i a l w e l l ) , t h e s c a t t e r i n g f u n c t i o n i s of t h e form m

Sinc(Q9

w) =

.A

( 9 2 ) 6(w)

E l a s t i c term

+

L

n=1

An (QR) L (mn)

(18)

Q u a s i - e l a s t i c terms

The EISF i s given by Ao(QR) = S ~ ~ ~ ( Q R / ~ ) / ( Q = Rj o/ 2~ (QR/2) )~ where R i s t h e i n t e r p l a n a r s p a c i n g and j i s t h e s p h e r i c a l Bessel f u n c t i o n of o r d e r zero. The q u a s i - e l a s t i c componegt c o n s i s t s of a series of L o r e n t z i a n s of width Awn and amplitude An(QR). I n p r a c t i c e , t h e sum converges r a p i d l y because of t h e form of t h e g e n e r a l amplitudes. However, f o r most purposes t h e model may be s i m p l i f i e d by a d d i n g a l l t h e q u a s i - e l a s t i c terms t o p e t h e r t o o b t a i n a n average l i n e w i d t h < Au > whose amplitude w i l l be q u i t e c l o s e t o 1 - A (QE). The v a l u e ofCdw> w i l l , i n g e n e r a l , be Q-dependent, s i n c e t h e mixing of t h e component l i n e s i s weighted by t h e i r Q-dependent amplitudes. F i g u r e 7 i l l u s t r a t e s t h e dependence of d A w b on ( Q R ) 2 on a douhle l o g a r i t h m i c scale f o r t h e p a r a l l e l - b o u n d a r y models. A t low Q (01?<2n) t h e i s approximately *independent. At e l a s t i c component dominates and (Aw> Q11>2* t h e c o n t i n o u s d i f f u s i o n model ( a ) and t h e jump model (b) tend r e s p e c t i v e l y t o t h e p r e d i c t i o n s of t h e unbounded F i c k i a n d i f f u s i o n model (c) and t o t h a t of t h e r e l a t e d unbounded jump d i f f u s i o n model, i.e. t o widths Am = DQ2 and Am = r where, a s b e f o r e , r = l/r. The one-dimensional s o l u t i o n s c a n r e a d i l y be g e n e r a l i s e d t o t h r e e dimensions t o d e s c r i b e r e s t r i c t e d random motions i n any p a r a l l e l - s i d e d box. For example, f o r jumps of mean l e n g t h r w i t h i n a cube of s i d e R i t c a n show t h a t

where Qx, Qy,

and Q

are t h e components of Q a l o n g t h e s i d e s of t h e cube and

L (Aw ) d e n o t e s a L o r e n t z i a n of HWHM ij k

C l e a r l y Aw

000

-

0, i.e.

t h e r e i s s t i l l a n e l a s t i c component.

( i i i ) R e o r i e n t a t i o n a l motions. For a molecule o r f u n c t i o n a l proup undergoing f i x e d r o t a t i o n without any- centre-of-mass d i f f u s i o n , t h e o r y a g a i n p r e d i c t s a n e l a s t i c s c a t t e r i n g component s i n c e a n atom may r e t u r n t o its s t a r t i n g posi ti o n a t long t i m e s . Two important cases a r e t h e models f o r

6 4 jump r e o r i e n t a t i o n between N e q u i d i s t a n t s i t e s on a c i r c l e (Barnes, 1973), and t h e s p h e r i c a l r o t a t i o n a l d i f f u s i o n model o f S e a r s (1967) which h a v e been used t o model u n i a x i a l and i s o t r o p i c r o t a t i o n a l p r o c e s s e s r e s p e c t i v e l y . As i n t h e case of r e s t r i c t e d d i f f u s i o n above, t h e Q-dependence of t h e e l a s t i c i n t e n s i t y ( i . e . t h e a m p l i t u d e of t h e EI S F ) e n a b l e s t h e s p a t i a l dimensions of t h e motion, i n t h i s c a s e t h e r a d i u s of t h e r o t a t i o n a l motion, t o be determined. F u r t h e r d i s c u s s i o n s of r o t a t i o n a l d i f f u s i o n models, i n c l u d i n g t h o s e s u b j e c t t o p e r i o d f r p o t e n t i a l , are g i v e n elsewhere Dianoux e t a l . , 1975; Dlanoux and V n l i n n , 1977; Ross and Hall, 1980).

(a) Jump d i f f u s i o n w i t h ( b l Jwnp d i f f u s i o n w i t h guassian distribu?-ion of lengths (Hall and Ross, 1981). icl Continuous Fickian d i f f u s i o n . Curves ( a ) and ibl are calculated f o r mean square j m p Fig.3.6 Quasi-elastic p(rJ

=

broadening curves.

r e x p ( - r / r ) ( S i n - m i and Sjtilander, 19601.

l e n t h s 3.f

?.

1

of q u a s i - e l a s t i c broadenings f o r one-dimensionai! r e s t r i c t e d d i f f u s i o n models. ( a ) Continuous ( b l Jump d i f f u s i o n on segmmt of length d i f f u s i o n on segment of length 9,. I. (c) Fickian d i f f u s i o n . (d). and ( e l are t h e o r e t i c a l asymptotic values AW = 0,2112 and ~w = r r e s p e c t z v e l y .

F i g . 9.7 DoubZe logarithmic plot of v a r i a t i o n w i t h (QI)

65

1.0

X

0.5

3

Fig.3.8 QENS d a t a f o r Ca2' m o n t m o r i l l o n i t e t w o - l a y e r h y d r a t e ( p / p = 0.76) a t a n e n e r g y r e s o l u t i o n of 38 peV. S o l i d c u r v e s a r e s i m u l a t i o n s g a s e d on the t w o - p h a s e j u m p d i f f u s i o n model d e s c r i b e d i n t h e t e x t . -------AE.

_____ X .

1.0

A

4

u

.d .y

v1

id

4

w

0.4

0.2

Q2

(113

0.6

0. 8

Fig. 3.9 &ENS data for Ca2+ montmorilZonite two-layer hydrate at high energy ------- B E . . A ------- Elastic fraction. resolut
6 6 3.3.3 Quasi-elastic scattering studies of the mobility of water associated with expanding lattice clays

In this section the contribution made by incoherent quasi-elastic neutron scattering (QENS) to the characterization of the rapid diffusive motions of water associated with expandin lattice clays is reviewed. In and M g ' + -exchanged montmorillonite and particular, QENS studies of Ca2+ vermiculite are discussed. These measurements, reported in more detail elsewhere (Hall et al., 1978, 1979b, 1981a; Ross and Hall, 1980; Tuck, 1981) can be interpreted in terms of a simple physical model of the motions of the absorbed water molecules. The nature of the model is described here and the physical parameters which it yields are compared with the results obtained using other techniques sensitive to rapid single-particle motions.

-

The QENS data were recorded on two different instruments ( f ) a medium resolution "time-of-flight'' spectrometer, and (ii) a high resolution "backscattering" spectrometer. These instruments are respectively referred to as the IN5 and IN10 spectrometers at ILL, Grenoble, France, and are described elsewhere (ILL, 1977; Ross and Hall, 1980). Computational line-profile fitting methods permitted each set of spectra to be separated into elastic and quasi-elastic components, each broadened by the resolution function of the instrument. For each instrument, the data was reproducible within the limits of experimental error between different measurements on samples prepared under identical conditions. However for identical samples the data differed significantly and reproducihly between the two instruments. Figures 3.8 and 3.9 illustrate the Q-dependence of the quasi-elastic broadenings and the ratio of quasi-elastic to total (elastic + quasi-elastic) intensity for a two layer Ca2+ lnontmorillonite (p/p = 0.76, do01 = 15.5A) for the IN5 and IN10 data respectively. Because of the relatively large magnitude of the incoherent scattering cross-section of the proton, the major contribution to the scattered intensity arises from the water fraction, the lattice protons making a smaller though measurable contribution. Table 3 . 2 lists the relative scattering intensities (calculated from the incoherent cross-section listed in Table3.l)for the clay lattice, hydration-shell water molecules (assuming six-fold co-ordination) and non-hydration shell water molecules. The data

f

I/////

/?B

/

/

fo 0-

0

02

OL

O6

X N5

Fig.3.10 Correlation between experimental v a l u e s o f X = Iquasi-elastic 1 and the t h e o e r e t i c a l f r a c t i o n a l s c a t t e r i n g 'elastic 'quasi-elastic predicted f o r t h e non-hydration s h e l l water molecules. +

TABLE 3 - 2 R e l a t i v e incoherent s c a t t e r i n g i n t e n s i t i e s f o r c l a y , hydration-shell and non hydrations h e l l water f r a c t i o n s c a l c u l a t e d from c.e.c. and adsorption data. X values a r e experimental q u a s i - e l a s t i c s c a t t e r i n g f r a c t i o n s (high- asymptotic values).

No. of

Clay

Cation valency

c.e.c.

Na+ mont.

1

100

2

1

100

2

Ca+

mont.

Mg 2+ mnt. CaZ+ verm.

water layers

Total water (mg g-l)

F r a c t i o n a l Incoherent S c a t t e r i n g

Measured X

lattice

hydration s h e l l water

non-hydration s h e l l water

190

0.20

0.45

0.35

0.52

2

280

0.15

0.33

0.52

-

100

2

220

0.18

0.20

0.62

0.56

2

100

3

280

0.15

0.16

0.69

0.65

2

100

2

220

0.18

0.20

0.62

0.56

2

100

3

280

0.15

0.16

0.69

0.66

2

130

2

194

0.20

0.36

0.44

0.40

m

U

6 8

are given for various different values of cation valency, exchange capacity and number of interlamellar water layers. The right-hand column contains the asymptotic values (at high Q) of the parameter X, the quasi-elastic intensity fraction. The significance of these values lies in the fact that at high Q the intensity of the EISF due to a rotational (or restricted volume) diffusion process becomes zero. The residual elastic fraction at high Q, i.e. 1 - X, therefore represents the scattering from protons which are not mobile on the QENS timescale (i.e. protons having correlation times >> seconds). From the tabulated values, it can be seen that in all cases the measured elastic fraction is significantly higher than that due to the lattice protons alone. The intensity of the asymptotic elastic fraction in fact agrees quite well with the calculated intensity due to the sum of the lattice and co-ordinated (i.e. hydration shell) protons, i.e. only the non-hydration shell water appears to be mobile on the QENS timescale. Figure 3.10 illustrates the correlation between the measured values of X and the fractional cross-section of the non co-ordinated water components. The restricted mobility of the hydration-shell water fraction is attributed to the strong crystal fields in the vicinity of the divalent cations. This finding is in accordance with the low mobility of hydrated cations in smectites and vermiculites determined by tracer diffusion and NMR (Lai and Mortland 1968; Hougardy et al., 1976), which indicate correlation times of ca. 10-8'seconds for these motions. The different results obtained using the two different energy resolutions ruled out any of the simple diffusion models described in Section 3.2. In particular, two-dimensional diffusion parallel to the interlayer plane could be ruled out since the data showed no significant anisotropy for directions of Q parallel and perpendicular to the plane of oriented samples (Hall et al., 1978; Ross and Hall, 1980). The IN5 broadening curves flatten at high Q,indicating a departure from Fickian diffusion, and also exhibit a decrease in the elastic scattering intensity with increasing Q. These facts indicate a spatially bounded jump diffusion process. Attempts to fit the data to isotropic or uniaxial rotational motions about fixed oxygen positions were unsuccessful, since the required radii of rotation were much larger than any physically realistic values. The data could, however, be explained on the basis of a model containing two phases of motion, of which the faster (giving the larger broadenings) is a spatially bounded random motion incorporating some centre-of-mass diffusion of oxygen atoms. The physical interpretation of the model is as follows: the water molecules outside the cation hydration shells undergo rapid jumps within a "cage" bounded by the silicate surfaces and the hydrated cations, together with somewhat slower and longer-range motions which may be pictured as more occasional jumps between adjacent cages. The different spectra observed at the two different energy resolutions may then be qualitatively explained as follows. Suppose the slower motions (smaller broadenings) are represented by a Lorentzian linewidth AES, while the faster motions are represented by a larger linewidth AEF and and EISF Ao(QX), where L is a length related to the linear dimension of the "cage". Then assuminp that the twu phases of the motion are independent, the scattering function will be a convolution in frequency or energy of the functions representing the individual phases of the motion. Using the properties of the convolutions of Lorentzians, it follows that

narrow line where L(x)

broad line

is a Lorentzian of HWMH x, and the asterisk denotes the

69

convolution, defined by c(x)

= f(x)*g(x)

=

1 f(x/)g(x

- x')

dx'

(22)

The scattering function i s thus predicted to be the sum of narrow and broad components, whose relative amplitudes are governed by the EISF of the faster motions. The experimental observation will depend on the relative magnitude of the characteristic quasi-elastic linewidths and the energy resolutions of the spectrometers. Let these be designated AE and % for the high and medium energy resolution cases respectively. H In the case A E s d

AEH4dAEF,

the high resolution measurement will selectively

show the narrow line, while the broad component will tend to merge with the background (Fig. 3 . 1 la). In contrast, if AEF~AEJWEs, then the medium resolution measurement will emphasize the broad component, while the narrow line may not be resolved at all, contributing to the elastic scattering at this resolution (Fig.3.llb). Moreover, since the EISF decreases with increasing Q, equation (21) predicts that the relative intensity of the broad quasi-elastic component will increase with Q while the intensity of the narrow component (as seen o n the high resolution instrument) will decrease with Q, which agrees with the experimental evidence.

a

F i g . 3.11 E f f e c t of variable energy resolution. ( a ) Narrow l i n e observed, broad l i n e merges w i t h background. (bl Broad l i n e observed, narrow l i n e unresolved.

The data could be accurately simulated using this model after averaging over the measured particle orientation distribution (Tuck, 1981, Hall et al., 1981a). For the two layer Ca2+ -montmorilloni;e, this simulation ielded mean jump lengths and residence times of rs = 1.7 A and T = 1.0 x seconds and TII = 7.0 x 10-l' seconds :or for the short range motions and rE = 5.0 the longer range motions, the cage having a diameter in the range 5 - 6 A . (The data for polycrystalline materials are not particularly sensitive to the shape of the cage, the value representing only a mean "pore" dimension). The cage size agrees quite well with the average spaces between the cation hydration shells estimated by subtracting from the average inter-cation separations (calculated from the c.e.c. values) the approximate size of the sixfold co-ordination shell of the divalent ions (Mathieson and Walker, 1954). For the two-layer Ca2+ vermiculite, the data required both correlation times to be ca. 50% longer than for the montmorillonite, accounted for by the more restricted intra- and inter-cage environments due

70 to the more closely packed cations in the more highly charged clay. For the montmorillonite, the simulations are represented by the solid lines in Fig.3.8. The agreement with the high-resolution data is also very good, except that the quasi-elastic intensities are a little larger in this case than the values predicted by the model. This may be due to an additional contribution to the scattering seen at this resolution, e . g . restricted motions of some of the co-ordinated water, or perhaps a contrihution from a proton "hopping" mechanism, as discussed by Fripiat and Stone (1978). It is difficult to assess the importance of this effect, since QENS measurements see only the motions of individual atoms, and cannot distinguish between H20 and H+ or H30+. According to the NMR data of Touillaw et al. (1968) and the conductivity measurements of Fripiat et al. (1965) the mean association time of the free proton with a given molecule is 5 x seconds and the degree of dissociation at the clay surface is.l%. Since the entire proton population contributes to incoherent scattering, and only a small fraction of this population would be expected to have 'hopped" in a time corresponding to the T = 1.0 x 10-l' seconds, under these longer correlation time, i.e. L conditions protonic diffusion would not appear to make a significant contribution to the neutron scattering intensity. However, if an equally high degree of dissociation were to occur in combination with a short proton association time, say seconds (the bulk water value) then the influence of the mechanism might be significant since most of the protons would then have made at least one jump in 10-l' seconds. Further experiments are needed to clarify this problem. High-resolution measurements on the Ca2+ -montmorillonite-water system were also made on the IN10 spectrometer at a range of relative humidities (Tuck, 1981; Hall et al., 1981). These measurements showed that the equivalent diffusion coefficients increased up to p/p = 0.33, but thereafter varied little with increasin relative humidity, all Galues lying in the ' s m These values agree quite well with values range 3.0 - 6.0 x 10-l' . for the Li+ montmorillonite system found by Cebula et al. (1979a), suggesting that the non-hydration-shell water has a mobility which is not very sensitive to the nature of the cation, though being significantly more restricted than For L f + bulk water, as first indicated by Olejnik and White (1972). montmorillonite, however, Cebula et al. (1979a) observed no extra e l a s t i c component attributable to an immobile, co-ordinated water fraction. Perhaps the lithium aquo-cations exhibit more rapid rotational motions than the hydrated divalent ions, as suggested by recent QENS measurements on Li+ hectorite by Conard (1979). Other measurements by neutron spectroscopy which yield information regarding the dynamics of water in clays are those of A d a m et al. (1979a). These workers measured a somewhat slower diffusion coefficient (D = 0.61 x 10-l' m2 8 - l ) for water in a Na+-montmorillonitedeuteropyridine-water system than the values characteristic of pure cation-water systems, presumably due to hindering by the organic ligands. The same workers (Adams et al., 1979b) also studied the rate of exchange of H20 and D20 in this system by kinetic diffraction measurements. They showed, by using both deuterated methanol and D20 as a deuterium source, that the exchange process was H/D atomic replacement rather than molecular diffusion. However, with D20 the half-life of the exchange process was approximately 100 minutes. The calculation of an equivalent diffusion coefficient using D = i 2 / 4 T , with = lo4 A as a typical penetration distance, yields an extremely small value of D, several orders of magnitude lower than given by the QENS measurements. It cannot therefore be concluded from these measurements that proton exchange is responsible for the quasi-elastic broadenings associated with water motions. A similar slow rate of H/D exchange is suggested by the small-angle scattering of Cebula et al. (1980). Adam et al. (1980) have also estimated a diffusion coefficient for pyridine in Na+ - montmorillonite by kinetic diffraction studies of CgHgN - CgD5N exchange which is extremely slow (ca. 10-l6 m2 s - ' ) .

7 1 From these studies, it would appear that the OENS measurements of the motions of water associated with montmorillonite and vermiculite are essentially seeing molecular jump motions of non-hydration shell water molecules, the faster component being a relatively localized "caged" motion. It would be misleading, however, to interpret this localized diffusion solely to motions of interlamellar water molecules in the spaces between the hydrated cations and the silicate surfaces. There is recent evidence that in Ca2+ -montmorillonite the fraction of adsorbed molecules located outside the interlamellar region may be significantly larger than the ratio of external to internal surfaces areas as determined by gas adsorption methods (Bergaya et al., 1979). Indeed, the distinction between external and internal surfaces is probably not clear cut, since stacking faults, bent or misaligned layers can produce external spaces of typical dimensions lm3 (Aylemore, 1977). It is suggested that the QENS data for clay-water systems might well contain a significant contribution from water in external micropores of dimensions smaller than or equal to this value, although the essential conclusions discussed above regarding the two phases of motion remains unchanged. In order to assess the relative contributions of external and internal water motions, experiments on montmorillonites with variable layer charges are required. 3.4 3.4.1

COHERENT NEUTRON SCATTERING INTERCALATES

- STRUCTURES OF

CLAY MINERALS A N D

Neutron diffraction studies

A brief account is given here of recent neutron diffraction studies of water-clay and organo-clay intercalates. More details concerning earlier studies are to be found in a previous review (Ross and Hall, 1980). The two pieces of information provided by ND measurements are: (i) conventional crystallographic data, including the location of the positions of hydrogen atoms, and (ii) characterisation of particle orientations by rocking curve measurements. The use of neutron diffraction to determine particle orientations is a simple experiment, since measurements in reflection and can be made usually at all sample orientations (i.e. transmission geometries) and the attenuation factors can be readily calculated and corrected for simple sample geometries (Hall et al., 1979b, In contrast for X-rays the muchlargerattenuation factors and Tuck 1981). the difficulty of using transmission geometries makes such measurements more difficult, although X-ray rocking curves for thin montmorillonite films have been reported (Taylor and Norrish, 1966; Roberson et al., 1968). Montmorillonite-water films of ca. 0.5 mm thickness prepared for the QENS experiments described previously were found to exhibit orientation 50° wide, distribution functions of gaussian shape and typically 40° together with a small component of randomly orientated or highly tilted platelets (Hall et al., 1979b; Cebula et al., 1979b). These clays wre,<2vm equivalent spherical diameter, but otherwise unfractionated with respect to particle size. More recently, it has been found that considerable enhancement of preferred orientation occurred in films of extremely fine an effect which was more pronounced in Na+ particles ( d O . 0 5 m e.s.d.), monhnorillonite than in divalent (Ca2+ or Mg2+) exchange forms (Harrison et al., 1981). It appears that only the finest fraction is truly composed of separate, planar flakes after drying. All larger fractions contain microaggregates, even in suspension, which are only partially broken down by ultrasonic waves. These results are in accordance with the earlier X-ray work of Roberson et ale. (1968). Characterization of the interlamellar water structure in montmorillonite by diffraction has not progressed far owing to the limited number of diffraction orders which can be resolved above the incoherent background. Replacing H20 by D20 doe's not improve matters due to a loss of contrast

-

72 between t h e l a t t i c e and t h e i n t e r l a m e l l a r region on account of t h e p o s i t i v e s c a t t e r i n g length of t h e deuteron. Hawkins and E g e l s t a f f (1980) have reported d i f f r a c t i o n d a t a f o r a Na+- montmorillonite in which t h e l a t t i c e protons were d e u t e r a t e d i n o r d e r t o reduce t h e incoherent background. However, t h e observation of only a small number of b a s a l r e f l e c t i o n s l i m i t s t h e s p a t i a l r e s o l u t i o n of t h e i r Fourier d e n s i t y map and renders i t l i a b l e t o termination-of-series e r r o r s . Their c a l c u l a t e d i n t e r l a m e l l a r d e n s i t y is i n f a c t r a t h e r uniform. These measurements, and t h e s t u d i e s of Cebula e t a l . (1979b), merely confirm t h e X-ray d a t a and t h e e s s e n t i a l l y l i q u i d - l i k e n a t u r e of water i n montmorillonites and provide l i t t l e information regarding s t r u c t u r a 1 ordering

.

-

I n c o n t r a s t , r e c e n t work on an o r i e n t a t e d Co2+ v e r m i c u l i t e f l a k e (two l a y e r hydrate) has i n d i c a t e d two p r e f e r e n t i a l hydrogen p o s i t i o n s from a onedimensional p r o j e c t i o n normal t o t h e s i l i c a t e s h e e t s . These p o s i t i o n s were c o n s i s t e n t with t h e bonding of one H atom per molecule t o an a d j a c e n t molecule i n t h e plane of t h e water oxygen atoms, while t h e o t h e r hydroxyl bond makes a l a r g e angle with t h e s i l i c a t e s h e e t s , c o n s i s t e n t with bonding t o a s u r f a c e oxygen atom (Adams and Riekel, 1980). While l i m i t e d t o one dimension, t h e d a t a a r e i n good agreement with X-ray c r y s t a l l o g r a p h i c s t u d i e s (Matthieson and Walker, 1954; Shirozu and Bailey, 1966). Other neutron d i f f r a c t i o n s t u d i e s have included deduction of t h e conformations of t h e i n t e r c a l a t e d molecules i n t h e kaolinite-formamide and montmorillonite-pyridine systems (Adams e t a l . , 1975, 1976). I n t h e l a t t e r system, t h e neutron d a t a i n d i c a t e d t h a t t h e C-C a x i s of t h e p y r i d i n e molecule l a y s p a r a l l e l t o t h e s i l i c a t e s h e e t s , a conclusion i n disagreement with previous i n f r a r e d d a t a ( S e r r a t o s a , 1966). 3.4.2

Small angle s c a t t e r i n g s t u d i e s

This technique has been b r i e f l y discussed e a r l i e r by Ross and An11 (1980), and provides a method of measuring t h e s i z e s of microscopi inhomogenities somewhat l a r g e r than c r y s t a l l a t t i c e dimensions. The p o t e n t i a l a p p l i c a t i o n s t o c l a y mineralogy l i e i n two a r e a s : ( a ) determination of t h e dimensions of c o l l o i d a l p a r t i c l e s i n suspensions, and (b) measurement of pore s i z e s i n compacted m a t e r i a l s such a s s h a l e s . It is a r e l a t i v e l y r e c e n t development i n t h e neutron s c a t t e r i n g f i e l d , although i n p r i n c i p l e i t i s d i r e c t l y analogous t o small angle X-ray s c a t t e r i n g (Guinier and Fournet, 1955). The t h e o r e t i c a l p r i n c i p l e s and a p p l i c a t i o n s of t h e technique have (1974) and Kostorz (1979), been discussed i n t h e reviews by Schmatz e t a l . while instrumentation and methods a r e documented by I b e l (1976), Mildner e t a l . (1981) and Allen and Ross (1981) among o t h e r s .

The i n t e n s i t y d i s t r i b u t i o n of small angle s c a t t e r i n g as a f u n c t i o n of s c a t t e r i n g angle o r Q ( = 4nsinB/X) can be modelled f o r p a i t i c l e s of various shapes (Kostorz, 1979). For disc-shaped p a r t i c l e s theory p r e d i c t s t h a t

1

By p l o t t i n g where R and H a r e t h e d i s c r a d i u s and thickness r e s p e c t i v e l y . l o g [ Q 2 1 ( Q ) ] a g a i n s t Q 2 , Cebula e t a l . (1980) obtained mean values of H of 10, 25 and 40 a f o r d i l u t e s o l s of Li+, F? and Cs+ montmorillonite r e s p e c t i v e l y , a t t r i b u t e d t o average s t r u c t u r e s c o n s i s t i n g of one, t w o and t h r e e l a y e r t h i c k n e s s e s , t h e l a r g e r spacings i n c o r p o r a t i n g two water l a y e r s between each p a i r of s i l i c a t e s h e e t s .

7 3 For s p h e r i c a l p a r t i c l e s t h e SANS s i n g l e - p a r t i c l e given by I(Q) = {3[sin(QR)

- QRCOS(QR)]/(QR)~]~

form f a c t o r is (22)

I n c a s e s where t h e r e is a d i s t r i h u t i o n o f where R is t h e s p h e r e r a d i u s . p a r t i c l e s i z e s t h e s c a t t e r i n g w i l l be a weighted average of e x p r e s s i o n s of Size d i s t r i b u t i o n f u n c t i o n s can t h e n i n t h i s form f o r each v a l u e of R. p r i n c i p l e be determined by s p e c i a l c o m p u t a t i o n a l f i t t i n g methods such a s t h e Using t h i s method H a l l e t a l . (1981b) have method of Vonk (1976). c a l c u l a t e d size d i s t r i b u t i o n f u n c t i o n s from SANS measurements on two New Zealand a l l o p h a n e s . These d i s t r i b u t i o n s d i f f e r from one a n o t h e r but a r e broadly c o n s i s t e n t w i t h p r e v i o u s e l e c t r o n microscopy d a t a , i n c l u d i n g p a r t i c l e d i a m e t e r s predominantly w i t h i n t h e range 30-558. ACKNWEDGEMENTS It is a p l e a s u r e t o r e c o r d h e r e my g r a t i t u d e t o numerous c o l l e a g u e s for t h e i r h e l p , a d v i c e , and c o l l a b o r a t i o n o r f o r s t i m u l a t i n g and p r o d u c t i v e d i s c u s s i o n s a t v a r i o u s t i m e s and p l a c e s i n c l u d i n g Birmingham, Oxford, H a r w e l l , Grenoble and Champaign, I l l i n o i s . These include:A.J. Allen, I.S. Anderson, C.J. C a r l i l e , D.J. Cebula, G.J. Churchman, A.J. Dianoux, D.H.C. H a r r i s and h i s t e c h n i c a l s t a f f , R. H a r r i s o n , M.H.B. Hayes, J. Hayter, A. Heidemann, W.S. Howells, M.W. Johnson, V. Rainey, C. R i e k e l , D.K. Ross, B.K.G. Theng, R.K. Thomas, J.J. Tuck, J . W . White and C.J. Wright.

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Volino, F., and Dianoux, A.J., 1980. Neutron incoherent scattering law for diffusion in a potential of spherical symmetry: general formalism and application to diffusion inside a sphere.Mo1. Phys., 41: 271-280. Vonk. C.J., 1976. On two methods for determination of particle size distribution functions by means of small-angle X-ray scattering. J. Appl. Cryst., 9: 433-440. Willis, B.T.M., 1973. Chemical Applications of Thermal Neutron Scattering, Oxford University Press, U.K.

77

Chapter 4 THE USE OF NMR IN THE STUDY OF CLAY MINERALS William E.E. STONE Section de Physico-Chimie Minerale (MRAC), Place Croix du Sud 1 , B-1348 Louvain-la-Neuve, Belgium. CONTENTS. 1. NMR. The Principles. 1.1. Nuclei in external fields. Relaxation effects. 1.4. Line1.2. Spin state populations. 1.3. widths. 2. NMR. Essentials. 2.1. The NMR absorption line. 2.1.1. The line-width. 2.1.2. The line-shape. 2.2. Spin-lattice relaxation. 3. Applications. 3.1. Water-clay systems. 3.1.1. High hydrated systems. 3.1.2. Low hydrated systems. 3.2. Structural distribution of ions in clay minerals. 3.2.1. Line shape analysis. Micas. 3.2.2. Second moment analysis. 3.3. Organic-clay systems. 4. References. 4.1

NMR.

THE PRINCIPLES

Nuclear Magnetic Resonance (NMR) is simGly a branch of the vast domain of absorption spectroscopy. It owes its existence to the property that certain nuclei possess a magnetic moment which interacts with applied magnetic fields. Certain isotopes have a nuclear spin angular moment equal to zero (such as Cl2 or 0l6) and therefore display no NMR spectra. The nuclear spin quantum number I is indeed and mI, its observafound to take the values 0, 1/2, 1, 3/2, 2 ble component, the corresponding (21 + 1) values. Nuclei with spins 1 I > 2 also have electric quadrupole moments. The relation between magnetic moment and angular momentum of a given nucleus is :

...

where gN is the nuclear g-factor, 3 is Planck's constant divided by 2a and pN is the nuclear magneton, uN = ne/2m, where mN is the mass of the nucleus. y is called the nuclear magnetoqyric ratio, its value depends on the type of nucleus and can either be positive or negative. Nuclei in external fields * Because of its convenience, a vector description is chosen here. When a nucleus is placed in an external static field g o , its 4.1.1

78

Zeeman interaction energy is given by:

--

E = - 1-1 * B o

An ordinary magnetic bar would simply align with B, but the quantum nucleus differs in possessing an angular momentum; the result is that the nucleus will precess about the Bo direction, at a certain angle, with a characteristic angular frequency w o , the Larrnor frequency, equal to:

This is the fundamental Larmor equation linking applied field to frequency. In fact, if we measure the energy of the system, it is found that it does not depend on but upon its time-averaged value in the Eo direction i.e. pz if Bo is applied in the Z-direction, thus 1

19

13

E = - p z B Z . For a nucleus of spin I = 1/2 (such as H , F , C ) there will be two energy levels corresponding to m = 1/2. The effect of placing a nucleus in an external field is to remove its

spin degeneracy. This is shown in Fig. 4.1. The lower energy state corresponds to a state where spins are parallel to B Z , the upper state (the excited state) to antiparallel spins.

Fig. 4.1.

Energy levels for spins I = 1/2.

7 9

The energy separation is given by AE = hv = RyBZ. If a quantum of energy equal to the energy separation is applied, transitions between the levels can be induced. Following our vector model, it is thus necessary to change the orientation of a magnetic moment precessing in the direction of BZ to the opposite direction. The required torque is obtained by applying a second electromagnetic field B1 in a direction perpendicular to BZ. If B1 is stationary and because is precessing at a frequency wo,no effective deflection is obtained. To produce a transition,B1 must therefore rotate in phase with the precession,'of i.e. at the Larmor frequency and such that E q . ( 3 ) is satisfied. Because the y values of nuclei are, in general, very different,the respective spectra can be recorded quite selectively. For the commonly used Bo, the resonant frequencies are located in 1 the convenient radiofrequency range (e.9. 90 MHz at 21.14 kG for H ) cm-I (kT The energies involved are of the order of 200 crn-l Q,

at 30O0K1 and places NMR at the very low energy side of many existing spectroscopies. In NMR,because it is easier to visualize the effect

on the spins of external applied perturbations (such as radiofrequency fields),it is often common to go from the fixed laboratory frame of reference to a rotating frame of reference which rotates about BZ at wo. By doing this, the fast Larmor frequency is suppressed and remains fixed in space. 4.1.2 Spin state populations Actually what is recorded during an NMR experiment is the behaviour of an ensemble of spins which are distributed between the energy levels. Provided the system is in thermal equilibrium, the ratio of the number of particles in the upper and lower levels of Fig. 4.1 is given by the Boltzmann law, as: n1/n2

%

exp

kT

2.

1

+

AE kT

(4)

The excess population in the lower state under normal conditions of 5 temperature and magnetic field is for H1 only about 1 in 10 As this difference is so minute and as the ability to detect a net

.

absorption of energy depends upon the excess population in the lower state, NMR by spectroscopic standards is inherently a very insensitive technique. A sample which is in thermal equilibrium,acquires a net total magnetization M,which i s the vectorial sum of N indivilies in the direction of the applied magnetic field. dual

r;

80

4.1.3 Relaxation effects As we have just seen, at thermal equilibrium, in a large applied

field, the spins are distributed among the energy levels according to a Boltzmann distribution. Any external process which disturbs this distribution (such as moving the sample in and out of Bo or irradiating the sample at w,) will accordingly modify the temperature of the spin system. After the perturbation, the nuclear spin system will tend to restore a Boltzmann distribution corresponding to the temperature of the lattice in which the nuclear spins are embedded. This relaxation process, characterized by a specific time constant T I , is the spin-lattice relaxation. T1 describes the return of the magnetization fi to its equilibrium value along B,. The efficiency of the coupling which restores thermal equilibrium between the nuclear spin system and its surroundings ( = lattice) is reflected in the value of T1. The shorter this time, the more efficient is the coupling and the quicker is the thermal equilibrium reached. The same problem exists in other fields of spectroscopy but is accentuated in magnetic resonance because of its small quantum of energy and population difference. The spin system can be considered as a thermostat relatively well isolated from the outside world. 4.1.4 Line-widths In solids containing a large number of spins at close distance,the

magnetic resonance line is broad. This finite line-width is due essentially to inter-nuclei magnetic interactions. Each nucleus is under the influence not only of the large external magnetic field Bo ( % 1 0 kG) but also of an internal local field EL ( % 5 G ) created by neighbouring spins. As a result, each energy level of Fig. 4.1 is no longer just a single state but is split into a large number of very close levels. The spectrum instead of being an infinitely sharp line now spreads out on both sides of the Larmor frequency w o , over a region proportional to wL = yBL. However, when molecules are in fast random motion, such as in normal liquids, the timeaveraged value of EL goes to zero and the line is sharp. Any motion whose characteristic frequency is in the range of wL will be effective in narrowing the NMR line. In a manner similar to spin-lattice relaxation, a spin-spin relaxation process may be introduced which measures the effect of spin-spin interactions in bringing the spin system into internal equilibrium. The spin-spin relaxation time (the so-called T2) characterizes, after a perturbation, the return

8 1

to zero of any finite component of the magnetization which lies in a plane perpendicular to the B, direction. Between the solid and the liquid, a gradation of behaviour of the observed spectra is found. 4.2

NMR.

ESSENTIALS

Certain specific points of NMR theory are presented below in more detail. These points were chosen because of their relevance with the applications to be discussed later on. The interested reader is however referred to the literature for more elaborate discussions (Abragam, J961; Slichter, 1963; Farrar and Becker, 1971; Fripiat, 1980 a). 4.2.1 The NMR absorption line 4.2.1.1 The line-width The main interaction responsible for the broadening of absorption lines in solids is the dipole-dipole coupling between a spin Ti and a spin I The main contribution of this interaction operator (the j' secular part) can be written simply as :

where r is the distance between Ti and 7 and 0 the angle between r j and B Z . The magnitude of this interaction therefore depends on the value of the magnetic moments involved and on their relative coordinates. When a given spin interacts with various different neighbours, each spin-spin coupling because of distance and orientation will contribute differently to the line. The experimental spectrum will therefore be the unresolved sum of a large number of lines and appear as a structureless signal. Interesting information can however be obtained from a "second moment" analysis of the lineshape. The s E n d moment of a normalized line f(B) is the mean square width AB2 measured from the center of the line. This second moment can be evaluated graphically from the experimental spectrum. Van Vleck (1948) showed that it is possible to link this macroscopic experimental parameter to a detailed microscopic description of the system being investigated. In the case where the dipolar local fields arise from n identical nuclei in equivalent positions, the Van Vleck equation is :

82

Various expressions can be derived to take into account non-resonant nuclei or polycristalline specimens. These equations have been used countlessly as they provide,among other things,extremely accurate values for internuclei distances. One should however be extremely cautious when comparing experimental values to second moments calculated from specific models. This is especially true when the signal-to-noise ratio is not sufficient, when the sample contains paramagnetic impurities,or when the distribution of nuclei over the crystal sites is not homogeneous. A s noted previously, because of the angular dependence of E q . ( 5 ) ,

whenever the molecules tumble rapidly in a random fashion, the dipolar coupling no longer contributes to the linewidth. However when certain degrees of freedom of the molecules remain blocked, such as in the case of a rapid motion about a fixed molecular axis, then the dipolar interaction is only partially averaged out. The result is a reduced line-width and a smaller second moment. This "motional narrowing" is the key to a number of techniques which are used to artificially narrow broad spectralleading to what is known as high resolution in solids. This narrowing of the line can be achieved by mechanically spinning the sample about an axis, inclined with respect to E, such that the angular factor of Eq. (5) equals zero (at the "magic angle", 8 = 54O44'). This method averages the dipolar interaction in spatial coordinate space. Another way of obtaining sharp lines is by motional averaging in spin space. This is done by irradiating the spin system with properly chosen radiofrequency pulse sequences so that the spins

are stirred in the rotating frame. Yet another approach commonly used is to observe the resonance of low abundant isotopes (C13,N15..) for which homonuclear dipole interactions are necessarily reduced 3 In this case, heteronuclear interactions and sensiti(HA l/r ) . Q

vity problems can be dealt with satisfactorily by various trick pulse sequences (Mehring, 1976). Once the large dipolar term has been reduced, the influence of smaller, more subtle effects usually linked with the immediate chemical environment can be approached (chemical shifts .) .

..

The line shape We will now examine a few cases where the spectrum instead of

4.2.1.2

being a single line shows some structure. (i) Dipolar doublet When 2 spins I = 1/2 are in dipolar interaction, then instead of considering the sum over all spins in Eq. ( 5 ) , only one term is conserved. In this case, Hi will modify the Zeeman energy levels

a3 of F i g . 4 . 1 i n s u c h a manner t h a t t h e o b s e r v e d s p e c t r u m w i l l c o n s i s t of a p a i r of l i n e s , a d o u b l e t , whose r e s o n a n c e f i e l d v a l u e s a r e g i v e n , f o r i d e n t i c a l s p i n s , by :

For u n l i k e s p i n s I = 1 / 2

( s u c h a s H1 and F19) t h e e x p r e s s i o n i s :

I n a c r y s t a l l i n e or p a r t i a l l y o r i e n t e d s a m p l e , t h e s e p a r a t i o n between t h e 2 l i n e s w i l l v a r y w i t h 8 and c o l l a p s e i n t o a s i n g l e l i n e a t t h e magic a n g l e .

I f t h e c r y s t a l l i n e s t r u c t u r e i s known o r p o s t u l a t e d , 8

(which c a n b e r e w r i t t e n i n t e r m s o f i n t e r n a l c r y s t a l l i n e a n g l e s ) o r

r can be d e t e r m i n e d .

I n g e n e r a l , each of t h e 2 l i n e s forming t h e

d o u b l e t i s broadened (see F i g . 4 . 2 )

by t h e i n t e r a c t i o n t h a t e a c h of

the 2 spins has with t h e i r o t h e r neighbours.

A doublet i n f a c t w i l l

o n l y b e o b s e r v e d i f t h e i n t e r a c t i o n s , t h a t e a c h member o f t h e s p i n p a i r has with o t h e r s p i n s , a r e l e s s important than t h e i n t e r a c t i o n I n t h e case o f powdered s a m p l e s , a d o u b l e t

between t h e 2 p a r t n e r s .

c a n s t i l l be o b s e r v e d , t h e l i n e s e p a r a t i o n i s t h e n o f c o u r s e 0 independent. ( i i ) N u c l e i w i t h q u a d r u p o l e moments A c e r t a i n number of n u c l e i of

or Li

7

i n t e r e s t , s u c h a s d e u t e r o n s ( I = 1)

( I = 3 / 2 ) , have s p i n s l a r g e r t h a n 1/2.

In t h i s case the nuclei

w i l l also h a v e a q u a d r u p o l e moment Q which may i n t e r a c t w i t h l o c a l

electric f i e l d gradients (eq).

I n a h i g h f i e l d e x p e r i m e n t , when t h e

n u c l e a r Zeeman'-energy i s l a r g e r t h a n t h e q u a d r u p o l e i n t e r a c t i o n and i n t h e c a s e of a x i a l symmetry, t h e f i r s t o r d e r 2 1 e n e r g y l e v e l s a r e g i v e n by :

The a l l o w e d t r a n s i t i o n s a r e t h o s e i n which t h e n u c l e a r quantum number

m c h a n g e s by a v a l u e I along

Bo

and

E

2

1.

The quantum number m i s t h e p r o j e c t i o n o f

i s t h e a n g l e between

the electric f i e l d gradient.

Go and t h e symmetry a x i s o f

Applying E q .

( 9 ) t o t h e case of d e u t e r o n s

l e a d s t o a s p e c t r u m c o n s i s t i n g of two p e a k s , s i m i l a r t o what i s o b s e r v e d i n t h e case of a d i p o l a r d o u b l e t (see F i g . 3 . 2 ) are:

and whose f i e l d v a l u e s

84

The separation between lines is again angle dependent and provided the quadrupole coupling constant e2 qQ/h is known, structural information can be obtained. In a perfectly symmetrical environment, the quadrupole interaction goes to zero and a single line is observed. (iii) Electron-nucleus coupling It is well known that in high resolution NMR spectroscopy of liquids, because of electron-nucleus couplings,the spectrum may be split into many lines. This fine structure originates from the characteristics of the immediate chemical environment of the investigated nuclei. For protons, for instance, the measured chemical shifts all lie in a small range of about 30 ppm and are therefore completely masked in the case of solids by the important dipole-dipole interaction. If the effect of electrons located on orbitals close to the studied nucleus are too small to be observed under the usual solid-like NMR conditions, the influence of unpaired electrons situated on nearby atoms (paramagnetic centers) can be considerable. Because the magnetic moment of an electron ue is % l o 3 larger than that of a nucleus, the local field created at the nucleus by this electron will shift the resonance condition by approximately the same factor (100 to 1000 G). However it is usually observed that the electron spin, at room temperature, reorients very quickly in the applied field ( > > w o ) and the 1-1, "seen" by a nucleus is a time-averaged value given by :

where 0 is a constant introduced to take into account weak interactions between electron spins. At room temperature and for normal G o values, ue is then only a few times larger than the nuclear magnetic moment and the shifts reduced to a few Gauss. Eq. (11) shows that the influence of e can be monitored by changing the field and the temperature. In a diamagnetic insulator, the magnetic part of the electronnucleus interaction can be written briefly as :

where HC is an isotropic contact term and H,, an anisotropic dipolar-

85 dipolar interaction between the nucleus and an electron similar in form to what was discussed previously, see Eq. (5). When examining the effect of paramagnetic ions on the NMR parameters, it is often advantageous to distinguish the influence of ions which are close from those which are further away from the studied nucleus. By close we mean such that the local fields produced by the ions at the nuclear site arelarger than the local fields due to nuclear spin interactions alone. Far away ions will only contribute an extra broadening to the line while nearby ions will shift the resonance line away from the average Lasmor field. Usually in powdered samples or crystals of high symmetry, these shifted lines go undetected because of background noise. In low symmetry crystals however, satellite lines due to nuclei 3+ at close distance to paramagnetic ions (such as Fe ) have been observed. The corresponding shifts can be evaluated by taking the anisotropic dipolar part of Eq. ( 1 2 ) as :

Fe

is given by Eq. ( 1 1 ) and r and 9 are the ion-nucleus coorwhere dinates relative to the magnetic field direction. 4.2.2 Spin-lattice relaxation As stated earlier, the spin-lattice relaxation process refers to the exchange of energy which takes place between the nuclear spin system and its surroundings (which consist of the various degrees of freedom of the system and loosely called the lattice). This process establishes or restores a thermal Boltzmann distribution of the spins on the energy levels. The mechanisms responsible for this relaxation are numerous. Generally speaking, relaxation can be caused by time dependent magnetic fields present at the nucleus and induced by random motions existing in the sample (such as diffusion or rotational movements, flipping of unpaired electrons ). These motions will be effective in inducing transitions between the spin energy levels provided they modulate the existing spin interactions and that these fluctuations cover the appropriate frequency range. In the description of random motions, a fundamental parameter is the correlation time, T ~ which , characterizes the time scale at which the motion takes place (e.g. the average time between 2 collisions). The correlation time will depend on temperature when the relaxation process is thermally activated. Depending on the type of motion and/or interactions present, various expressions for the spin-lattice

...

8 6

relaxation time can be derived. -1

T1

=

A

2

In general:

F(T~)

where A represents the magnitude of the spin interactions being modulated and F(rc) is of the form :

In liquids, T1 can typically rang'e from sec to lo2 sec while for solids the upper limit can be much larger when the sample is pure.

Figure 4.2 shows a schematic dependence of relaxation times

, relaxation determined by dipoleon the correlation time, T ~ for dipole interactions.

Fig. 4 . 2

Schematic dependence of T1 and T2 on rC.

Ordinate and abscissa values are of course only approximate. On this figure was added the variation of the spin-spin relaxation time T2 as it is assumed, in this case, that an identical relaxation mechanism is responsible for both T1 and T2. This is usually the case when studying normal liquids. In the short correlation time region i.e. in the fast motion, high temperature range,

a7

w o ~ c< < 1 and according to Eqs. (14) and (15) TY1 % T ~ . This is a region where T1 is independent of the used resonance frequency and where T1 = T2. As the motion slows down, T1 and T2 decrease until the value w , 2.~ 1 ~is reached where T1 passes through a minimum. This minimum corresponds to the region where the considered motion is the most efficient in establishing thermal equilibrium with the lattice. If this minimum can be experimentally observed it provides, at a given temperature, a value for T ~ . Below this point, T1 increases while T2 goes on decreasing. In the long correlation region where W,T C > > 1, T1 becomes proportional to W ,2 T ~ . As the temperature is decreased, the relaxation mechanism becomes less and less efficient for the spin-lattice relaxation. At a certain point, the motion no longer influences the line width, this hagpens when w T 1 where wL = yHL (see section 1.4) and corresponds to the L C rigid lattice region. The correlation time T~ can be written as : T

=

T,

exp U/kT

(16)

where U is the activation energy of the motion responsible for relaxation and which can be obtained from the slope of the T1 curve. The experimental determination of spin relaxation times as function of temperature and/or frequency can therefore provide, given an appropriate model, interesting information concerning the dynamics of the investigated system. 4.3

APPLICATIONS

As the reader will soon realize, a large proportion of the work dealing with NMR applied to the study of clay minerals concerns water-clay systems. This is certainly due to the importance of the subject in many domains and also because of the relative ease with which, in this case, NMR signals could be detected. The author has tried to cover most of the recent literature. He apologizes for any unintentional omissions. Water-clay systems In soil, environmental and life processes, water is an essential and necessary component. An appreciation of the nature and properties of water in the vicinity of a surface or at interfaces is therefore extremely important. In water-clay systems, parameters such as size, charge and water content can relatively well be controlled and, as such, constitute in this complex area an important and interesting case. Well documented reviews related to water 4.3.1

in heterogeneous systems have been published by Drost-Hansen (1969), Packer ( 1 9 7 7 ) and Farmer ( 1 9 7 8 ) . For these systems, the problem is to determine how the behaviour of vicinal water molecules is modified by the surface and how far out do these modifications extent. NMR is a technique which samples local molecular arrangements and whose time scale covers an extremely large range (from 10-l' sec to 1 sec). It is therefore well adapted for the problem at hand. Because of their high internal surface and crystalline regularity, most of the attention has been focussed on expanding layer silicates. Results obtained on these clays and other,systems are discussed below. High and low hydration levels are examined separately because the properties of these two types of systems are quite different. High hydrated systems (i) Some general considerations Essentially, 3 types of movements are found in liquid water : translational diffusion, rotational motions and intermolecular proton exchange. The first two appear closely related with correlation times in the range 1 0-11 sec. (Eisenberg and Kauzmann, 1969). The self-diffusion coefficient at room temperature is about 2.3 lo-' m2/sec. The exchange of protons between water molecules occurs, at pH 7 , at a much lower pace, of the order of sec (Meiboom, 1 9 6 1 ) . In bulk water, a water molecule experiences an anisotropic instantaneous interaction potential. The axes which define this anisotropy move at least as fast as the molecule itself and therefore, on the NMR time scale, this anisotropy is completely averaged out. However, when a surface is present, some fraction of the water molecules, in some way, will interact at the interface in a highly anisotropic manner. The difference now will be that even if these molecules go on moving, the anisotropic potential that they will sample remains constant for at least several molecular reorientations (this interaction potential may of course vary from point to point along the surface). Once again the time scale is of importance. If the rate of exchange of molecules between various sites or between bound and free state is fast, then this time effect will be reflected upon the NMR parameters being measured. One of the NMR parameters, which can be used to visualize the anisotropy induced by the surface,is the lineshape of the studied nuclei. In the case of water, when the molecule is preferentially oriented with respect to the surface, the dipole-dipole coupling between the two protons is no longer averaged to zero as it is in bulk water. This results in a splitting of the proton resonance 4.3.1.1

89

into two lines, a doublet as given by Eq. (7). The field separation between the two lines is a measure of the dipolar coupling, the angular factor reflects the way in which the molecule is oriented with respect to the surface in the applied magnetic field. Experimentally it is observed that the splittings are always much smaller than those found in crystalline hydrates or ice. This is because the angular factor is time averaged due to molecular motions (see section 2.1.1). The preferential orientation of water molecules at the surface constitutes in fact a dynamic ordered state. When the surface is randomly distributed with respect to the applied field such as in powders, then terms are also space-averaged. A rapid exchange of protons between different water molecules has the effect of damping out the observed doublet; this is because the incoming proton is not necessarily in the same spin state as the one it replacestwhich leads to an extra modulation of the proton-oroton interaction. This exchange process does not however affect the doublet which is observed when deuterated water is used,as in that case,the donblet is due solely to the interaction of the quadrupole moment of the deuteron with the electric field gradient which exists within the water molecule (see Eq. 10). The deuteron doublet is therefore often found a more reliable indicator of the presence of preferential ordering of the water molecules on the clay surface. The use of deuterons also has other advantages; for protons, the spin-lattice relaxation T1 depends on both intra- and intermolecular interactions, for deuterons the latter are negligeable. This also means that extraneous effects such as the influence of paramagnetic impurities on the relaxation process is greatly reduced in the case of heavy water. As stated above, a fast molecular exchange process generally prevails; this leads to a single weighted average value for T1 which for a simple two phase-system is written as -1 -1 -1 Tl = a Tlf + b Tlb where a and b are the fractions of free and bound water (Zimmerman and Britten, 1957; Brownstein and Tarr, 1977). (ii) Results Highly hydrated water-clay systems such as montmorillonite, hectorite, vermiculite, illite and kaolinite have been investigated in detail by Woessner. He essentially studied deuterated water systems with the aid of spin-echo pulse sequences, a method which gives access to doublet splitting parameters such as separation, magnitude and width of the two lines (Woessner and Snowden, 1969 a and b; Woessner, 1974; Woessner, 1977). Part of his results were

90

obtained on oriented clay samples. Certain proton doublet splittings were also measured for a Na-hectorite oriented sample. In this case, and for reasons given in the previous paragraph, a doublet could only be observed when the water content was low ( Q 0.2 cm3/ g dry clay) or the temperature lowered (Woessner, 1969 a). In the same clay system, a doublet for deuterons is however observed, at room temperature, for water contents up to 8.4 cm3 H20/g clay (approximately 85 sheets of H20 between consecutive clay layers). A plot of the doublet splitting constant, B, versus the clay concentration is perfectly linear (see Fig. 4.3).

10’

B

0

Deuteron

o Proton

(SeC*)

10’

gms clay /c(rfwater

Fig. 4.3. The concentration dependence of the proton and deuteron doublet splitting constants of oriented Na-hectorite (following Woessner, 1 9 7 5 ) . B comprises all the constant terms of Eq. (10) including the part of

the angular factor which relates the symmetry axis of the intra-water molecule electric field gradient to the normal to the surface. The product of B by the amount of H20 per gram of dry clay, B ’ , remains virtually constant over the entire concentration range (Woessner, 1975). This observation can be interpreted as follows: because of rapid diffusion, all the water molecules experience the same average preferential orientation in a time shorter than 1/B (10-2-10-3 sec) and this preferential orientation only resides in the first water layers. There is no long-range effect as measured by this

91

orientation-sensitive NMR parameter. The use of powdered samples renders the interpretation of results more difficult because of space averaging effects. Also the possibility for water molecules to diffuse from one interlamellar space to another should be accounted for. The results that emerge (Woessner, 1 9 7 5 ) are that the B', at least 3 up to water contents around 2.5 cm /g are essentially independent of concentration. Furthermore, whatever the clay (hectorite, montmorillonite, vermiculite) or nature of exchangeable cation, all the values are remarkably similar. To compare these results with those obtained on non-expandable clays such as kaolinites or illites, the observed splitting constants are normalized to unit surface area. It turns out that this parameter is then similar for all the studied clays. This rather surprising result has led Woessner ( 1 9 7 5 ) to suggest that it is the mere presence of a static interface and not its electrical or chemical characteristics which is the determinant factor in producing the preferential orientation of the water molecules. This proposal has recently been given some support by the work of Walmsley and Shporer ( 1 9 7 8 ) . They calculated a surface induced anisotropy in the orientational probability distribution of the water molecules and its influence on NMR parameters. This anisotropy, provided there is sufficient water (i.e. bulk plus surface water), is independent of the nature of the substrate. At this point, it seems interesting to recall certain results obtained by different methods by Low and coworkers. It is suggested (Low, 1 9 7 9 ) that unless the number of water layers is small, the influence of the interlayer cation is secondary compared to the effect of the surface on the structure (in terms of bond lengths and bond angles) of the interlayer water in clays. Low shows that for Na-montmorillonite various macroscopic-type properties (such as viscosity, specific heat capacity ...) of interlayer water differ from those of bulk water although the measured deviations depend strongly on the type of property being measured. It is proposed that these deviations originate essentially from water-surface interactions although these interactions need not necessarily be long-ranged to induce long-range effects (Low and Margheim, 1 9 7 9 ) . Contrary to what is suggested by Woessner, Low ( 1 9 7 9 ) finds that specific factors closely associated with the nature of the montmorillonite (as its b dimension) are important in considering the properties of vicinal water. Results,to be discussed below on low hydrated systems with water squeezed between two close adjacent clay layers also show that factors such as nature of the clay and type of

9 2

counter-ion are of importance. In a recent published work,Woessner ( 1 9 8 0 ) on the same oriented Na-hectorite-D20 samples as used previously, determined the range of the surface influence on the rotational properties of the water molecules. This was done by measuring the deuteron T1 as function of water layer thickness (from 3 to 108 molecular layers). The conclusions of this work are again that, at room temperature, the hectorite surface influences and slows down the rotation of only the first layer or two of water molecules. The surface causes the nearby molecules to rotate approximately five times slower than those of bulk water. A factor much higher than five has recently been found by Fripiat ( 1 9 8 0 c). These results should be contrasted with a work by Ovcharenko et al. ( 1 9 7 4 ) who + measured the H spin-lattice relaxation in kaolinite dispersions. Contrary to what was observed for hectorite by Woessner ( 1 9 8 0 ) a break occurs in the curve of relaxation rate versus clay concentration at about 6 5 % kaolinite which interestingly correlates with structural-mechanical characteristics (filtration coefficients ... ) of the dispersion. The proton relaxation rate mechanisms were however not fully analysed. From the ensemble of results, it seems that from a dynamic point of view, a certain low fraction of the water molecules, those who are close to the clay surface, have properties which are distinct from those of bulk water. There is also evidence that in highly hydrated systems a certain proportion of the water does not freeze until low temperature. This is usually inferred by NMR by examining as function of temperature the line shape and relaxation times of the water molecules. According to the experimental conditions, these parameters will either only exhibit ice type features when well below freezing temperature or give rise to a two phase behaviour (see for instance, Annanyan, 1 9 7 8 ) . The role of the surface which, as shown above, modifies the dynamics of certain water molecules in unfrozen systems has still to be understood when the highly cooperative process of freezing sets in. A model of ice nucleation has been proposed by Anderson ( 1 9 6 7 ) which in the case of silicate-water interfaces visualizes the process by an increase in the life-times and sizes of H-bonded domains which facilitates the nucleation of ice. The same author in a review article with Morgenstern ( 1 9 7 3 ) indicates that,when clay-water systems freeze,the ice only crystallizes in the extralamellar space. In frozen soils, two interfaces are therefore defined: the interlamellar space and a silicate-water-ice interface

93 which surrounds the particles

By studying the H

i

NMR spectra of

Na-montmorillonite at various water contents, the authors come to the conclusion that the water molecules at this latter interface have still liquid-like propert es but aremore structured than the interlamellar water. The amount of unfrozen water found around mineral surfaces is discussed by Banin and Anderson (1975),who find an extremely simple empirical relation between temperature and thickness of this water layer. The problem of the state of water in the interlamellar space is taken up in the following section. 4.3.1.2 $ow hydrated systems (i) Some general considerations For obvious reasons, most of the reported NMR work has been carried out on layer silicates.

The systems discussed below have a water

content which corresponds roughly, to one or two water sheets located between the layers (i.e. an x-ray spacing of respectively % 1.2 nm and 1.4-1.5 nm). In the interlamellar space, the state of the water molecules can be considered as somewhere between what is found in crystalline hydrates and ionic solutions.

It is however extremely

important to consider the particulars of each studied system.

The

range of stability, structure and properties of the interlayer water will depend not only on the type of exchangeable cation but also on the intrinsic composition and properties of the silicate layer. Certain points to be considered are : 1) relative importance of the negative surface charge; 2 ) origin of this charge, the site at which ionic substitutions occur within the layer (tetrahedral substitutions in the case of vermiculite, octahedral for montmorillonite and hectorite) directly affects the localization of the charge at the surface: 3) average dimensions of the clay platelets.

Differences

between hydration properties of clays is well known and has been confirmed by numerous detailed work using either infra-red (e.g. Farmer, 1978) or water isotherms associated with x-rays. The latter typify the existing differences. It is found in the case of hectorite and montmorillonite that whereas the spacing increases by steps, the sample absorbs water in a continuous manner (Prost, 1975; Keren and Shainberg, 1975). On the contrary, in the case of vermiculite, plateaux in spacing correspond to well defined steps in water content (Van Olphen, 1965; Hougardy et al., 1977). It is anticipated, therefore, that vermiculite will produce a more homogeneoustrelatively well defined system compared to the other clays which form a more complex situation with possibly different water populations, interstratification and microporosity (Prost, 1975; Mamy, 1968; Eqer

94 et al., 1979). The heterogeneity of montmorillonite systems is moreover exemplified by the large distribution of layer charges found in these clays (Stul and Mortier, 1974; Lagaly and Weiss, 1 9 7 5 ) .

As

it will be shown below,the observed differences between various waterclay systems are also reflected in the NMR results. It should be mentioned however that the aim in studying these systems has not only been to try and determine some of the physical properties of the adsorbed water in relation to its particular environment but also to try and gain access to part of the chemistry which takes place in the interlamellar space i.e. replacement reactions, proton transfers, catalytical reactions, enhanced acidity (Fripiat 1976; Theng,1974).

...

In this context, there is still place for much work. (ii) Results As in section 3.1.1.(ii), we will first examine the information obtained by NMR concerning the structure of the interlamellar water. The words "structure" or "order" should still be considered in a dynamic context. Also because of heterogeneity, detected order may only be of local character and not follow a regular pattern throughout the entire sample. Schematically, it is easy to imagine at least 3 possible states for adsorbed water: 1) water in the first hydration shell of the cation; 2 ) water not directly linked to a cation but still in the interlamellar space; 3 ) water on the external surface or in micropores. The relative percentage of these various types of water strongly depends on the nature of both the clay and the cation. When the NMR relaxation times for these water populations aresufficiently different and exchange between them slow, it is possible to obtain a rough estimate of their relative distribution.

Different

states for adsorbed water have been identified for certain clays by infra-red (Farmer, 1978; Prost, 1976). A paper by Hougardy et al. (1976) examines the properties of water in a two-layer hydrate of Na-Llano vermiculite. This clay represents a somewhat model case as it has a high value of CEC (200 meq./100 g ) , large individual flakes, small microporosity, a water isotherm (Van Olphen, 1965) with two well defined steps corresponding to one or two sheets of water. Its iron impurity content (1700 ppm) however makes a non negligeable contribution to the spin-lattice relaxation T 1 mechanism. The well characterized isotherm strongly suggests an homogeneous arrangement of water molecules strongly coordinated to the cation with very little water outside the hydration shell. For the two layer hydrate, there is, on the average, 6 water molecules + per Na cation and the basal spacing (14.8 A) is sufficient to 0

9 5

Fig. 4.4. Spectra for the two layer hydrate of Na-vermiculite at room temperature. Top : decomposed spectrum for H 2 0 , showing doublet and central line. Bottom : spectrum for D20, showing the absorption doublet and its fir.st derivative (Hougardy et al. , 1 9 7 6 ) . accomodate an octahedral arrangement of these water molecules around the sodium ion. The deuteron and proton spectra run on oriented samples seem to corroborate this assumption. The spectra (see Fig. 4.4) show the characteristic aspect of a doublet, indicative of preferential orientation of the water molecules. The line perfectly follows the ( 3 cos2 0-1) law of E q . (7) when the sample is rotated in the magnetic field. An analysis of the line shape is consistent with an octahedral arrangement,such aS shown in Fig. 4.5 in which each water molecule rotates rapidly around the C 2 axis (coordinative bond) and the entire ensemble rotates around the C 3 axis (which is

96

Fig. 4.5. Octahedral arrangement of H20 molecules around the Na ion in the two layer hydrate of Na-vermiculite (Hougardy et al. 1976). parallel to the normal to the layer c*). The frequency at which these two rotations take place should be faster than 20 kHz. This x double rotation maintains a constant angle between C2 and c and therefore dynamic order for the water molecules. It is interesting to note that Giese and Fripiat (1979) using a simple ionic bonding model, have calculated that a combination of these 2 rotations "saves" a great deal of activation energy and reduces the energy.of rotation from above 50 kcal/mole to below 20 kcal/mole. From T1 measurements performed on this system (Hougardy et al., 1976) the activation energy associated with these motions are of the order of 10 kcal/mole. It should be noted that if the relatively simple structural model given above fits the experimental data for the 2 layer Na-vermiculite, other more complex and unexpected NMR spectra have been observed for the same mineral in the case of the onelayer Na-hydrate and two-layer Li-hydrate (Hougardy et al., 1977). For these samples, two doublets are found which could not be resolved unambigously by the simple use of a classical description in terms of dipolar doublets. The authors were therefore not in a position to propose, as above, a structural model for the water molecules. Finally, it is interesting to note that in the case of the Mg twolayer hydrate, the octahedral arrangement of water molecules around the cation proposed by x-ray studies (Shirozu and Bailey, 1966; Alcover and Gatineau, 1980) is consistent with the observed NMR spectralprovided some fast averaging motion occurs,which allows the

97

Mg2+ ion to be shared between the empty and full octahedral water shells (Hougardy et al., 1 9 7 7 ; Giese and Fripiat, 1 9 7 9 ) . Let us now consider more heterogeneous systems, namely less charged clays such as montmorillonite and hectorite. A work by Hecht and Geissler ( 1 9 7 3 ) studies the proton (but not deuteron) NMR spectra of water adsorbed on a Na-synthetic fluor montmorillonite. Both the single and double layer samples showed a doublet spectrum together with a relatively large central line (see below). The doublet was interpreted in terms of random jumps between different possible molecular orientations within a water structured frame-work similar to the one found in ice (tetrahedral disuosition). This type of structure has been shown (Mamy, 1 9 6 8 ) to be a reasonable assumption for the single layer system as in that case, the exchangeable Nacation neatly takes the place of one of the water molecules without disturbing the ice-like structure. For the two layer system, such a disposition is however more difficult to conceive. Matyash et al. + ( 1 9 7 4 ) have indeed shown by following the Na NMR spectrum that the model proposed by Mamy ( 1 9 6 8 ) for monohydrate montmorillonite is correct but that as soon as the 2 layer state is reached, the Na is contained in a symmetric hydration octahedral shell. Various papers deal with the water-Li hectorite system for which proton, deuteron and Li NMR spectra have been recorded both on powdered samples and oriented films. Hectorite is interesting because both contributions from lattice OH and iron impurities are low. The one-layer state of 0 Li-hectorite (spacing 12.6 A; 6H20/Li) which is the only well defined state, has been studied by Fripiat et al. ( 1 9 8 0 ) . The proton and deuteron spectra both show a doublet whose separation follows the expected angular variation as the oriented samples are turned in the magnetic field. A close examination of the spectra, recorded at various angles, however reveals a slightly more complicated shape, suggesting that the line is probably the sum of several contributions. This seems to be especially true for the one-layer Ba hydrate (KadiHanifi, 1 9 8 0 ) for which a quite visible central line is also observed. For the Li case, the authors suggest a dynamic pyramidal ordering of 3 water molecules surrounding each Li' cation with again two rotational motions similar to those found for the Na 2 layer vermiculite

.

case (i.e. around C2 and cx) The rapid diffusion motion of rnonovalent cations as measured by Calvet ( 1 9 7 3 ) is also certainly an important factor in the structural averaging process of the water

+

frame-work. A detailed study by Conard ( 1 9 7 6 a and b) on the Li NMR spectra of this same powdered sample at low water content (s 5%)

98

concludes to a nearly flat trihydrated cation fixed by 3 hydrogen bonds to the six oxygens of the hexagonal cavity. The author suggests that the 3 protons are in fact tunnelling among the various possible sites which would concilate the known acidity of these systems with the proposed double rotation of the three water molecules. Finally, an interesting case has been studied by Cruz et al. (1978) which concerns the monolayer of water in the interlamellar space of halloysite. As in this sample there is no charge balancing cation, it constitutes a useful check concerning the role played by this ion on the water structure. The resuLts are clear inasmuch as neither the H20 or D20 NMR lines show a doublet structure. This absence at least for times longer than 5 sec, of preferential orientation of the water moleculesistherefore conclusive about the importance of coordinative links between water molecules and cation which were present in the other cases. The surface only plays a secondary role. Following their spin-lattice relaxation analysis, the authors however show that an hexagonal arrangement of water molecules similar to the one proposed many years ago by Hendricks and Jefferson (1938) is not incompatible with their results. The water molecules would then be submitted to a double orthogonal motion : a 180'

flip around C 2 and a tumbling motion about an axis perpendicular to C2. The measured correlation times for these 2 motions are, at room temperature, In this case, the respectively of 2.4 10-l' sec and %2 lo-' sec. main effect of the surface is thus to induce an anisotropy in the molecular motions. Before leaving these line-shape analyses, a few words should be said concerning the central region of the spectrum (i.e. the region around coo, between the two lines of the doublet) as it has been the subject of many discussions. As mentioned above for water adsorbed on a synthetic fluor montmorillonite, Hecht and Geissler (1973) observe a large central line which was attributed to a local dissociation of the water molecules around the exchangeable cation, the dissociated protons being in rapid motion and involved in an exchange process with the nearest water molecules.

An alternative

explanation was proposed by Resing and Davidson (1976) which suggested that the simultaneous presence of the broad and narrow lines was due to an "apparent phase transition" caused by a broad distribution of correlation times connected with an isotropic reorientation motion (Resing, 1968). As these experiments were not run with D20 replacing completely H20, these hypotheses could not be tested: furthermore, isotropic motion can only take place for relatively free water located

9 9

on external surfaces or within existing pores.

For vermiculite,

Hougardy et al. ( 1 9 7 6 ) also observed a central line in the proton spectra (see Fig. 4 . 4 ) which was however absent in the deuteron spectra. Since an exchange of deuteron nuclei does not affect the deuteron doublet, the absence of a deuteron central line therefore clearly confirms the model of preferentially oriented water molecules with a proton exchange process collapsing the proton doublet

of some of the present molecules. If, in vermiculite, this process is probably the dominating factor responsible for the central line, in other cases, several other contributions may play a role. This is exemplified, for instance, by the spectra observed on oriented samples of Li and Ba-hectorite (Kadi-Hanifi, 1 9 8 0 ; Fripiat et al., 1980). For these cases, the central region of the spectra follows a much more erratic behaviour linked, most probably,with the nature of the clay itself. Contributions to this part of the spectrum are certainly due to the presence of water in micropores, disorganization of the microcrystals leading to doublets of various splittings or such that the angular factor ( 3 cos2 0 - 1 ) is near zero. Even after a thorough NMR line shape analysis, the models proposed hereabove for the structure of interlamellar water are naturally of a schematic nature and not necessarily unique. They should, when possible,be confronted with results obtained by other methods taking into account the time and space domain to which each method is sensitive. NMR, EPR and I.R. provide snap-shot pictures of short range patterns. A detailed review of EPR results may be found elsewhere in this book. This method, when the charge balancing cation is chosen to be paramagnetic (Cu2+, pln2+ . . . ) , provides through the study of hyperfine splitting values, interesting structural information,some of which are close to the ones proposed by NPlR. Long-range sensitive methods and macroscopic type studies on waterclay systems are well documented and only some of the recent papers will be cited here. A recent neutron diffraction paper by Adams and Rickel ( 1 9 8 0 ) carried out on a divalent cation exchanged 2 layer vermiculite hydrate reveals the existence of highly structured water layers consistent with the NMR results reported above. In this case, the neutron scattering density profiles are well defined and should be contrasted with results obtained by Hawkins and Egelstaff (1980) on a Na-montmorillonite by the same technique and for which the diffraction patterns show, as expected,much less structured profiles. A detailed analysis by isotherm curves,TG and DTA techniques carried out by Suquet et al. ( 1 9 8 0 ) on saponite and vermiculite concludes

100

to a fairly structured framework for the interlamellar water. Recent thermodynamic calculations from water adsorption isotherms have been carried out by Tardy et al. ( 1 9 8 0 ) for hectorite and by Keren and Shainberg (1980) for montmorillonite which show that the hydration water is in a state intermediate between liquid water and ice. The latter authors find that,as the water content increases,the influence of the surface is not felt beyond a few molecular layers of water. A l l techniques therefore tend to confirm the organized character of water in low hydrated systems. What NMR teaches us is first, that in the preferential organization of the water molecules the coordinative links with the cation are of primary importance and secondly, that the order which is established is essentially of dynamic character. NMR is not only capable of proposing local structural models but also has the advantage of being able to measure correlation times characterizing the various existing motions. This is done by measuring as function of temperature the spin-lattice relaxation time T1 of the concerned nuclei (section 2.2). The motions, whose correlation times are measured by T1,should hopefully be consistent with the dynamic structural models proposed through the study of the line shapes. For all the above cited systems, spinlattice relaxation measurements have been performed and certain of the results already been given. A s clay-water systems are by no means simple when considered from a microscopic dynamic scale, it is not surprising that the attribution of the various T1-detected motions is not always easy or straightforward. Comparing different systems is, as usual, often very instructive. It is not our purpose here to go into all the details concerning the calculations, corrections (essentially for paramagnetic impurities) or modeling which can be found in the various cited papers. Suffice it to say that for natural clay-water systems, the measured T1 often reveal, as function of temperature (Fig. 4.2), the presence of several minima (i.e. several motions) and that these minima are very often shallow (this is usually linked with the presence of a distribution of correlation times indicative of the heterogeneity of encountered situations). It is perhaps interesting to reproduce here (Fripiat, 1980 b) a diagram giving the correlation times measured for various systems (see Fig. 4.6). Curves la and 2a have been attributed to the rotational diffusion motion of the hydration shell about the cx axis for respectively the 2 layer Na-vermiculite and 1 layer Lihectorite. This attribution seems to be corroborated by cation autodiffusion coefficients measured independently (Calvet, 1973).

101

Fig. 4.6. Temperature dependence of various measured correlation times: la and lb, 2 layer Na-vermiculite; 2a and 2b, one layer Li-hectorite; 3, one layer halloysite (following Fripiat 1 9 8 0 b). As the values of

T~ for curves lb, 2b and 3 all have similar values around the region corresponding to the respective experimental T1 minima and that this minimum value conditions the type of motion involved, it is suggested that the T~ have a similar origin namely the intramolecular rotation of the water molecule around its C2 axis. The activation energies found are appropriate i.e. in 4 to 6 kcal mole-’ range. The T~ measured for this assumed rotation are at least an order of magnitude shorter than what is found for free water. A value of T~ for molecular rotation in a 2 layer Ca-mont-11 morillonite hydrate has also been found, T~ % 10 sec, by Kvlividze et al. (1975) using the 017 spectrum of H2017 molecules. It is well know, in the case of bulk water, that rotational and translational motions of water molecules are highly correlated. These last values for T~ could therefore also be associated with a translational diffusion motion for certain given systems i.e. systems with interlamellar non-coordinated water or/and with water in micropores. These T~ would then be in the appropriate range of the water self diffusion coefficients measured from high resolution neutron scattering experiments (Hall et al., 1979) on different clay water systems. For the hectorite sample discussed above (Fripiat et al., 1 9 8 0 ) , it has been possible,because the respective T1 were sufficiently different,to distinguish between coordinated and non-coordinated

102

water.

The latter population increased from about 20% at room

temperature up to

%

50% at 200'K.

The exchange rate between the

two types of water is slow in this case.

Finally in the case of

the 2 layer Na-vermiculite hydrate (Hougardy et al., 1976) the proton exchange rate between adjacent water molecules has been estimated to be, at room temperature, between and sec. This means that the exchange is between 10 and 100 times faster than in bulk water.

Such fast exchange processes

(%

an order of

magnitude) were also found for a series of clay-water system by Woessner (1974) who also showed that the rate depended on the + type of exchangeable cation (T, shorter for Mg2+ than for Na ) . This enhanced proton exchange frequency is therefore linked with the strongly polarizing effect of the cation and is certainly connected to the known higher degree of dissociation of interlayer water (Touillaux et al., 1968). If in liquid water a fairly simple relation links the proton activity to an exchange rate (Meiboom, 1961), in the very particular case of interlamellar water such a relation has still to be established. This relation would certainly depend very strongly on such parameters as: nature of exchangeable cation, water structure, magnitude and localization of the negative charge

...

4.3.2 Structural distribution of ions in clay minerals NMR is very useful in resolving structural problems. This is because the NMR spectrum contains information not only about the type of nucleus responsible for the line but also, in the case of dipolar interactions, on the relative arrangement of elements in the lattice (see Eq. 5). The interpretation of results is relatively straightforward when the sample is homogeneous i.e. the distribution of elements throughout the whole structure is identical. As this is not always the case for natural systems, the ability to monitor as many variables as possible is quite an asset.

-

4.3.2.1 Line shape analysis Micas A series of papers published by Sanz and Stone (1977, 1979,1980) investigates by NMR the problem of the distribution of ions in the octahedral sheet of trioctahedral 1M micas. The OH group which forms part of the octahedra are coordinated to 2 different cationic 2+ sites: one M1 site and two M 2 sites occupied essentially by Mg 2+ . ions. These can be replaced by Fe ions up to few percent in the case of phlogopites, up to 20% in the case of biotites.

Other

cations such as A13+, Fe3+, Ti4+ and vacancies are also present (to a lesser extent). The OH groups can be isomorphously replaced

103 to various degrees, by F- ions.

Because of the heterogeneity in the

composition of the octahedral sheet, micas could be thought as forming multivariable systems wherein random distribution of the ions on the various sites should prevail. This shall in fact be proven not to be the case. The obtained results benefited by 3 important features : 1) the possibility of studying a collection of samples with a continuous range of ionic concentration; 2 ) the use of crystalline samples which allowed appropriate orientations to be chosen: 3 ) the possibility of studying both the OH and the fluorine signals. Experimentally, it was found that the H+ spectra of phlogopites and biotites are formed by a main central line flanked by smaller satellite lines. The intensity and number of side-lines increased with iron content, their position depended on the sample orientation in B, ( 2 typical spectra are given in Fig. 4.7). A detailed analysis showed that these side-lines are due to OH groups which are either first, second or third neighbour to Fe2+ ions located in the M1 and/or M2 sites. The parameters of the side-lines were shown in fact to obey quite well E q s (11) and (13) of section 2.1.2 and could therefore easily be identified. In Fig. 4.7, lines 2+ . I1 and I2 correspond to protons which are adjacent to a Fe ion From the relative areas located respectively in a M1 and M2 site. of these peaks, it can be deduced that on the average the Fe2+ ions

are randomly distributed between the two possible sites.

The lines

1112, 112 and I11 correspond to associations around a given OH of 2+ 2+ (M1 + M2); 1 Mg2+ and 2 Fe respectively, 1 Mg2+ and 2 Fe (M2); and 3 Fe2+. The main line at w, corresponds to 3 Mg2+. By using the protons as internal probe, it is thus possible to follow from one sample to'another, the various associations as function of 2+ chemical composition. It turns out that the association of Fe ions around the OH groups is more important than what is expected from a random distribution. In other words, Fe2+ ions have a net tendency to cluster around the OH groups. Side-lines associated with vacancies have also been observed and in the case of biotites an ordering of A1 cations around the OH seems also possible. Interesting correlations with I.R. spectra in the OH stretching region have also been established (Stone and Sanz, 1980 and results to be published). This segregation of Fe2+ ions around the OH groups is confirmed by spectra run on the same samples for fluorine. It is found that unlike the OH groups, the F- are not directly coordinated to Fe2+ (only side-lines corresponding to second or third neighbours are observed).

This is corroborated by data

104

P-18, H ( 1 4 M H Z )

b axis

rp

= 30"

8-10

+

Fig. 4.7. Two typical H spectra of micas, at the same orientation about the b axis (14 MHz, room temperature). Top : a phlogopite (2.6% iron). Bottom: a biotite (13.8% iron) (Sanz and Stone,1977) showing that, at low Bo fields, the F- signals consist of doublets of F-H and F-F pairs. It is possible to evaluate the doublet separations as function of orientation by E q s . (7) and (8); the experimental points follow the prediction completely. The number of F pairs obtained from the spectra and those expected on a random basis are shown in Fig. 4.8. It is seen that the number of F pairs increases much faster than expected. This is interpreted as evidence for the existence of homogeneous F-rich domains from which the Fe2+ ions are excluded. NMR therefore shows that the OH- and F- ions are highly differentiated with respect to cationic associations. This short

105

F /site

Fig. 4.8. F-F percentage a s function of the F content: ( 0 ) different statistically-calculated values. (Sanz and phlogopite samples: (---I Stone, 1 9 7 9 ) . range order (undetected by x-ray methods) could play an important role in the vermiculitization process of micas and is certainly connected with the observations made concerning the definite relation which is found between the amounts of F, Mg and Fe in silicates (Ramberg, 1 9 5 2 ) . Another study carried out on the proton signal of a series of powdered glauconites (dioctahedral Fe3+ rich micas) has been published by Kohler and Burkert ( 1 9 7 6 ) . The data reveal highly asymmetric signals which were decomposed into two lines of approximately equal areas. The observed shift between these two lines is found to increase with Fe3+ content. The authors propose that the narrow line corresponds to structural OH groups while the other more broader line is related to iron containing impurities or interlayered hydroxy iron complexes. They do not consider the possibility of shifts brought about by structural Fe3+ ions. The use of powdered samples makes the interpretation of results difficult. The use of shifted lines for structural problems can also be found in a paper by Kalinichenko et al. ( 1 9 7 4 ) who studied the H+ line of a diocta0 hedral 10 A type mineral - chernykhite - which contains a large amount of paramagnetic vanadium in'the octahedral sheet. The authors consider that, in this case, the isotropic contact term is the dominant contribution in the electron-nucleus interaction (i.e. Hc of

106

They propose that in the octahedral sheet of this mineral, two chains of V cations alternate with one chain of A1 cations (in

Eq. 12).

a direction parallel to 1101 or 1111).

The analysis was carried out

on a single sample in the powdered form: the experimental shift was twice the calculated value. This powdered spectrum is compared with an oriented sample at one unspecified orientation in the ab plane. 3.2.2. Second moment analysis When the NMR lines are structureless or when powdered samples are used, information can still be obtained (under certain conditions as discussed in section 2.1.1) by a second moment (S2) analysis of the line. A few examples related to clay minerals or associated minerals are reviewed below. Gastuche et al. (1963) found for kaolinite an experimental S 2 value which was in reasonable agreement with a calculated value taking into account H-H and H-A1 interactions with lattice parameters obtained by x-ray determinations. Fripiat et al. (1969) followed the temperature variation of S 2 in the case of boehmite. The decrease of S 2 between 300°K and 430°K was associated with a proton diffusion mechanism. With the aid of I.R. and S 2 measurements, Vivien et al. (1973) were able to propose in the case of gibbsite a model for the localization of the OH groups between

sheets of oxygen. A structural model of allophane was proposed by Okada et al. (1975) using x-ray radial distribution and fluorescence. The OH contents calculated from these models were shown to compare favourably with those estimated by an S 2 analysis of the proton absorption lines. Because the lattice structure of kaolinite is similar to the one of halloysite, Cruz et al. (1978) find that the S 2 value determined previously by Gastuche et al. (1963) agrees

quite well with their results obtained for the broad Gaussian line found for dehydrated endellite. In their work on the 2 layer hydrate sample of Na-vermiculite, Hougardy et al. (1976) computed an S 2 value for the water molecules taking into account the various parameters imposed by the proposed structural model for H20 (section The calculated value agrees quite well with the experimental 2 S 2 value of 2 Gauss . As the temperature is lowered below O°C, S 2 increases until it reaches the value of 19 Gauss2 at -100°C. This 3.1.2).

should be compared with the value found for ice which is about 37 Gauss2 The difficulties encountered in the study of natural

.

samples is well illustrated in a recent paper by Scala et al. (1978) where a complex series of aluminosilicates is studied by Na and A1 NMR coupled with EPR results. References to connected systems can also be found in this paper.

107

Organic-clay systems The study of interactions between organic molecules and soil or clay material is an extremely important and complex area. NMR is one of the techniques which combined with others may be helpful in characterizing these systems. Because of the complexity of the problems involved, the NMR literature on the subject is scarce and restricted to relatively model cases i.e. simple organic molecules intercalated in layered silicates. Such studies are however of interest as there is now evidence for the interlayer formation of highly complex substances (humin-like) at the expense of smallersized molecules in near natural conditions (Cloos et al., 1981). Although the use of x-ray diffraction has elucidated considerable detail about these intercalates, detailed information concerning the physics and chemistry of these systems has to be sought by techniques such as IR, EPR, neutron scattering and UV/visible spectroscopy (see the various chapters in this book). A few examples of the use of NMR are given below. Having determined by one-dimensional electron density maps the position of tetrahydrofuran molecules in a series of different cationexchanged montmorillonites, Adams and Breen (1978) investigated by H+ NMR the behaviour of this same molecule on a synthetic Na-fluorohectorite. This support was chosen so as to reduce the influence of paramagnetic impurities and structural hydroxyls on the proton signal. The linewidth of the spectra were studied as function of temperature in order to obtain some information concerning the motion of the interlamellar species (see sections 1.4 and 2.2). Motional narrowing was effective down to 210'K but the temperature at which molecular motion sets in depended on the water content of the material: the lower the water content, the higher this particular temperature. At room temperature, the organic molecules are extremely mobile inasmuch thatthe smaller contribution of the water molecules to the line could not be separated from that of the organic molecules. The precise type of motion was not determined but an estimate for the activation energy was found to be around 5 kcal/mole. In a effort to compare x-ray results with information obtained by NMR, Stohrer and Noack (1975) measured H+ spin-lattice relaxation times as function of temperature and frequency for n-alkylammonium-beidellite + n hexadecanol samples. These results were compared with similar measurements performed on pure odd-numbered alkanes considered as reference model for the intercalated organic molecules. A detailed evaluation of the relaxation mechanisms however revealed quite large differences 3.3.

108

in the behaviour of the two systems.

For the pure alkane system,

two transition temperatures are observed, one corresponding to a rotational phase transition the other to the melting point. For the intercalated beidellite, only one transition temperature, at a higher value, is observed indicative of the modifications brought about by the restricted interlamellar space. It is well known that for liquid organic systems C13 NMR spectro+ scopy, compared to H NMR, gives better resolved spectra (because the chemical shifts are larger and the internuclear broadenings smaller) For highly mobile surface species, C1 NMR techniques appropriate to liquids can still be used. Nevertheless, when the adsorbed species are relatively immobile, the residual molecular motion may be too slow to sufficiently reduce proton-carbon dipolar

.

interactions in order to observe well resolved lines. As it was hinted at in section 2.1.1, high resolution spectra for such systems can now be obtained; the main source of line broadening, in this case, is the chemical shift anisotropy. Chemical shifts are in fact tensors whose principal axes are directly related to the molecular axis system.

In the case of axial symmetry, there is a one-to-one

correspondence between shift and molecular orientation. A study of the chemical shift can therefore be used to obtain structural information for molecules adsorbed on orientable surfaces. An interesting paper by Kesing et al. (1980) considers in detail the use of this technique in the case of benzene adsorbed on an oriented Agexchanged hectorite sample. The spectra were examined as function of temperature and for various orientations of the platelets in the applied magnetic field. The conclusions are that the benzene molecules "stand" on edge between the clay layers with their hexad axis tipped up, out of the ab plane, by about 15'. The molecules are submitted to a double rotation: one about the hexad axis and one about the normal to the clay. This latter motion is completely quenched at 77'K but not the former. A close examination of the line reveals several types of disorder present in the clay-benzene systems some of which are linked to the turbostratic character of the clay. These new techniques have therefore proven to be of great potential interest for the study of this kind of clay system. One of the drawbacks is still, at the moment, sensitivity. Below 2 0 0 m2/g, C13 enrichment of the molecules is necessary.

The study of humic material, as such, is also now been carried out by high resolution liquid C13 NMR. The possibility of differentiating humic acids of various originsfthe determination of the

109

aromaticity factor and the analysis of the various present chemical groups are being investigated. Some recent papers concerning this subject have been published by Wilson et al. (1978), Dereppe et al. (1980) and Ruggiero et al. (1979). 4.

REFERENCES

Abragam, A., 1961. The principles of nuclear magnetism. Oxford University Press. Adams, J.M. and Breen, C., 1978. Surface and Intercalate Chemistry of layered silicates. VIII. A study of synthetic hectorite : Tetrahydrofuran intercalate by NMR. J. Chem. Research ( S ) , 172-173. Adams, J.M. and Riekel, C., 1980. One-dimensional neutron diffraction study of a vermiculite. Clays Clay Minerals, 28: 444-445. Alcover, J.F. and Gatineau, L., 1980. Structure de l'espace interlamellaire de la vermiculite Mg bicouche. Clay Minerals, 15 : 25-35. Ananyan, A.A., 1978. Study of non-freezing water in clays. Colloid J. USSR, 40: 1165-1168. Anderson, D.M., 1967. The interface between ice and silicate surfaces. J. Colloid Interface Sci., 25: 174-191. Anderson, D.M. and Morgenstern, N.R., 1973. Physics, chemistry and mechanics of frozen ground : a review. In "Permafrost. Second International Conference". Nat. Acad. of Sci. Washington D.C., pp 257-288. Banin, A. and Anderson, D.M., 1975. A similar law may govern water freezing in minerals and living organisms. Nature, 255: 261-262. Brownstein, K.R. and Tarr, C.E., 1977. Spin-lattice relaxation in a system governed by diffusion. J. Magn. Reson., 26: 17-24. Calvet, R., 1973. Hydratation de la montmorillonite et diffusion des cations compensateurs. Ann. Agron., 24: 73-214. Cloos, P., Badot, C. and Herbillon, A., 1981. Interlayer formation of humin in smectites. Nature (to be published). Conard, J., 1976 a. Structure of H 2 0 and hydrogen bonding on clays studied by Li7 and H 1 NMR. In "Magnetic resonance in colloid and interface Science". Resing, H.A. and Wade, C.G. (Editors). ACS Symposium Series 34, pp 85-93. Conard, J., 1976 b. Etude structurale de l'eau adsorbde sur l'hecIn "Proceedings International Clay Conference torite Li par RMN. 1975". Bailey, S.W. (Editor). Appl. Publ. Ltd., p~ 221-230. Cruz, M.I., Letellier, M. and Fripiat, J.J., 1978. NMR study of adsorbed water. 11. Molecular motions in the monolayer hydrate of halloysite. J. Chem. Phys., 69: 2018-2027. Dereppe, J.M., Moreaux, C. and Debyser, Y., 1980. Investigation of marine and terrestrial humic substances by H1 and C13 NMR and I.R. Organic Geochemistry, 2: 117-124. Drost-Hansen, W., 1969. Structure of water near solid interfaces. Ind. Eng. Chem., 61: 10-47. Eger, I., Cruz-Cumplido, M.I. and Fripiat, J.J., 1979. ouelques donndes sur la capacit6 calorifique et les propri6t6s de l'eau dans divers systPmes poreux. Clay Minerals, 14: 161-172. Eisenberg, D. and Kauzmann, W., 1969. The structure and properties of water. Clarendon Press, Oxford. Farmer, V.C., 1978. Water on particle surfaces. In : "The Chemistry of Soil Constituents.'' Greenland,D.J. and Hayes, M.H.B. (Editors). Wiley, pp 405-448.

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Farrar, T.C. and Becker, E.D., 1 9 7 1 . Pulse and Fourier Transform NMR. Academic Press. Fripiat, J.J. and Touillaux, R., 1 9 6 9 . Proton mobility in solids. Trans. Faraday SOC., 6 5 : 1 2 3 6 - 1 2 4 7 . Fripiat, J.J., 1 9 7 6 . The NMR study of proton exchange between adsorbed species and oxides and silicates surfaces. In "Magnetic resonance in colloid and interface science". Resing, H.A. and Wade, C.G. (Editors). ACS Symposium Series 3 4 , pp 2 6 1 - 2 7 4 . Fripiat, J.J., Kadi-Hanifi, M., Conard, J. and Stone, W.E.E., 1 9 8 0 . NMR study of adsorbed water. 111. Molecular orientation and protonic motions in the one-layer of a Li-hectorite. In : "Magnetic resonance in colloid and interface science". Fraissard, J.P. and Resing, H.A. (Editors). Reidel Publ. C o , pp 5 2 9 - 5 3 5 . Fripiat, J . J . , 1 9 8 0 a. The application of NMR to the study of clay minerals. In: "Advanced chemical methods for soil and clay minerals research". Stucki, J.W. and Banwart, PJ.L. (Editors). Reidel Publ. C o , pp 2 4 5 - 3 1 5 . Fripiat, J.J., 1 9 8 0 b. Organisation des molecules d'eau dans les silicates de grande surface specifique. Bull. Mineral., 1 0 3 : 440-443.

Fripiat, J.J., 1 9 8 0 c. Private communication. Gastuche, M.C., Toussaint, F., Fripiat, J.J., Touillaux, R. and Van Meersche, M., 1 9 6 3 . Study of the intermediate stages in the kaolin + metakaolin transformation. Clay Minerals Bulletin, 5 : 227-236.

Giese, R.F. and Fripiat, J.J., 1 9 7 9 . Water molecule positions, orientations and motions in the dihydrates of Plg and Na-vermiculites. J. of Coll. Interf. Sci., 7 1 : 4 4 1 - 4 5 0 . Hall, P.L., ROSS, D.K., Tuck, J.J. and Hayes, M.H.B., 1 9 7 9 . Neutron scattering studies of the dynamics of interlamellar water in montmorillonite and vermiculite. In: "Proceedings of the Intern. Clay Conference. Oxford". Mortland, M.M. and Farmer, V.C. (Editors). Elsevier, pp 1 2 1 - 1 3 0 . Hawkins, R.K. and Egelstaff, P.A., 1 9 8 0 . Interfacial water structure in montmorillonite from neutron diffraction experiments. Clays Clay Minerals, 2 8 : 1 9 - 2 8 . Hecht, A.M. and Geissler, E., 1 9 7 3 . Nuclear spin relaxation in a single and double layer system of adsorbed water. J. Coll. Interf. Sci., 4 4 : 1 - 1 2 . Hendricks, S.B. and Jefferson, M.E., 1 9 3 8 . Structures of kaolin and talc pyrophyllite hydrates and their bearing on water sorption. Am. Mineral., 23: 8 6 3 - 8 7 5 . Hougardy, J., Stone, W.E.E. and Fripiat, J.J., 1 9 7 6 . NMR study of adsorbed water. I. Molecular orientation and protonic motions in the two-layer hydrate of a Na vermiculite. J. Chem. Phys., 64:

3840-3851.

Hougardy, J., Stone, W.E.E. and Fripiat, J.J., 1 9 7 7 . Complex proton NMR spectra in some ordered hydrates of vermiculites. J. Magn. Reson., 2 5 : 5 6 3 - 5 6 7 . Kadi-Hanifi, M., 1 9 8 0 . Proton NMR studies of one layer hydrates of oriented hectorite. Clays Clay Minerals, 2 8 : 6 5 - 6 6 . Kalinichenko, A.M., Matyash, I.V., Rozhdestvenskaya, I.V. and Frank, V.A., 1 9 7 4 . Refinement of the structural characteristics of chernykhite by NMR. Sov. Phys. Crystall., 1 9 : 7 0 - 7 1 . Keren, R. and Shainberg, I., 1 9 7 5 . Water vapour isotherms and heat of immersion of Na/Ca-montmorillonite systems. I. Homoionic clay. Clays Clay Minerals, 2 3 : 1 9 3 - 2 0 0 . Keren, R. and Shainberg, I., 1 9 8 0 . Water vapour isotherms and heat of immersion of Na/Ca-montmorillonite systems. 111. Thermodynamics. Clays Clay Minerals, 2 8 : 2 0 4 - 2 1 0 .

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Kohler, E.E. and Burkert, P.K., 1976. Kernmagnetische resonanzmessungen an glaukoniten. Clay Minerals, 11: 303-310. Kvlividze, V.I. and Krasmuskin, A.V., 1975. Mobility of H20 on the surface of clay minerals with 0 1 7 NMR. Dokl. Aka. Nauk. SSSR, 222: 388-392. Lagaly, G. and Weiss, A., 1975. The layer charge of smectitic layer silicates. In: "Proceedings of the Intern. Clay Conference. Mexico" Bailey, S.W. (Editor). Appl. Publishing Ltd. ,pp 157-172. Low, P.F. and Margheim, J.F., 1979. The swelling of clay. I. Basic concept and empirical equations. Soil Sci. SOC. Am. J., 43: 473-481. Low, P.F., 1979. Nature and properties of water in montmorillonite water systems. Soil Sci.Soc. Am. J., 43: 651-658. Mamy, J., 1968. Recherches sur l'hydratation de la montmorillonite: proprietes dielectriques et structure du film d'eau. Ann. Agron., 19: 175-292. Matyash, I.V., Litovchenko, A.S. and Vasilev, N.G., 1974. NMR spectra of the exchange cation of montmorillonite at different degrees of hydration. Colloid J. USSR, 36: 576-578. Mehring, M., 1976. High resolution NMR in solids. In: NMR-Basic principles and progress. Springer-Verlag, Vol. 11. Meiboom, S., 1961. NMR study of the proton transfer in H2O. J. Chem. Phys., 34: 375-388. Okada, K., Morikawa, H., Iwai, S., Ohira, Y. and Ossaka, J., 1975. A structure model of allophane. Clay Sci., 4: 291-303. Ovcharenko, F.D., Brekhunets, A.G., Plank, V.V. and Pereverzeva, I.N., 1974. Relaxation and filtration characteristics of aqueous dispersions of kaolinite. Colloid J. USSR, 36: 1177-1179. Packer K.J., 1977. The dynamics of water in heterogeneous systems. Phi Trans. Roy. SOC. London, B278: 59-87. Prost, R., 1975. Etude de l'hydratation des argiles: interactions eau mineral et mdcanisme de la retention d'eau. Ann. Aqron., 26: 463-535. Prost, R., 1976. Interactions between adsorbed water molecules and the structure of clay minerals: hydration mechanism of smectites. In: "Proceedings Intern. Clay Conference 1975". Bailey,S.W., (Editor). Applied Publ. Ltd., pp 351-359. Ramberg, H., 1952. Chemical bonds and the distribution of cations in silicates. J. Geol., 60: 331-355. Resing, H.A., 1968. NMR Relaxation of molecules adsorbed on surfaces. Adv. Mol. Relaxation Processes, 1: 109-154. Resing, H.A. and Davidson, D.W., 1976. Commentary on the NMR apparent phase transition effect in natrolite, fluor-montmorillonite and other systems. Can. J. Phys., 54: 295-300. Resing, H.A., Stotfeld-Ellingsen, D., Garroway, A.N., Weber, D.C., Pinnavaia, T.J. and Unger, K., 1980. C13 chemical shifts in adsorption systems: molecular motions, molecular orientations, qualitative and quantitative analysis. In: "Magnetic resonance in colloid and interface science". Fraissard, J.P. and Resing, H.A. (Editors). Reidel Publ. C'. Ruggiero, P., Interesse, F.S. and Sciacovelli, O., 1979. H 1 and C 1 3 NMR studies on the importance of aromatic structures of fulvic and humic acids. Geoch. Cosmochimica Acta, 43: 1771-1775. Sanz, J. and Stone, W.E.E., 1977. NMR study of micas. I. Distribution of Fe2+ ions on the octahedral sites. J. Chem. Phys. , 67: 3739-3743. Sanz, J. and Stone, W.E.E., 1979. .NMR study of micas. 11. Distribution of Fez+, F- and OH- in the octahedral sheet of phlogopites. Am. Mineral., 64: 119-126.

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Scala, C.M., Hutton, D.R. and McLaren, A.C., 1978. NMR and EPR studies of the chemically intermediate plagioclase feldspars. Phys. Chem. Minerals, 3: 33-44. Shirozu, M. and Bailey, S.W., 1966. Crystal structure of a 2 layer Plg-vermiculite. Am. Mineral., 51: 1124-1143. Slichter, C.P., 1963. Principles of magnetic resonance. Haruer. New York. Stohrer, M. and Noack, F., 1975. Magnetische Relaxationsspektroskopie an gequollenen beidellit. Progr. Colloid and Polymer Sci., 57: 61-68. Stone, W.E.E. and Sanz, J., 1980. Distribution of ions in the octahedral sheet of micas. In: "Advanced chemical methods for soil and clay minerals research". Stucki, J.W. and Banwart, W.L. (Editors). Reidel Publ. C o , pp 317-329. Stul, M.S. and Mortier, W.J., 1974. The heterogeneity of the charge density in montmorillonites. Clays Clay Minerals, 22: 391-396. Suquet, H., Malard, C. and Pezerat, H., 1980. Etude des contenus en eau des saponites et vermiculites Na et Ca. Bull. Mineral., 103: 230-239. Tardy, Y., Lesniak, P., Duplay, J. and Prost, R., 1980. Energies d'hydratation des argiles. Application 5 l'hectorite. Bull. Mineral., 103: 217-223. Theng, B.K.G., 1974. The chemistry of clay-organic reactions. J. Wiley, Chapter 7. Touillaux, R., Salvador, P., Van der Meersche, C. and Fripiat, J.J., 1968. Study of H 2 0 layers adsorbed on Na and Ca montmorillonite by NMR. Israel J. Chem., 6: 337-348. Van Olphen, H., 1965. Thermodynamics of interlayer adsorption of water in clays. I. Na-vermiculite. J. Coll. Interf. Sci., 20: 822-837. Van Vleck, J.H., 1948. The dipolar broadening of magnetic resonance lines in crystals. Phys. Rev., 74: 1168-1183. Vivien D., Stegmann, M. and Mazieres, C., 1973. Contribution 5 l'gtude par IR et RMN des hydroxydes d'aluminium. J. Chimie Phys., 70: 1502-1508. Walmsley, R.H. and Shporer, M., 1978. Surface induced NMR line splittings and augmented relaxation rates in water. J. Chem. Phys., 68: 2584-2590. Wilson, M.A., Jones, A.J. and Williamson, B., 1978. NMR spectroscopy of humic materials. Nature, 276: 487-488. Woessner, D.E. and Snowden, B.S., 1969 a. NMR doublet splitting in aqueous montmorillonite gels. J. Chem. Phys., 50: 1516-1523. Woessner, D.E. and Snowden, B.S., 1969 b. A study of the orientation of adsorbed water molecules on montmorillonite clays by pulsed NMR. J. Coll. Interf. Sci., 30: 54-68. Woessner, D.E., 1974. Proton exchange effects on pulsed NMR signals from preferentially oriented H 2 0 molecules. J. Elagn.Reson., 16: 483-501. Woessner, D.E., 1975. NMR studies of preferentially oriented water at interfaces. In: Mass Spectrometry and NMR Spectroscopy in pesticide chemistry. Haque, R. and Biros, F.J. (Editors). Plenum Press, pp. 279-304. Woessner, D.E., 1977. Nuclear magnetic relaxation and structure in aqueous heterogeneous systems. Molec. Phys., 34: 899-920. Woessner, D.E., 1980. An NMR investigation into the range of the surface effect on the rotation of water molecules. J. Magn. Reson., 39: 297-308. Zimmerman, J.R. and Britten, W.E.,1957. NMR studies in multiple phase systems: lifetime of a H 2 0 molecule in an adsorbing phase on silicagel. J. Phys. Chem., 67: 1328-1333.

113 Chapter 5

MOSSBAUER SPECTROSCOPY

B.A.

GOODMAN

Department o f S p e c t r o c h e m i s t r y , The Macaulay I n s t i t u t e f o r S o i l Research C r a i g i e b u c k l e r , Aberdeen, AB9 2QJ, S c o t l a n d . 5 . 1 INTRODUC I ON The aim o

t h i s c h a p t e r i s t o p r e s e n t an a c c o u n t o f t h e c u r r e n t a p p l i c a t i o n s

o f Mossbauer s p e c t r o s c o p y i n t h e s t u d y o f c l a y m i n e r a l s . I t i s n o t i n t e n d e d as a r e v i e w a r t c l e , p a r t l y because a comprehensive r e v i e w a r t i c l e would be beyond t h e scope o f t h i s book and, more i m p o r t a n t l y , because t h e Mossbauer 1 i t e r a t u r e has been we1 -covered elsewhere. A p a r t f r o m t h e Mossbauer E f f e c t Reference and Data J o u r n a l (Stevens e t a l . ) , which l i s t s a l l papers p u b l i s h e d on Mossbauer spectroscopy, a number o f r e v i e w s o f a p p l i c a t i o n s t o t h e s t u d y o f c l a y m i n e r a l s has

been p u b l i s h e d i n r e c e n t y e a r s (e.g.

Heller-Kallai,

B a n c r o f t , 1973; 1979; Coey, 1975;

1980; Goodman, 1980). Thus, r e f e r e n c e s used h e r e w i l l be t o

i l l u s t r a t e t h e d i s c u s s i o n o r t o g i v e examples o f areas o f a p p l i c a t i o n , and a l a r g e number o f i m p o r t a n t p u b l i c a t i o n s w i l l n o t be r e f e r r e d t o . For t h i s I a p o l o g i s e i n advance, b u t , a p a r t f r o m i n d i c a t i n g t h e g e n e r a l areas i n which Mossbauer s p e c t r o s c o p y has p r o v e d u s e f u l , i t i s my i n t e n t i o n t o d e v o t e some o f t h e space a v a i l a b l e t o a c r i t i c a l e x a m i n a t i o n o f some o f t h e approaches t h a t have been g e n e r a l l y adopted and t o i n d i c a t e some e x p e r i m e n t s t h a t s h o u l d be performed i n t h e f u t u r e .

I n o r d e r t o do t h i s s a t i s f a c t o r i l y , i t will a l s o be

necessary t o i n c l u d e an account o f some aspects o f t h e t h e o r y , so t h a t t h i s c h a p t e r can be r e a d on i t s own w i t h o u t c o n t i n u a l r e f e r e n c e s t o t e x t s on t h e subject. Mossbauer s p e c t r o s c o p y i s a branch o f a b s o r p t i o n s p e c t r o s c o p y i n which t r a n s i t i o n s between t h e energy l e v e l s i n an a t o m i c n u c l e u s a r e s t u d i e d by means o f t h e r e s o n a n t a b s o r p t i o n o f y - r a d i a t i o n . A l t h o u g h w i d e l y separated, t h e n u c l e a r energy l e v e l s a r e v e r y p r e c i s e l y d e f i n e d and t h e i r changes w i t h v a r i a t i o n s i n t h e environment o f t h e n u c l e u s a r e v e r y s m a l l . I t i s t h u s necessary t o use r a d i a t i o n t h a t has an e x c e e d i n g l y s m a l l spread o f e n e r g i e s i n o r d e r t o observe t h e h y p e r f i n e s t r u c t u r e i n Mossbauer s p e c t r a . T h i s i s achieved b y u s i n g as t h e e x c i t i n g r a d i a t i o n y - r a d i a t i o n t h a t has been e m i t t e d b y n u c l e i i n t h e i r e x c i t e d s t a t e s as t h e y decay t o t h e i r ground s t a t e s . T h i s energy may t h e n be modulated by i m p a r t i n g a Doppler v e l o c i t y t o the- source n u c l e u s and a Mossbauer spectrum i s o b t a i n e d as t h e v a r i a t i o n o f t h e percentage a b s o r p t i o n o r t r a n s m i s s i o n r e l a t i v e t o t h e magnitude o f t h e Doppler v e l o c i t y . F o r t h i s reason v e l o c i t y (mm s - l o r cm s

-1

114 i s c o n v e n t i o n a l l y used as t h e energy u n i t i n Mossbauer spectroscopy. Several f a c t o r s l i m i t t h e number o f i s o t o p e s t h a t e x h i b i t t h e Mossbauer e f f e c t o r can be c o n v e n i e n t l y s t u d i e d by i t . F i r s t l y , i t i s necessary f o r a s o u r c e t o e x i s t t h a t can decay t o produce an i s o t o p e o f t h e n u c l e u s under i n v e s t i g a t i o n i n an e x c i t e d s t a t e . T h i s i m m e d i a t e l y e l i m i n a t e s a l l elements o f l o w a t o m i c number (i.e.

<19). Secondly, f o r r o u t i n e a p p l i c a t i o n s ,

i t i s necessary t h a t t h e h a l f -

l i f e of t h e source n u c l e u s i s a t l e a s t s e v e r a l days and p r e f e r a b l y months o r years. T h i r d l y , f o r a p p l i c a t i o n s i n c l a y mineralogy, the n a t u r a l occurrence o f t h e Mossbauer i s o t o p e s h o u l d be h i g h enough f o r t h e r e s o n a n t a b s o r p t i o n t o be detected w i t h a conventional spectrometer (i.e. F o u r t h l y , on e m i s s i o n o f t h e y - r a y ,

>O.OlX o f t h e t o t a l material).

t h e source n u c l e u s tends t o r e c o i l ( c o n s e r v -

a t i o n o f momentum), and t h e Massbauer e f f e c t can o n l y be observed i f t h i s r e c o i l energy can be spread o v e r t h e l a t t i c e as a whole, by c o u p l i n g w i t h t h e l a t t i c e v i b r a t i o n s . Thus, a f r a c t i o n o f e m i s s i o n s can o c c u r w i t h o u t n u c l e a r r e c o i l and i t i s t h e s e y - r a y s t h a t produce t h e Mossbauer spectrum. Since a s i m i l a r process

o c c u r s on a b s o r p t i o n , t h e r e c o i l - f r e e f r a c t i o n ( o r f - f a c t o r ) f r o m t h e source and a b s o r b e r must be as h i g h as p o s s i b l e i n o r d e r t o observe a w e l l - d e f i n e d G s s b a u e r spectrum. The problem o f r e c o i l i n c r e a s e s w i t h i n c r e a s i n g energy o f t h e y - i r a d i a t i o n and an upper l i m i t o f about 140 KeV e x i s t s f o r t h e o b s e r v a t i o n o f t h e Mossbauer e f f e c t . However, f o r e n e r g i e s g r e a t e r t h a n about 50 KeV i t i s necessary t o work a t l o w temperatures ( i . e .

b o i l i n g p o i n t o f l i q u i d helium) i n order t o

observe r e c o i l l e s s e m i s s i o n and a b s o r p t i o n . F i n a l l y , t h e w i d t h o f t h e Mossbauer a b s o r p t i o n peak i s determined b y t h e l i f e t i m e o f t h e n u c l e a r e x c i t e d s t a t e a c c o r d i n g t o t h e Heisenberg u n c e r t a i n t y p r i n c i p l e : I'T

=

where

h/2n r i s the l i n e width,

(1) T

i s t h e mean l i f e o f t h e e x c i t e d s t a t e and h i s

P l a n c k ' s c o n s t a n t . Thus a s h o r t h a l f - l i f e o f t h e e x c i t e d s t a t e l e a d s t o broad a b s o r p t i o n peaks. A f t e r t a k i n g a c c o u n t o f t h e above-mentioned f a c t o r s , t h e o n l y i s o t o p e s u i t a b l e f o r r o u t i n e i n v e s t i g a t i o n i n c l a y m i n e r a l s i s 57Fe, which has a n a t u r a l abundance o f 2.245% o f t h e i r o n . The n u c l e a r p r o p e r t i e s o f 57Fe a r e such t h a t i t s f i r s t e x c i t e d s t a t e , which has a s p i n o f 3/2, l i e s a p p r o x i m a t e l y 14.4 KeV above i t s ground s t a t e , which has a s p i n o f 1/2.

I n t e r a c t i o n s w i t h t h e environment o f t h e n u c l e u s can cause t h e s e

energy l e v e l s t o s h i f t o r s p l i t as shown i n F i g . 5.1. The s h i f t i n t h e c e n t r e of t h e absorption from t h a t o f a reference (Fig. 5.la) 6, t h e d o u b l e t s p l i t t i n g ( F i g . 5 . l b )

i s known as t h e isomer s h i f t ,

as t h e e l e c t r i c quadrupole i n t e r a c t i o n , o r

quadrupole s p l i t t i n g , A , and t h e s e x t e t s p l i t t i n g ( F i g . 5 . 1 ~ ) as t h e magnetic hyperfine interaction. S p e c t r a can be observed f r o m e i t h e r t r a n s m i t t e d o r s c a t t e r e d r a d i a t i o n . The f o r m e r i s used i n most c o n v e n t i o n a l arrangements b u t b a c k - s c a t t e r e x p e r i m e n t s a r e u s e f u l i n t h e s t u d y o f s u r f a c e r e g i o n s o f a sample and a r e o f p a r t i c u l a r

115 value i n corrosion studies.

-6; m

I

m

I

7

7-

nl

-2

- -1 2

-2

1

Y d

C

F i g . 5.1 The h y p e r f i n e i n t e r a c t i o n s i n Mb'ssbauer spectroscopy. ( a ) The isomer s h i f t s 6 , ( b ) t h e WadruPOle s p l i t t i n g , A , ( c ) t h e m a g n e t i c h y p e r f i n e i n t e r a c t i o n , and ( d ) combined magnetic and quadrupole i n t e r a c t i o n s . 5 . 2 B A S I C THEORY 5.2.1

Isomer s h i f t , 6

The isomer s h i f t observed i n a Mossbauer e x p e r i m e n t i s determined by t h e e l e c t r o n d e n s i t y a t t h e nucleus, and i s g i v e n b y : 6 = (2*/5) Ze 2 i l $-( ~ ) 1 2 a - 1 ~ ( o 2) rl 1 (Re 2 - Rg 2 1 where Z i s t h e a t o m i c number, e i s t h e e l e c t r o n i c charge,

(2) 1'

and

1'

a (0) r a r e t h e e l e c t r o n d e n s i t i e s a t t h e n u c l e u s o f t h e a b s o r b e r and r e f e r e n c e m a t e r i a l

Re and R a r e t h e r a d i i o f t h e n u c l e a r e x c i t e d and ground s t a t e s , 9 r e s p e c t i v e l y . I n i r o n t h i s e l e c t r o n d e n s i t y can v a r y by two mechanisms: ( i ) d i r e c t respectively,and

changes i n 4s e l e c t r o n d e n s i t y t h r o u g h t h e i n v o l v e m e n t o f 4s o r b i t a l s i n m o l e c u l a r o r b i t a l s and ( i i ) i n d i r e c t changes i n t h e e l e c t r o n d e n s i t y o f t h e c o r e s - e l e c t r o n s Mechanism ( i ) can sometimes be ( i . e . 15, 2s and p a r t i c u l a r l y 3s e l e c t r o n s ) . i m p o r t a n t i n t h e h i g h l y c o v a l e n t l o w s p i n compounds b u t i s u s u a l l y small when i r o n i s i n t h e h i g h s p i n s t a t e . Thus, i n s i l i c a t e minerals,mechanism

( i i ) i s the pre-

dominant source o f t h e isomer s h i f t . -It o c c u r s because t h e r a d i a l d i s t r i b u t i o n s o f a t o m i c o r b i t a l s a r e such t h a t e l e c t r o n s i n t h e 3d o r b i t a l s spend a f r a c t i o n of t h e i r time c l o s e r t o t h e nucleus than electrons i n t h e core s - o r b i t a l s .

When t h i s

occurs, t h e a t t r a c t i v e Coulomb p o t e n t i a l between t h e n u c l e u s and t h e s - e l e c t r o n s

116 decreases. Therefore, decreases I $ t e r m Re2-R

1'

any i n c r e a s e i n t h e number o f d - e l e c t r o n s ,

effectively

and r e s u l t s i n a change i n 6, which i s p o s i t i v e because t h e

2 ( y ~e q u a t i o n ( 2 ) i s n e g a t i v e . The number o f d - e l e c t r o n s on t h e i r o n

9 i s determined n o t o n l y b y t h e o x i d a t i o n s t a t e b u t a l s o by t h e n a t u r e o f any molec-

u l a r o r b i t a l s i n which t h e d - e l e c t r o n s a r e i n v o l v e d . 5.2.2

The e l e c t r i c quadrupole i n t e r a c t i o n

I f t h e n u c l e u s has a n o n - u n i f o r m charge d e n s i t y , which i s t h e case when t h e

nuclear spin, I >

3,

then t h e energy l e v e l s may be s p l i t by an i n t e r a c t i o n w i t h

a non-cubic e l e c t r i c f i e l d g r a d i e n t . The magnitude o f t h e s p l i t t i n g , i l l u s t r a t e d i n Fig. 5 . l b , i s

k n o w n a s t h e quadrupole s p l i t t i n g , A , and i s p r o p o r t i o n a l t o t h e

p r i n c i p a l component o f t h e e l e c t r i c f i e l d g r a d i e n t t e n s o r . Thus, 2 A = $e q Q ( 1 + n2/3)'

(3) where Q i s t h e n u c l e a r quadrupole moment, e i s t h e charge on a p r o t o n , -eq i s t h e z-component o f t h e e l e c t r i c f i e l d g r a d i e n t t e n s o r (Vzz) parameter and i s equal t o (Vxx

-

Vyy)/Vzz.

and II i s t h e a s y m e t r y

Axes a r e c h o s e n so t h a t 0

n

Q

1. The

e l e c t r i c f i e l d g r a d i e n t i s composed o f terms f r o m t h e valence e l e c t r o n s and f r o m t h e charge d i s t r i b u t i o n i n t h e s u r r o u n d i n g c r y s t a l l a t t i c e . Thus, =

(1

where

q l a t t + (1 - R ) q v a l and R a r e t h e S t e r n h e i m e r f a c t o r s (Sternheimer, 1963) and qlatt

-Ym) y,

(4) and qval

a r e c o n t r i b u t i o n s f r o m t h e c r y s t a l l a t t i c e and t h e v a l e n c e e l e c t r o n s , r e s p e c t i v e l y The p r i n c i p a l ( V z z ) components o f t h e qval

terms a r e g i v e n i n T a b l e 5.1 f o r t h e

five d orbitals. TABLE 5.1 The c o n t r i b u t i o n s t o Vzz o f an e l e c t r o n i n each o f t h e 3d o r b i t a l s ( i n t h e i r usual forms each o f t h e s e o r b i t a l s has Wavefunction 1

d L z2 2 dx -Y d XY dxz dYz

0

= 0).

Vzz*

41 7 -417 -4/7

217 2/7

* i n u n i t s o f e < r-3 >, where e i s t h e charge o f an e l e c t r o n and i s t h e mean v a l u e o f r-3 f o r t h e o r b i t a l s

117 Ift h e l a t t i c e t e r m can be c o n s i d e r e d as a r i s i n g f r o m a s e t o f p o i n t charges, t h e n

f o r each charge q a t a d i s t a n c e r f r o m t h e n u c l e u s -3 2 2 V x x = q r ( k i n ecos Q - 1 ) 2 . 2 v = qr-3(3sin esin 4 - 1) yy -3 2 Vzz = q r ( k o s e - 1 ) vXy = vyx = q r - 3 ( 3 s i n 2 e s i n Q c o s $ )

vXz

=

vYz =

vZx v ZY

= qr-3(3sinecosecosQ) = qr-3(3sinecosesin~)

where t h e p q l a r c o o r d i n a t e s r,

e

(5) and ,$ have t h e i r usual meaning.

Quadrupole i n t e r a c t i o n i n Fe3+. As t h e h i g h s p i n Fe3+ i o n has 1 e l e c t r o n i n each o f the d-orbitals,

t h e t o t a l qval

t e r m i s zero. Thus, f o r a c o m p l e t e l y i o n i c

s i t u a t i o n , t h e quadrupole s p l i t t i n g would be d e t e r m i n e d s o l e l y by l a t t i c e terms. However, i n a r e a l s i t u a t i o n t h e bonds w i l l be p a r t l y c o v a l e n t and t h e p o p u l a t i o n s o f these d - o r b i t a l s w i l l no l o n g e r g e n e r a l l y be u n i t y , and, f o r any s i t u a t i o n i n which t h e Fe3+ i o n i s n o t bound t o i d e n t i c a l groups i n c u b i c symmetry, t h e r e may be a c o n t r i b u t i o n t o A f r o m valence terms. W i t h low s p i n Fe3+ a non-zero qval i s expected (see T a b l e 5.1),but t h e s i t u a t i o n i s c o m p l i c a t e d b y t h e h i g h degree o f covalency i n t h e m o l e c u l a r o r b i t a l s o f such s p e c i e s . Quadrupole i n t e r a c t i o n i n Fez+. The h i g h s p i n Fez+ i o n has 1 e l e c t r o n o v e r t h e h a l f - f i l l e d s e t o f d - o r b i t a l s and would, t h e r e f o r e , b e expected t o have a l a r g e valence e l e c t r o n c o n t r i b u t i o n t o A . T h i s i s t h e case f o r t h e ground s t a t e i o n , b u t u s u a l l y t h e r e i s an e x c i t e d e l e c t r o n i c s t a t e o f o n l y a s l i g h t l y h i g h e r energy t h a n t h e ground s t a t e which produces a c o n t r i b u t i o n t o A o f t h e o p p o s i t e s i g n . Thus, i f a t t h e t e m p e r a t u r e o f o b s e r v a t i o n o f t h e spectrum, an e l e c t r o n i c e x c i t e d s t a t e i s populated, A w i l l be composed of v a l e n c e terms f r o m b o t h s t a t e s , i n a d d i t i o n t o any l a t t i c e terms - a f a c t o r t h a t can c o m p l i c a t e t h e i n t e r p r e t a t i o n o f t h e spectrum (see s e c t i o n 5.3.3).

The l o w s p i n Fe2+ i o n would be expected t o have a z e r o qval

t e r m (see T a b l e ' 5 . 1 ) , l a r g e effect.

b u t as f o r

low s p i n Fe3+, c o v a l e n t i n t e r a c t i o n s may have a

However, s i n c e t h e l o w s p i n i o n s a r e e x t r e m e l y r a r e i n m i n e r a l s ,

t h e y w i l l n o t be c o n s i d e r e d f u r t h e r here. The i n t e n s i t i e s o f t h e two peaks o f a quadrupole s p l i t spectrum v a r y w i t h t h e angle, 8 , between t h e p r i n c i p a l a x i s o f t h e e l e c t r i c f i e l d g r a d i e n t and t h e d i r e c t i o n o f t h e y - r a y as shown i n

T a b l e 5.2.

Thus, f o r 0 = 0, t h e i n t e n s i t y r a t i o .

TABLE 5.2 Angular dependence o f t h e peaks i n a q u a d r u p o l e - s p l i t spectrum Transition

5

4 +&

3/2

+t++t

* e

Relative Intensity*

1 5/3

+

-

cos

2

e

2 cos e ~

_

_

_

i s t h e a n g l e between t h e y - r a y and t h e z - a x i s o f t h e e l e c t r i c f i e l d g r a d i e n t

118 o f t h e peaks i s 3:1,

whereas f o r

e

= 90°,

t h e r a t i o i s 3:5. Consequently, w i t h a

s i n g l e c r y s t a l , t h e d i r e c t i o n of t h e p r i n c i p a l a x i s o f t h e e l e c t r i c f i e l d g r a d i e n t can be r e a d i l y determined. I n a p o l y c r y s t a l l i n e sample w i t h randomly o r i e n t e d c r y s t a l l i t e s , a summation o v e r a l l angles i s r e q u i r e d and an i n t e n s i t y r a t i o o f 1 : l i s o b t a i n e d . I n most work on c l a y m i n e r a l s p o l y c r y s t a l l i n e samples a r e used and an i n t e n s i t y r a t i o o f 1 : l f o r t h e peaks i s o f t e n assumed. However, i f t h e r e i s p r e f e r e n t i a l o r i e n t a t i o n i n t h e a b s o r b e r t h e n t h e peaks may n o l o n g e r have t h e same i n t e n s i t y , and l a c k o f r e c o g n i t i o n by t h e r e s e a r c h e r can l e a d t o t h e number of components i n t h e spectrum b e i n g i n c o r r e c t l y r e p o r t e d . However, i t has been shown by E r i c s o n and Wappling (1976) t h a t o r i e n t a t i o n o f t h e p l a n e o f t h e a b s o r b e r a t 54.7'

t o t h e d i r e c t i o n o f t h e y-beam r e n d e r s t h e peak i n t e n s i t y r a t i o s a g a i n

equal. 5.2.3

The magnetic h y p e r f i n e i n t e r a c t i o n

A c o u p l i n g o f t h e n u c l e a r magnetic moment w i t h any l o c a l o r a p p l i e d m a g n e t i c f i e l d a t t h e n u c l e u s causes a s p l i t t i n g o f t h e energy l e v e l s as shown i n F i g . 5 . l ~ Where t h e r e i s a combined magnetic and e l e c t r i c quadrupole i n t e r a c t i o n t h e n u c l e a r s p l i t t i n g s a r e d i r e c t l y r e l a t e d t o t h e combined terms ( F i g . 5 . l d ) ,

but u n l e s s t h e

a n g l e between t h e p r i n c i p a l axes o f t h e magnetic f i e l d and t h e e l e c t r i c f i e l d g r a d i e n t i s known, i t i s n o t p o s s i b l e t o d e t e r m i n e t h e magnitudes o f b o t h f r o m such a spectrum. The r e l a t i v e e n e r g i e s and i n t e n s i t i e s o f t h e t r a n s i t i o n s shown

i n F i g . 5 . l d a r e g i v e n i n T a b l e 5.3 f o r t h e case where t h e energy o f t h e m a g n e t i c h y p e r f i n e i n t e r a c t i o n i s much g r e a t e r t h a n t h a t o f t h e e l e c t r i c f i e l d g r a d i e n t . TABLE 5.3 R e l a t i v e e n e r g i e s and i n t e n s i t i e s o f t h e peaks i n a spectrum o f a sample h a v i n g a l a r g e magnetic f i e l d whose e n e r g i e s a r e p e r t u r b e d by t h e presence o f an e l e c t r i c f i e l d gradient. Transition

R e l a t i v e Energy

Relative Intensity

i s t h e a n g l e between t h e m a g n e t i c f i e l d and t h e p r i n c i p a l a x i s o f t h e e l e c t r i c f i e l d gradient e i s t h e a n g l e between t h e m a g n e t i c f i e l d and t h e y - r a y

JI

The parameters go and ge a r e t h e ground and e x c i t e d s t a t e gyromagnetic r a t i o s ,

119 respectively

-

t h e e n e r g y l e v e l s b e i n g s e p a r a t e d b y ggnH b y t h e magnetic f i e l d ,

ti, where 6, i s t h e n u c l e a r magneton. F o r a s i n g l e c r y s t a l when t h e a n g l e

t h e magnetic f i e l d and t h e y - r a y d i r e c t i o n i s Oo, a r e 3:0:1:1:0:3

and f o r e = 90'

e between

t h e i n t e n s i t y r a t i o s o f t h e peaks

t h e s e become 3:4:1:1:4:3.

Summing o v e r a l l angles

f o r a p o l y c r y s t a l l i n e sample g i v e s a r a t i o o f 3:2:1:1:2:3. 5.2.4

L i n e Shapes

Over a range o f e n e r g i e s , E , t h e a b s o r p t i o n , u, i s g i v e n b y + 4(E-Eo) 2 / r 2 J

u = uo[ 1

where Eo i s . t h e energy o f t h e t r a n s i t i o n ,

r

(6) i s d e f i n e d i n e q u a t i o n ( 1 ) and uo i s

given by u0

=-l.h;lcz.

2 1 , + 1 . 1

(7)

Eo'

21 t 1 1 t a 9 where h i s P l a n c k ' s c o n s t a n t , c i s t h e v e l o c i t y o f l i g h t ,

2n

o f t h e n u c l e a r e x i c t e d and ground s t a t e s , r e s p e c t i v e l y , and

Ie and Ia are the spins i s th;

internal

conversion c o e f f i c i e n t . The L o r e n t z i a n l i n e shape d e s c r i b e d by e q u a t i o n ( 6 ) i s s t r i c t l y v a l i d o n l y f o r i n f i n i t e l y t h i n absorbers. However, i n o r d e r t o o b t a i n a reasonable s i g n a l - t o - n o i s e r a t i o f a i r l y t h i c k absorbers a r e g e n e r a l l y used ( u s u a l l y between 0.05 and 0.2 mg 2 o f 57Fe p e r cm ) , and t h e t r u e l i n e s h a p e i s m o d i f i e d by s a t u r a t i o n e f f e c t s . N e v e r t h e l e s s , i t i s g e n e r a l l y assumed t h a t peaks a r e o f L o r e n t z i a n shape, and, i n t h e a n a l y s i s o f p a r t i a l l y r e s o l v e d s p e c t r a , t h i s may l e a d t o i n a c c u r a c i e s i n t h e e s t i m a t i o n o f t h e parameters ( i . e .

p o s i t i o n s , w i d t h s and i n t e n s i t i e s ) o f t h e

componrnt peaks. T h i s problem can b e overcome by t h e use o f t h e t r a n s m i s s i o n i n t e g r a l i n t h e c o m p u t a t i o n procedures (Shenoy

Gal. 1975), b u t

i t r e q u i r e s much

more computer t i m e t h a n t h e s i m p l e L o r e n t i z i a n l i n e shapes and has n o t so f a r been g e n e r a l l y adopted i n c l a y - m i n e r a l work. Another case where t h e L o r e n t z i a n shape does n o t h o l d i s f o r r e l a x a t i o n s p e c t r a i.e.

-

where t h e r a t e o f f l u c t u a t i o n o f t h e h y p e r f i n e parameters i s comparable w i t h

the time o f the t r a n s i t i o n

(a.10-7-10-9

s ) . An example o f t h e dependence o f

t h e shape o f magnetic h y p e r f i n e s t r u c t u r e on t h e r e l a x a t i o n t i m e i s g i v e n i n Fig.5.2. T h i s b e h a v i o u r may r e s u l t e i t h e r f r o m t h e i n v e r s i o n i n d i r e c t i o n o f t h e magnetic h y p e r f i n e f i e l d i n a paramagnetic m a t e r i a l as a r e s u l t o f a s p i n - f l i p process o r by t h e c o l l e c t i v e r e o r i e n t a t i o n o f t h e m a g n e t i c moment d i r e c t i o n i n v e r y small p a r t i c l e s o f m a g n e t i c a l l y - o r d e r e d m a t e r i a l s . T h i s l a t e r process i s v e r y i m p o r t a n t

i n t h e s t u d y o f t h e c l a y m i n e r a l components o f s o i l s where s m a l l p a r t i c l e s a r e

comnon and w i l l be r e f e r r e d t o i n more d e t a i l l a t e r .

120

iW T ’ q -10

10 -10 VELOCITY/rnm

10

5-l

Fig.5.2

The dependence o f t h e shape o f t h e magnetic h y p e r f i n e s t r u c t u r e on r e l a x -9 a t i o n time, ( a ) t = 1 0 - l 2 s , ( b ) t = lO-’s, ( c ) t = 2.5 x lO-’s, ( d ) t = 5 x 10 s , ( e ) t = 7.5 x lO-’s, ( f ) t = 2.5 x 10-8s, (adapted f r o m Wickman, 1966).

5.2.5

-8

( 9 ) t = 7.5 x 10

s (h) t =

A b s o r p t i o n areas and t h e r e c o i l - f r e e f r a c t i o n The magnitude o f t h e resonance a b s o r p t i o n i s dependent on t h e e f f e c t i v e t h i c k -

ness, t, o f t h e absorber.

t =

(8)

nfoO

where n i s t h e number of atoms o f t h e MEssbauer i s o t o p e p e r u n i t area, f i s t h e r e c o i l - f r e e f r a c t i o n and u0 i s t h e a b s o r p t i o n c r o s s s e c t i o n as d e f i n e d i n e q u a t i o n ( 7 ) . On t h e assumption t h a t a L o r e n t z i a n shape i s v a l i d t h e a r e a under t h e a b s o r p t i o n peak i s g i v e n by

(9)

A = (n/2)fsrt where fs i s t h e r e c o i l - f r e e f r a c t i o n o f t h e source. F o r f a i r l y t h i n absorbers

2 ( < about 0.1 mg 57Fe/crn ) t h e L o r e n t z i a n shape i s a r e a s o n a b l e a p p r o x i m a t i o n t o

t h e e x p e r i m e n t a l peaks, b u t t h e l i n e w i d t h i s dependent upon t h e a b s o r b e r t h i c k n e s s . Thus , a c c o r d i n g t o B a n c r o f t ( 1 973)

rex = ra + rs

t

0.27 r t

(10)

121 where rex i s t h e e x p e r i m e n t a l peak w i d t h and ra and

rs

are the widths f o r t h i n

a b s o r b e r and source, r e s p e c t i v e l y . The e x p e r i m e n t a l l i n e w i d t h s may be broadened b y a number o f f a c t o r s o t h e r t h a n t h e a b s o r b e r t h i c k n e s s and some o f these can be e x t r e m e l y i m p o r t a n t i n t h e s t u d y o f c l a y m i n e r a l s . I n p a r t i c u l a r t h e e x i s t e n c e o f i n h o m o g e n e i t i e s i n t h e sample r e s u l t s i n t h e p r o d u c t i o n o f a range o f components which may n o t be m u t u a l l y r e s o l v e d . Broadened peaks, which may no l o n g e r approximate t o L o r e n t z i a n shape, r e s u l t and, i f t h e problem i s n o t r e c o g n i z e d , may l e a d t o erroneous c o n c l u s i o n s when c o m p u t e r - f i t t i n g o v e r l a p p i n g peaks. The magnitude o f t h e f - f a c t o r can v a r y a p p r e c i a b l y f r o m one sample t o a n o t h e r and may be expressed as 2 2 2 <x >/A )

f = exp (-4n

(11 1 where A i s t h e wavelength o f t h e y - r a y and <x2> i s t h e mean square d i s p l a c e m e n t o f t h e Mossbauer atom f r o m i t s e q u i l i b r i u m p o s i t i o n under thermal v i b r a t i o n . The f-factor,

t h e r e f o r e , v a r i e s w i t h t e m p e r a t u r e and decreases r a p i d l y a t h i g h temper-

a t u r e s . A l s o <x2 > may v a r y . a l o n g d i f f e r e n t d i r e c t i o n s i n a c r y s t a l ( G o l d a n s k i i e t al.,

1963), w i t h t h e r e s u l t t h a t unequal peak h e i g h t s can be o b t a i n e d i n a

quadrupole s p l i t spectrum f r o m a randomly o r i e n t e d p o l y c r y s t a l l i n e sample. F o r t u n a t e l y , t h i s phenomenon i s q u i t e r a r e and i s b e l i e v e d n o t t o be i m p o r t a n t i n t h e s t u d y o f c l a y m i n e r a l s a t ambient and l o w e r temperatures. 5.3 APPLICATIONS I N THE STUDY OF CLAYS

I n t h i s , t h e main s e c t i o n o f t h i s c h a p t e r , t h e p r i n c i p a l t y p e s o f a p p l i c a t i o n o f Mossbauer s p e c t r o s c o p y w i l l be discussed. F o r convenience t h e s e have been grouped under 3 main headings:( i ) q u a l i t a t i v e a n a l y s i s , which w i l l be concerned w i t h t h e i d e n t i f i c a t i o n o f o x i d a t i o n s t a t e s o r p a r t i c u l a r m i n e r a l phases; ( i i ) q u a n t i t a t i v e a n a l y s i s , where some o f t h e problems o f o b t a i n i n g q u a n t i t a t i v e i n f o r m a t i o n w i l l be discussed; (iii)

s t r u c t u r a l analysis,

i n which t h e n a t u r e o f t h e s t r u c t u r a l i n f o r m a t i o n

t h a t can be o b t a i n e d f r o m a Mossbauer spectrum w i l l be c r i t i c a l l y assessed. 5.3.1 (i)

Applications i n Q u a l i t a t i v e Analysis I d e n t i f i c a t i o n o f Oxidation States.

The most comnon a p p l i c a t i o n o f

Miissbauer s p e c t r o s c o p y i s i n t h e i d e n t i f i c a t i o n o f t h e o x i d a t i o n s t a t e s o f i r o n . As i n d i c a t e d i n S e c t i o n 5.2.1,

t h e h i g h s p i n i o n s Fe2+ and Fe3+ can be r e a d i l y

d i s t i n g u i s h e d b y t h e magnitudes o f b o t h t h e i r isomer s h i f t s and t h e i r quadrupole s p l i t t i n g s . The isomer s h i f t i s t h e more r 6 l i a b l e parameter, s i n c e p o p u l a t i o n o f t h e 4s e l e c t r o n i c l e v e l i s n e v e r l i k e l y t o be l a r g e and t h e v a r i a t i o n i n t o t a l e l e c t r o n d e n s i t y i n t h e 3d o r b i t a l s , as a r e s u l t o f changes i n t h e c o v a l e n t c h a r a c t e r o f t h e bonds, i s much s m a l l e r t h a n t h e d i f f e r e n c e s between t h e two s t a t e s .

122 Thus isomer s h i f t s a t room t e m p e r a t u r e r e l a t i v e t o i r o n m e t a l a r e i n t h e ranges

-

0.1

0.5 mm

5-l

f o r Fe3+ and 0.7

-

1.3 mm s-'

f o r Fez+. W i t h i n these groups t h e r e

a r e v a r i a t i o n s w i t h c o o r d i n a t i o n number ( d i s c u s s e d i n S e c t i o n

5.3.3 below) and w i t h

t h e n a t u r e o f t h e atom bound t o t h e i r o n , so t h a t f o r c l a y m i n e r a l s , where t h e i r o n i s u s u a l l y bound t o oxygen atoms, t h e isomer s h i f t s f o r m d i s c r e t e groups w i t h i n t h e ranges mentioned above. The quadrupole s p l i t t i n g i s o f t e n a good g u i d e t o t h e o x i d a t i o n s t a t e , p a r t i c u l a r l y when t h e c r y s t a l l o g r a p h i c s i t e s c o n t a i n i n g t h e i r o n a r e n o t g r e a t l y d i s t o r t e d f r o m c u b i c symmetry. I n such cases t h e quadrupole s p l i t t i n g f o r Fe3+ tends t o be small (<1

IMII

5 - l ) and t h a t f o r Fez+ l a r g e ( 2 . 5

-

3.5 mm s - ' ) .

However, as i n d i c a t e d e a r l i e r , t h e Fez+ v a l u e can be g r e a t l y reduced i f e x c i t e d e l e c t r o n i c s t a t e s a r e p o p u l a t e d . Such a phenomenon o f t e n o c c u r s a t q u i t e l o w temperatures (sometimes w e l l below a m b i e n t ) so t h a t i n t e r p r e t a t i o n o f t h e quadrupole s p l i t t i n g i s n o t always s t r a i g h t f o r w a r d . T h i s w i l l be d e a l t w i t h i n more d e t a i l i n s e c t i o n 5.3.3,

where t h e i m p l i c a t i o n i n s t r u c t u r a l a n a l y s i s w i l l be discussed.

With l o w e r symmetries, t h e r e may a l s o be a non-zero v a l e n c e c o n t r i b u t i o n t o t h e quadrupole s p l i t t i n g i n Fe3+, which, combined w i t h t h e l a r g e r l a t t i c e c o n t r i b u t i o n , can o c c a s i o n a l l y g i v e values o f A ( > 1.5 mm s - l ) comparable w i t h those o b t a i n e d from Fez+ i n low symmetry, where t h e valence and l a t t i c e c o n t r i b u t i o n s a r e u s u a l l y

o f opposite sign. Two p o t e n t i a l problems e x i s t , which cause d i f f i c u l t i e s i n t h e s i m p l e i d e n t i f i c ation o f oxidation states: (a)

I f a s p e c i e s has a low r e c o i l - f r e e f r a c t i o n a t

t h e temperature o f i n v e s t i g a t i o n , t h e n t h a t s p e c i e s w i l l make 1 i t t l e c o n t r i b u t i o n t o t h e o v e r a l l Mossbauer a b s o r p t i o n . An example h e r e i s t h e case o f a Fez+exchanged m o n t m o r i l l o n i t e (Fig.5.3).

A t 77K b o t h f e r r o u s and f e r r i c components

1.80

.83

3 1.78 E

.82

(o

0

K a 0

1.76 .81

-2

0

2 Velocity

/

-2

0

2

mm's-1

F i g . 5.3 S p e c t r a o f Fe*+-exchanged m o n t m o r i l l o n i t e a t ( a ) 77K and ( b ) ambient temperatures (Helsen and Goodman, u n p u b l i s h e d r e s u l t s ) .

123 can be c l e a r l y seen, b u t a t ambient temperature t h e f e r r o u s component, which a r i s e s e n t i r e l y f r o m i r o n i n exchange s i t e s , i s no l o n g e r v i s i b l e because o f i t s small f - f a c t o r .

On t h e b a s i s o f t h i s spectrum a l o n e i t c o u l d have been concluded

t h a t Fez+ was absent. ( b )

The l o w s p i n i o n s b o t h have parameters t h a t o v e r l a p

a p p r e c i a b l y w i t h those f r o m h i g h s p i n Fe3+. T h e r e f o r e , i f t h e r e i s t h e p o s s i b i l i t y o f low s p i n i o n s b e i n g p r e s e n t i n a sample, t h e n t h e i d e n t i f i c a t i o n o f t h e o x i d a t i o n s t a t e s i s less straightforward. Fortunately, low s p i n ions are u n l i k e l y t o o c c u r i n c l a y m i n e r a l s t r u c t u r e s , b u t where samples have been t r e a t e d w i t h c e r t a i n s t r o n g l y complexing r e a g e n t s , t h e p o s s i b i l i t y o f t h e i r f o r m a t i o n should n o t be i g n o r e d . (ii)

I d e n t i f i c a t i o n o f S p e c i f i c M i n e r a l Species.

I n t h e o r y , Mossbauer

spectroscopy has t h e p o t e n t i a l t o r e v e a l t h e e x i s t e n c e o f i r o n - c o n t a i n i n g m i n e r a l phases a t l e v e l s below t h e i r l i m i t s o f d e t e c t i o n by o t h e r more c o n v e n t i o n a l techniques such as XRD. T y p i c a l parameters f o r Fe i n some s i l i c a t e m i n e r a l s ( T a b l e 5.4) show t h a t t h e r e i s a l a r g e o v e r l a p o f parameters f r o m one s p e c i e s t o a n o t h e r . Thus Mossbauer s p e c t r o s c o p y has 1 i t t l e p o t e n t i a l f o r i d e n t i f y i n g a TABLE 5 . 4

T y p i c a l Mossbauer parameters f o r some l a y e r s i 1 i c a t e s a t ambient temperatures Fe3+ Kaol in it e Chamosi t e Muscovite Illite Glauconite Nontronite t Montmorillonite Biotite

*

6*

A*

0.36 0.38 0.40 0.33 0.36 0.36 0.32 0.40

0.52 0.78 0.72 0.65 0.40 0.30 0.52 0.52

Fe2+ 6*

0.38 0.35 0.36 0.37 0.38

A*

1.21 1.05 0.62 1.15 0.92

6*

A*

6*

A*

1.14 1.13 1.14 1.15

2.57 3.00 2.75 1.7-2.7

1.12 1.12

2.20 2.20

1.07 1.11

2.90 2.62

1.09

2.18

A l l parameters i n mm s - I ,

'Also

isomers s h i f t s r e l a t i v e t o Fe metal -1 and A = 0.5 3 r d component sometimes o b t a i n e d w i t h 6 i= 0.2 mm s

-

0.6 mn s

-1

s p e c i f i c s i l i c a t e m i n e r a l w i t h o u t a t l e a s t c o n s i d e r a b l e h e l p f r o m o t h e r techniques. On t h e o t h e r hand, parameters f o r c e r t a i n o t h e r groups o f m i n e r a l s may be d i s t i n c t enough f o r Mossbauer spectroscopy t o be used as a b a s i s f o r i d e n t i f i c a t i o n . An example i s t h e c a r b o n a t e m i n e r a l s where t h e Fez+ quadrupole s p l i t t i n g i s mm s-',

m. 1.9

b u t t h e main a p p l i c a t i o n s l i e w i t h t h o s e m i n e r a l s t h a t o r d e r m a g n e t i c a l l y .

To i l l u s t r a t e t h i s , t h e u s e f u l n e s s o f t h e t e c h n i q u e i n t h e i d e n t i f i c a t i o n of t h e v a r i o u s o x i d e s and h y d r o x i d e s i s d i s c u s s e d i n some d e t a i l . The parameters and magnetic p r o p e r t i e s f o r t h e h i g h l y - c r y s t a l l i n e m i n e r a l s a r e g i v e n i n Table 5.5.

124 TABLE 5.5 Mossbauer parameters f o r some c r y s t a l l i n e oxides and hydroxides o f i r o n Sample a-Fe203

Magnetic Properties

A

6-Fe2o3

Ordering Temperature ( K )

Experimental Temperature ( K )

956

300 77

107

295

H(T)

s(mm s - l )

51.6 52.7

0.36 0.48

4.2

49.6 51.9

0.324 0.331

y-Fe203

Fi

300

48.8 49.9

0.27 0.41

Fe3°4

Fi

300

49.3 46.0

0.25 0.65 0.37 0.48

a-FeOOH

A

393

300 77

38.4 50.4

6-FeOOH

A

295

300

-

77

v-FeOOH a-FeOOH

A

FO

A = antiferromagnetic

73

77 4.2

(455) (420) . ,

2 96

ao

Fo = ferromagnetic

A(IIIII

s-l)

0.72 0.97

0.37 0.38 0.52 0.48 0.48

0.55

46.0

-

0.48 0.51

0.55

38.0 52.5 50.5

0.35 0.7 0.7

47.3 46.3 43.7

F i = ferrimagnetic

Hematite (a-Fe203) i s c o n v e n i e n t l y i d e n t i f i e d by comparison o f s p e c t r a a t ambient and low temperatures because o f t h e e x i s t e n c e o f a Morin t r a n s i t i o n a t 260 K, which changes t h e angle between t h e p r i n c i p a l a x i s o f t h e e l e c t r i c a l f i e l d g r a d i e n t and t h e magnetic f i e l d . The spectrum o f magnetite, Fe304, a t ambient temperature has two components which r e p r e s e n t Fe3+ i n t e t r a h e d r a l s i t e s and a combination o f Fez+ and Fe3+ i n octahedral s i t e s . For the l a t t e r a s i n g l e s e t o f peaks i s observed because e l e c t r o n exchange between Fez+ and Fe3+ i s r a p i d . A t low temperatures, where t h e exchange i s slower, a f a r more complex spectrum i s obtained. The types o f magnetic o r d e r i n g shown i n Table 5.5,

can be d i s t i n g u i s h e d by o b t a i n i n g

s p e c t r a i n the presence o f a l a r g e a p p l i e d magnetic f i e l d . T h i s i s because f e r r o and f e r r i - m a g n e t i c samples r e o r i e n t i n t h e f i e l d w i t h subsequent l o s s o f i n t e n s i t y from peaks 2 and 5, whereas t h e a n t i f e r r o m a g n e t i c samples are u n a f f e c t e d . F o r ferromagnetic m a t e r i a l s , where t h e magnetic moments a r e a l i g n e d , t h i s r e o r i e n t a t i o n occurs w i t h much s m a l l e r f i e l d s than those r e q u i r e d f o r f e r r i m a g n e t i c m a t e r i a l s ,

125 where t h e magnetic moments a r e canted. When such experiments a r e combined w i t h t h e c h a r a c t e r i s t i c Mossbauer parameters, each o f t h e m i n e r a l s can be r e a d i l y d i s t i n g u i s h e d from t h e o t h e r s . Examples o f t h e i d e n t i f i c a t i o n o f h e m a t i t e and g o e t h i t e i n s o i l s have been g i v e n b y C h i l d s

Gal. (1978) and

(1978) and o f some o t h e r o x i d e s by Longworth

e.(1979).

Belozerskii

et.

However, i n n a t u r a l

samples, t h e problem i s much l e s s c l e a r c u t , s i n c e t h e e x i s t e n c e o f s m a l l p a r t i c l e s and isomorphous s u b s t i t u t i o n can have a marked e f f e c t on t h e magnetic p r o p e r t i e s of t h e m i n e r a l . As mentioned i n S e c t i o n 5.2.4,

the rate o f collective reorientation

o f m a g n e t i c moments i n m i c r o c r y s t a l s may be comparable t o t h e t i m e o f t h e Mossbauer t r a n s i t i o n a t temperatures w e l l below t h a t o f o n s e t o f t h e magnetic o r d e r i n g

-

a

phenomenon t h a t r e s u l t s i n a c o l l a p s e o f t h e 6-peak Mossbauer spectrum as shown i n Flg.5.2. W i t h isomorphous s u b s t i t u t i o n o f d i a m a g n e t i c i o n s i n t h e c r y s t a l l a t t i c e t h e s i z e s o f t h e domains o f m a g n e t i c o r d e r i n g decrease w i t h t h e same e f f e c t as a r e d u c t i o n i n p a r t i c l e s i z e . A c o m b i n a t i o n o f a h i g h l e v e l o f s u b s t i t u t i o n w i t h a s m a l l p a r t i c l e s i z e can l e a d t o v e r y d r a m a t i c r e d u c t i o n s i n t h e temperature a t which m a g n e t i c a l l y o r d e r e d s p e c t r a can be observed ( f o r example, Goodman and Lewis, 1981, have r e p o r t e d a c o l l a p s e d spectrum f r o m g o e t h i t e a t 77K). There i s a l s o an apparent r e d u c t i o n i n t h e magnetic f i e l d s f o u n d i n t h e s u b s t i t u t e d minerals (Janot

%,,

1971; Golden

u., 1979).

The presence o f a m i x t u r e o f phases may make t h e q u a l i t a t i v e i d e n t i f i c a t i o n o f t h e components d i f f i c u l t when t h e above-mentioned p a r t i c l e s i z e and isomorphous s u b s t i t u t i o n e f f e c t s a r e p r e s e n t . I n such circumstances t h e 6- and y-oxyhydroxides may n o t be d i s t i n g u i s h e d f r o m a-FeOOH. A l s o t h e p o o r l y - d e f i n e d m i n e r a l s f e r r i h y d r i t e and f e r r o x y h i t e b o t h o r d e r m a g n e t i c a l l y a t l o w temperatures w i t h magnetic f i e l d s (Murad and Schwertmann, 1980; C a r l s o n and Schwertmann, 1980) s i m i l a r t o t h a t o b t a i n e d f r o m aluminous g o e t h i t e , g i v i n g f u r t h e r p o s s i b i l i t i e s f o r m i s i d e n t i f i c a t i o n . I t would t h u s appear t h a t as t h e m i n e r a l s p e c i e s become l e s s w e l l - d e f i n e d ,

their

Mossbauer s p e c t r a become more a1 ike, and hence Mossbauer s p e c t r o s c o p y becomes l e s s u s e f u l f o r d i a g n o s t i c purposes. k s s b a u e r s p e c t r o s c o p y has f o u n d c o n s i d e r a b l e a p p l i c a t i o n s i n t h e s t u d y of s u r f a c e s when t h e s p e c t r a a r e o b t a i n e d i n t h e s c a t t e r i n g geometry (see e.g. T r i c k e r , 1977). I n a d d i t i o n t o t h e 14.4 KeV v - r a y ,

x - r a y s o r e l e c t r o n s e m i t t e d as

a r e s u l t o f t h e Mossbauer t r a n s i t i o n , may a l s o be d e t e c t e d . The mean escape depth o f t h e r a d i a t i o n v a r i e s w i t h i t s energy, t h u s a l l o w i n g s u r f a c e r e g i o n s t o be s e l e c t i v e l y s t u d i e d . W i t h t h e 14.4 KeV y - r a y t h e s p e c t r a a r i s e p r e d o m i n a n t l y f r o m the top

m o f t h e m a t e r i a l , whereas w i t h t h e l o w e r energy x - r a y s r e s u l t s a r e

o b t a i n e d f r o m t h e t o p few hundred nm. W i t h c o n v e r s i o n e l e c t r o n s i t may even be p o s s i b l e t o s t u d y monolayers p r o v i d e d t h i i r s p e c t r a do n o t o v e r l a p w i t h those from t h e i r substrates (Petrera

et., 1976).

Thus, w i t h b a c k s c a t t e r experiments

i t i s p o s s i b l e t o i d e n t i f y t h e i r o n - c o n t a i n i n g phases on t h e s u r f a c e s o f b u l k y m a t e r i a l s and examples o f a p p l i c a t i o n s i n c o r r o s i o n r e s e a r c h have been reviewed

126 by Graham and Cohen (1976) and i n t h e s t u d y o f f i n e a r t s by Keisch (1976).

5.3.2 Q u a n t i a t i v e A n a l y t i c a l A p p l i c a t i o n s Because o f t h e n a t u r e o f t h e t e c h n i q u e , any q u a n t i t a t i v e i n f o r m a t i o n o b t a i n e d by Mossbauer s p e c t r o s c o p y must r e l a t e s o l e l y t o t h e d i s t r i b u t i o n o f i r o n . Thus, i n a mixed-phase system i t i s o n l y p o s s i b l e t o o b t a i n t h e q u a n t i t a t i v e d i s t r i b u t i o n o f t h e i r o n components between t h e phases. W i t h o u t a knowledge o f t h e i r o n c o n t e n t s o f each phase i t i s i m p o s s i b l e t o determine t h e amounts o f each phase p r e s e n t . The q u a n t i t a t i v e a n a l y t i c a l a p p l i c a t i o n s a r e t h e r e f o r e e x t r e m e l y 1 i m i t e d , b u t t h e r e a r e areas, p a r t i c u l a r l y i n t h e d e t e r m i n a t i o n o f f e r r o u s / f e r r i c

ratios i n

u n s t a b l e phases, where t h e t e c h n i q u e i s a b l e t o make i n v a l u a b l e c o n t r i b u t i o n s . Examples here i n c l u d e redox r e a c t i o n s i n s i l i c a t e s (Rozenson and H e l l e r - K a l l a i , 1976 a and b; R u s s e l l

u., 1979), where Mossbauer s p e c t r o s c o p y has been success-

f u l l y used. One m a j o r d i f f i c u l t y i n t h i s t y p e o f work concerns t h e p o s s i b i l i t y

t h a t t h e r e c o i l - f r e e f r a c t i o n i s n o t i d e n t i c a l f o r a l l components i n t h e system b e i n g i n v e s t i g a t e d . Most workers have assumed t h a t by w o r k i n g a t low temperatures (e.g. 77K) any problems r e l a t i n g t o t h e f - f a c t o r s w i l l be i n s i g n i f i c a n t , o t h e r s have i g n o r e d t h e e f f e c t even a t room t e m p e r a t u r e . However, as t h e s p e c t r a i n Fig.5.3 show, m i s l e a d i n g r e s u l t s can sometimes be o b t a i n e d , p a r t i c u l a r l y when t h e n a t u r e s o f t h e l a t t i c e s i n which t h e i r o n i s h e l d d i f f e r a p p r e c i a b l y . A l t h o u g h t h i s example m i g h t be c o n s i d e r e d an extreme case, s i n c e t h e sample c o n t a i n e d b o t h i n t e r l a y e r and s t r u c t u r a l i r o n , q u a n t i t a t i v e problems r e l a t e d t o f - f a c t o r d i f f e r ences a r e by no means r a r e . TO f u r t h e r i l l u s t r a t e t h i s , F i g . 5.4 shows t h e d i f f e r e n c e s i n h e m a t i t e : g o e t h i t e r a t i o s d e t e r m i n e d f r o m Mossbauer s p e c t r a a t ambient t e m p e r a t u r e and 77K. I n t h i s case t h e m a j o r p r o b l e m a r i s e s f r o m t h e f a c t t h a t t h e g o e t h i t e peaks a t 77K a r e n o t t h e L o r e n t z i a n shape t h a t was assumed b y t h e c o m p u t e r - f i t t i n g program ( s e e S e c t i o n 5.2.4),

w i t h t h e r e s u l t t h a t some o f t h e a r e a under h e m a t i t e peaks i s

assigned t o g o e t h i t e . The room t e m p e r a t u r e r a t i o s may a l s o n o t be r e l i a b l e s i n c e f f a c t o r s a r e i n f l u e n c e d by p a r t i c l e s i z e and i n t h i s spectrum (Fig.5.4a)

there i s

evidence t h a t t h e g o e t h i t e p a r t i c l e s may be m i c r o c r y s t a l l i n e . I t has been shown r e c e n t l y t h a t Mossbauer s p e c t r o s c o p y can be used t o g i v e

a c c u r a t e q u a n t i t a t i v e i n f o r m a t i o n f r o m m u l t i p h a s e systems ( C o l l i n s , 1978, 1979; Bahgat, 1979). T h i s i n v o l v e d making measurements a t s e v e r a l t e m p e r a t u r e s and t h e n e x t r a p o l a t i n g t h e a b s o r p t i o n areas t o a t e m p e r a t u r e T* a p p r o p r i a t e f o r a z e r o v a l u e 2 f o r t h e mean square displacement, < x >, i n e q u a t i o n ( 1 1 ) . However, because o f t h e l a r g e number o f s p e c t r a r e q u i r e d t o c h a r a c t e r i z e one sample, t h e procedure i s v e r y t i m e consuming.

127

-10 -Fig.5.4

Vetocity

1

mm s-1

10

S p e c t r a from a m i x t u r e o f h e m a t i t e (a-Fe203) and g o e t h i t e (a-FeOOH) i n

a b a u x i t e sample ( a ) a t ambient temperatue and ( b ) a t 77K (Goodman, i i n p u b l i s h e d results). 5.3.3

Applications i n structural analysis

T h i s i s a v e r y wide f i e l d t h a t has developed o v e r t h e p a s t 20 y e a r s and, i n t h i s s e c t i o n , I s h a l l d i s c u s s s e v e r a l aspects o f t h i s t y p e o f a p p l i c a t i o n a l o n g w i t h t h e t h e o r e t i c a l j u s t i f i c a t i o n o f t h e procedures g e n e r a l l y adopted. These

w i l l be grouped under t h e f o l l o w i n g headings: ( i ) d e t e r m i n a t i o n o f c o o r d i n a t i o n number, ( i i ) d i s t r i b u t i o n o f i m p u r i t y i o n s i n o x i d e s , ( i i i ) i d e n t i f i c a t i o n o f the s i t e s containing octahedral i r o n i n s i l i c a t e s , ( i v ) e l u c i d a t i o n o f mineral a l t e r a t i o n r e a c t i o n s , and ( v ) t h e o r e t i c a l c o n s i d e r a t i o n s f o r s i l i c a t e s . By f a r t h e l a r g e s t amount o f e x p e r i m e n t a l work has been devoted t o t h e c h a r a c t e r i z a t i o n of i r o n - c o n t a i n i n g s i t e s , b u t t h i s w i l l be g i v e n p r o p o r t i o n a l l y l e s s space t h a n t h e o t h e r s u b - s e c t i o n s , because o f i t s b e t t e r coverage i n r e v i e w s pub1 i s h e d elsewhere ( B a n c r o f t , 1973; Coey, 1975; H e l l e r - K a l l a i , ( i ) D e t e r m i n a t i o n o f c o o r d i n a t i o n number.

1980).

I n s e c t i o n 5.2.1

i t was s t a t e d t h a t

t h e isomer s h i f t i s d e t e r m i n e d p r i n c i p a l l y ' b y t h e changes i n s h i e l d i n g o f t h e i n n e r - s h e l l s - e l e c t r o n s t h a t o c c u r as a r e s u l t o f v a r i a t i o n s i n d - e l e c t r o n d e n s i t y . T h i s a l l o w s t h e h i g h s p i n o x i d a t i o n s t a t e s o f i r o n t o be r e a d i l y d i s t i n g u i s h e d as

128 shown i n S e c t i o n 5.3.1.

However, t h e e f f e c t i v e d - e l e c t r o n d e n s i t y is dependent

upon t h e degree o f c o v a l e n c y i n t h e m o l e c u l a r o r b i t a l s i n which t h e d - e l e c t r o n s a r e i n v o l v e d . S i n c e t h e s e o r b i t a l s a r e e i t h e r non-bonding o r anti-bonding,any i n c r e a s e i n t h e degree o f c o v a l e n c y w i l l r e s u l t i n a decrease i n t h e d - e l e c t r o n d e n s i t y , and c o n s e q u e n t l y a s m a l l e r v a l u e f o r t h e isomer s h i f t . I n g e n e r a l f o r i r o n , where t h e t e r m R e 2 +

*

i n e q u a t i o n ( 2 ) i s n e g a t i v e , t h e r e appears t o be a 9 s t e a d y i n c r e a s e i n 6 w i t h i n c r e a s e i n c o o r d i n a t i o n number f o r a p a r t i c u l a r g r o u p bound t o t h e i r o n and, a l t h o u g h some workers have doubted t h e r e l i a b i l i t y o f u s i n g t h e isomer s h i f t as an i n d i c a t o r o f c o o r d i n a t i o n number (e.g.

Brown and

P r i t c h a r d , 1969; Richardson, 1975), i t has r e c e n t l y become i n c r e a s i n g l y a p p a r e n t t h a t i n s i l i c a t e m i n e r a l s i r o n i n t e t r a h e d r a l c o o r d i n a t i o n has i s o m e r s h i f t s 0.15

-

E.

0.20 mn s - l l o w e r t h a n t h o s e o f o c t a h e d r a l i r o n . Examples o f t h e u s e f u l n e s s

o f t h i s parameter have been i n t h e i d e n t i f i c a t i o n o f t e t r a h e d r a l i r o n i n n o n t r o n i t e s (Goodman

u., 1976 and Fig.5.5)

I

1

-1 F i g . 5.5

and i n p h l o g o p i t e (Sanz

1

e., 1978).

L

1

0 Velocity/mrn

s-1

2

1

Mossbauer spectrum o f a n o n t r o n i t e a t 77K ( f r o m Goodman

s., 1976).

There have been s u g g e s t i o n s t h a t t h e quadrupole s p l i t t i n g m i g h t be used as a g u i d e t o c o o r d i n a t i o n number (e.g.

Taylor

et., 1968),

b u t , a l t h o u g h t h e r e may

be some e m p i r i c a l r e l a t i o n s h i p o v e r a s m a l l group o f samples, t h e t h e o r y o f t h e o r i g i n crf t h e e l e c t r i c quadrupole i n t e r a c t i o n ( S e c t i o n 5.2.2)

eliminates the

129 p o s s i b i l i t y o f any g e n e r a l r e l a t i o n s h i p between A and c o o r d i n a t i o n number. ( i i ) D i s t r i b u t i o n o f i m p u r i t y i o n s i n o x i d e s . I n S e c t i o n 5.3.1 i t was mentioned t h a t b o t h isomorphous s u b s t i t u t i o n o f d i a m a g n e t i c i o n s and t h e presence o f s m a l l p a r t i c l e s i z e s can have a marked e f f e c t on t h e t e m p e r a t u r e a t which 6-peak s p e c t r a a r e o b t a i n e d from m a g n e t i c a l l y o r d e r e d m i n e r a l s . A c o n s i d e r a b l e amount o f work has been c a r r i e d o u t i n o r d e r t o use t h e s e e f f e c t s i n t h e c h a r a c t e r i z a t i o n o f t h e o x i d e and o x y h y d r o x i d e m i n e r a l s , p a r t i c u l a r l y h e m a t i t e and g o e t h i t e . J a n o t and G i b e r t (1970) and J a n o t

M. (1973)

have shown t h a t t h e degree o f aluminium s u b s t i t u t i o n

has a measurable e f f e c t on t h e s i z e o f t h e magnetic f i e l d i n aluminous h e m a t i t e s and g o e t h i t e s . T h i s theme has been f u r t h e r extended by Golden

Gal. (1979), who

developed e q u a t i o n ( 1 2 ) f o r t h e r e l a t i o n s h i p between t h e computed magnetic f i e l d ( H ) , aluminium s u b s t i t u t i o n ( A A l ) , and s u r f a c e area (SA) f o r g o e t h i t e a t 77K

-

H ( T ) = 49.8 - o.136(:oA1) O.Oll(SA) (12) Although t h i s e x p r e s s i o n appears t o h o l d w e l l f o r many samples o f g o e t h i t e i t should be used w i t h c a u t i o n f o r t h e f o l l o w i n g reasons: ( a ) The r e l a t i o n s h i p i s e m p i r i c a l and i s n o t , t h e r e f o r e , v a l i d f o r samples w i t h c h a r a c t e r i s t i c s o u t s i d e t h e range o f t h o s e used i n i t s d e r i v a t i o n ; ( b ) As shown by Goodman and Lewis (1981), many s p e c t r a o f samples o f h i g h s u r f a c e a r e a s and moderate aluminium s u b s t i t u t i o n show s i g n s o f r e l a x a t i o n a t 77K. Peaks a r e , t h e r e f o r e , n o t o f L o r e n t z i a n shape and such an assumption l e a d s t o v a l u e s o f t h e magnetic f i e l d s f r o m computer f i t s t h a t a r e a p p r e c i a b l y l o w e r t h a n t h e t r u e f i e l d s . I n such c i r c u m s t a n c e s s p e c t r a s h o u l d be r e c o r d e d a t l o w e r temperatures (e.g.

t h e b o i l i n g p o i n t o f l i q u i d helium, 4.2K)

i n o r d e r t h a t e f f e c t s o f isomorphous s u b s t i t u t i o n on t h e magnitude of t h e i n t e r n a l magnetic f i e l d may be more f u l l y understood. (iii)

I d e n t i f i c a t i o n of s i t e s containing octahedral i r o n i n s i l i c a t e s . This

area o f work has been w i d e l y developed o v e r t h e p a s t 15

-

20 y e a r s . The b a s i c

p h i l o s o p h y has been t h a t i f t h e c o o r d i n a t i n g groups o r degree o f d i s t o r t i o n f r o m c u b i c symnetry vapy f o r d i f f e r e n t c r y s t a l l o g r a p h i c s i t e s , t h e n t h e e l e c t r i c f i e l d g r a d i e n t s a t those s i t e s w i l l a l s o v a r y . G e n e r a l l y - a c c e p t e d assignments o f compone n t s i n t h e Mossbauer s p e c t r a t o t h e c r y s t a l l o g r a p h i c s i t e s have been d e r i v e d t h r o u g h t h e s t u d y o f many samples o f each m i n e r a l s p e c i e s . T y p i c a l examples o f work on some l a y e r and c h a i n s i l i c a t e s i l l u s t r a t e t h e r e s u l t s t h a t can be o b t a i n e d . From t h e b a s i c s t r u c t u r e f o r a 2 : l l a y e r s i l i c a t e (Fig.5.6),

i t can be seen t h a t

two d i f f e r e n t t y p e s o f c o o r d i n a t i o n e x i s t a t t h e o c t a h e d r a l s i t e s , one s i t e h a v i n g

cis c o o r d i n a t i o n and b e i n g t w i c e as abundant as t h e o t h e r s i t e groups a r e i n a trans arrangement. F o r bond angles o f 90’ i t can

two OH groups i n where t h e OH

e a s i l y be shown, u s i n g t h e e x p r e s s i o n s f o r t h e components o f t h e e l e c t r i c f i e l d g r a d i e n t t e n s o r g i v e n i n s e c t i o n 5.2.2, t h a t t h e magnitude of,q5,tt f o r the trans s i t e . Thus, where two f e r r i c components with s i t e i s twice t h a t f o r the isomer s h i f t s c h a r a c t e r i s t i c o f o c t a h e d r a l c o o r d i n a t i o n have been observed i n t h e Mo’ssbauer s p e c t r a o f l a y e r s i l i c a t e s , t h e one w i t h t h e l a r g e r quadrupole s p l i t t i n g

130

Fig.5.6

The b a s i c s t r u c t u r e o f 2 : l l a y e r s i l i c a t e s .

has been assigned t o t h e

trans s i t e ,

(Rozenson and H e l l e r - K a l l a i ,

-e t a l . , 1976

and Fig.5.5),

commonly l a b e l l e d M1. Examples f o r g l a u c o n i t e

1978; McConchie

Gal., 1979), n o n t r o n i t e

m o n t m o r i l l o n i t e (Rozenson a n d - H e l l e r - K a l l a i ,

(Goodman 1977) and

b i o t i t e (Annersten, 1974; B a n c r o f t and Brown, 1975) i l l u s t r a t e t h i s approach. Assignments f o r Fez+ i o n s a r e u s u a l l y based on t h e arguments t h a t t h e c o n t r i b u t i o n t o t h e e l e c t r i c f i e l d g r a d i e n t from t h e from qval;

(Ilatt

term i s o f o p p o s i t e s i g n t o t h a t

thus, t h e component w i t h t h e s m a l l e r v a l u e o f A i s assigned t o t h e M1

s i t e . Experimental j u s t i f i c a t i o n f o r t h i s assignment has been g i v e n by Haggstrom

e t al.

(1969) who showed t h a t w i t h i r o n - r i c h b i o t i t e t h e amount o f i r o n correspond-

i n g t o t h e area o f t h e component w i t h t h e l a r g e r A was t o o g r e a t f o r i t t o be accomnodated i n t h e M1 s i t e s . A t y p i c a l spectrum o f a b i o t i t e i s shown i n F i g . 5.7, along w i t h the assignments o f the Fez+ s i t e s . A s i m i l a r approach has been adopted w i t h t h e c h a i n s i l i c a t e s . F o r pyroxenes,

t h e two f e r r o u s components observed i n t h e Mossbauer s p e c t r a can be assigned t o t h e two d i s t i n c t c r y s t a l l o g r a p h i c s i t e s on t h e b a s i s o f t h e degree o f d i s t o r t i o n from c u b i c symmetry (see e.g.

V i r g o and Hafner, 1969), and f o r amphiboles, components

can be observed (Goldman, 1979) t h a t can be assigned t o t h e f o u r types of octahedral s i t e i n t h e s t r u c t u r e . However, f o r both pyroxenes.and amphiboles t h e r e have been suggestions t h a t next-nearest-neighbour c a t i o n s may have some e f f e c t

131

5.2

< (0

0

In

4-

C

3

0

0

5 .l

-2 F i g . 5.7

0 Velocity/mm

2 s-1

The Mossbauer spectrum o f a b i o t i t e a t ambient temperature.

on t h e quadrupole s p l i t t i n g s and t h a t t h i s may l i m i t t h e accuracy w i t h which t h e s i t e d i s t r i b u t i o n s can be determined (Dowty and L i n d s l e y , 1973; Goldman 1979). T h i s problem o f n e x t - n e a r e s t - n e i g h b o u r e f f e c t s w i l l be d e a l t w i t h more f u l l y when t h e o r e t i c a l a s p e c t s a r e c o n s i d e r e d a t t h e end o f t h i s s e c t i o n . (iv)

The s t u d y o f m i n e r a l a1 t e r a t i o n r e a c t i o n s .

Because Mossbauer spectroscopy

has t h e a b i l i t y t o i d e n t i f y t h e c r y s t a l l o g r a p h i c s i t e s t h a t c o n t a i n i r o n i n m i n e r a l s as w e l l as t h e o x i d a t i o n s t a t e s o f t h a t i r o n , i t has c o n s i d e r a b l e p o t e n t i a l i n t h e s t u d y o f m i n e r a l r e a c t i o n s . Examples o f n a t u r a l w e a t h e r i n g i n c l u d e t h e s t u d y o f b i o t i t e (Yassoglou

e., 1972;

Goodman and Wilson, 1973) and hornblende

(Goodman and Wilson, 1976). Also, because o f t h e e f f e c t s o f temperature on t h e d i s t r i b u t i o n and o x i d a t i o n s t a t e s o f i r o n i n d i f f e r e n t m i n e r a l s p e c i e s , t h e r e have been s u g g e s t i o n s t h a t Mossbauer s p e c t r o s c o p y m i g h t be used as a geothermometer. Examples h e r e i n c l u d e t h e s t u d y o f pyroxenes ( V i r g o and Hafner, 1969) and cummingt o n i t e (Ghose and Weidner, 1972). The e f f e c t s o f p r e s s u r e have a l s o been s t u d i e d , (e.g.

Burns

Gal., 1972). R e v e r s i b l e

chemical r e a c t i o n s t h a t can t a k e p l a c e

w i t h i n m i n e r a l s t r u c t u r e s r e p r e s e n t an a r e a o f r e s e a r c h where Mossbauer s p e c t r o s copy can make an e x t r e m e l y v a l u a b l e c o n t r i b u t i o n . Reactions can b e f r o z e n a t i n t e r m e d i a t e stages, b y r a p i d c o o l i n g t o t h e t e m p e r a t u r e o f l i q u i d n i t r o g e n , a t which t h e Mossbauer s p e c t r a a r e o b t a i n e d . The s t u d y o f t h e r e d u c t i o n o f s m e c t i t e s b y v a r i o u s r e d u c i n g agents g i v e s examples o f t h e c h a r a c t e r i z a t i o n o f a i r - s e n s i t i v e

132 s p e c i e s formed d u r i n g redox r e a c t i o n s (Rozenson and H e l l e r - K a l l a i , Gal., 1979).

1976 a and b;

Russell

Thermal a l t e r a t i o n s o f m i n e r a l s i n a i r have been e x t e n s i v e l y s t u d i e d b o t h f r o m t h e academic p o i n t o f v i e w o f c h a r a c t e r i z i n g d e c o m p o s i t i o n r e a c t i o n s (examples i n c l u d e t h e s t u d y o f c h l o r i t e b y Goodman and Bain, 1978; and b i o t i t e , p h l o g o p i t e and v e r m i c u l i t e by T r i p a t h i

c,, 1978), and i n t h e a p p l i e d sense f o r

identifying

t h e t r e a t m e n t s t o which samples o f a r c h a e o l o g i c a l i n t e r e s t had been s u b j e c t e d . T h i s l a t t e r a r e a o f work has been r e v i e w e d b y K o s t i k a s

e. (1976) and has been

shown t o be s u c c e s s f u l i n t h e c h a r a c t e r i z a t i o n o f a n c i e n t p o t t e r y o f known provenance and s t y l e , and i n t h e i d e n t i f i c a t i o n o f changes i n manufacuuring t e c h n o l o g y . (v)

Theoretical considerations.

I n t h e d i s c u s s i o n above on t h e i d e n t i f i c a t i o n

o f i r o n - c o n t a i n i n g s i t e s i n s i l i c a t e m i n e r a l s t h e r e a r e a number o f weaknesses i n t h e arguments t h a t have been p r e s e n t e d t o s u p p o r t t h e i n t e r p r e t a t i o n s o f t h e s p e c t r a . F i r s t l y , most s p e c t r a a r e o b t a i n e d a t room temperature. There a r e a few examples

o f s p e c t r a a t 77K, b u t r e s u l t s f r o m l o w e r t e m p e r a t u r e s a r e e x c e e d i n g l y r a r e . T h i s i s an i m p o r t a n t o m i s s i o n s i n c e , as d i s c u s s e d i n S e c t i o n 5.2.2, o f f e r r o u s quadrupole s p l i t t i n g s t h e qval

i n any c o n s i d e r a t i o n

t e r m i n room t e m p e r a t u r e s p e c t r a may be

c o m p l i c a t e d by t h e p o p u l a t i o n o f e x c i t e d e l e c t r o n i c s t a t e s . Such an e f f e c t can be e s t a b l i s h e d i f t h e quadrupole s p l i t t i n g shows a s i z e a b l e t e m p e r a t u r e dependence, e.g.

i n a b i o t i t e i n c r e a s e s i n A f r o m 2.61 t o 2.91 mm s - l f o r t h e o u t e r d o u b l e t

and f r o m 2.26 t o 2.59 mm s - l f o r t h e i n n e r d o u b l e t have been observed (Goodman, u n p u b l i s h e d r e s u l t s ) . T h e r e f o r e , i n o r d e r t o draw any c o n c l u s i o n s a b o u t t h e n a t u r e o f t h e qlatt

term, l o w t e m p e r a t u r e s p e c t r a must be c o n s i d e r e d . With f e r r i c i r o n

t h e r e i s n o t t h e same d i f f i c u l t y w i t h p o p u l a t i o n o f e x c i t e d s t a t e s a t ambient temperatures and t h e quadrupole s p l i t t i n g i s d e t e r m i n e d by a c o m b i n a t i o n o f qlatt and t h e qval

t h a t a r i s e s f r o m t h e unequal p o p u l a t i o n o f t h e d o r b i t a l s i n m o l e c u l a r

o r b i t a l s . I n many cases (e.g. Fig.5.7)

t h e f e r r i c components a r e l e s s c l e a r l y r e s o l v e d

t h a n t h e .ferrous a t room temperature, f u r t h e r i l l u s t r a t i n g t h e i m p o r t a n c e o f t h e e x c i t e d e l e c t r o n i c s t a t e s i n d e t e r m i n i n g t h e s i z e o f t h e l a t t e r components.

I n any d i s c u s s i o n r e l a t i n g t h e s i z e o f f e r r i c quadrupole s p l i t t i n g s t o t h e c r y s t a l s t r u c t u r e i t i s necessary t o be s u r e t h a t t h e i r o n i s i n f a c t i n t h e m i n e r a l . T h i s may be o f p a r t i c u l a r i m p o r t a n c e i n t h e s t u d y o f specimens o f l o w i r o n c o n t e n t , where a c o m p a r a t i v e l y m i n o r amount o f an i r o n - r i c h i m p u r i t y phase c o u l d make a s i g n i f i c a n t c o n t r i b u t i o n t o t h e Mossbauer spectrum. I t has been suggested b y Goodman (1978a) on t h e b a s i s o f EPR work, t h a t some m o n t m o r i l l o n i t e s m i g h t have a c o n s i d e r a b l e percentage o f t h e i r i r o n i n a phase adsorbed on t h e i r s u r f a c e s . Even a f t e r s a t i s f y i n g t h e problems mentioned above, t h e r e a r e c e r t a i n d i f f i c u l t i e s i n j u s t i f y i n g t h e assignment o f components i n t h e Mossbauer s p e c t r a t o s p e c i f i c c r y s t a l l o g r a p h i c s i t e s . E x p e r i m e n t a l l y t h i s s i t u a t i o n has been most e v i d e n t i n t h e s t u d y o f pyroxenes. Dowty and L i n d s l e y (1973) r e p o r t e d , i n t h e i r work on c a l c i c pyroxenes, anomalies i n t h e r e l a t i v e areas a s s i g n e d t o t h e two s i t e s f r o m

133 two-component f i t s t o s p e c t r a compared t o r e s u l t s f r o m XRD. However, b y assuming t h a t each M1 s i t e was c h a r a c t e r i z e d b y f o u r components c o r r e s p o n d i n g t o f o u r d i f f e r e n t arrangements o f Fe and Ca i n t h e t h r e e n e a r e s t neighbour M2 p o s i t i o n s , i t was p o s s i b l e t o o b t a i n computer f i t s t h a t gave r e l a t i v e s i t e p o p u l a t i o n s

c o n s i s t e n t w i t h t h e XRD r e s u l t s . A l d r i d g e

u.

(1978) a l s o found i t necessary

t o use a s i m i l a r model t o i n t e r p r e t t h e s p e c t r a o f omphacite c l i n o p y r o x e n e s . The e s t a b l i s h m e n t o f t h e e f f e c t s o f n e x t - n e a r e s t - n e i g h b o u r i o n s on t h e quadrupole s p l i t t i n g i n one group o f s i l i c a t e s r a i s e s t h e q u e s t i o n o f t h e i r importance i n other minerals. I f a c r y s t a l s t r u c t u r e i s

a c c u r a t e l y known and i f a n e t charge

can be a s s i g n e d t o each atom w i t h i n t h e s t r u c t u r e t h e n i t i s a s i m p l e c o m p u t a t i o n a l problem t o use e q u a t i o n s ( 5 ) t o e s t i m a t e t h e qlatt

values f o r any p a r t i c u l a r s i t e

i n t h e s t r u c t u r e . I t i s a l s o a s i m p l e m a t t e r t o examine t h e e f f e c t s o f v a r i a t i o n s i n t h e charges on n e i g h b o u r i n g c a t i o n s i t e s on t h e ‘Ilattv a l u e s . I n c a l c u l a t i o n s

on b i o t i t e Mineeva (1978) has shown t h a t t h e range o f v a l u e s f o r A f o r each c r y s t a l l o g r a p h i c s i t e i s g r e a t e r t h a n t h e d i f f e r e n c e s between t h e s i t e s . Also, Goodman (1976) showed t h a t f o r f e r r i a n n i t e an e l e c t r i c f i e l d g r a d i e n t d i s t r i b u t i o n c o u l d be g e n e r a t e d by a Monte C a r l o method f o r each c r y s t a l l o g r a p h i c s i t e , and t h a t f o r t h i s m i n e r a l each r e s u l t i n g envelope c o u l d b e f i t t e d f a i r l y w e l l by 2 components, t h e r e l a t i v e i n t e n s i t i e s depending on t h e r a t i o o f d i v a 1 e n t : t r i v a l e n t ions:vacancies.

By a p p l y i n g t h e s e r e s u l t s t o t h e a n a l y s i s o f t h e spectrum o f b i o t i t e

shown i n F i g . 5.7 Goodman (1976) o b t a i n e d t h e computer f i t shown i n F i g . 5.8 which produced v e r y d i f f e r e n t c o n c l u s i o n s c o n c e r n i n g t h e r e l a t i v e p r e f e r e n c e o f Fe

2t

f o r t h e two s i t e s i n t h e s t r u c t u r e . A l s o i n t h e spectrum o f n o n t r o n i t e ( F i g . 5.5) i t has been suggested (Goodman 1978b) t h a t t h e two o c t a h e d r a l components do n o t

n e c e s s a r i l y a r i s e f r o m two d i f f e r e n t t y p e s o f c r y s t a l l o g r a p h i c s i t e , and t h a t t h e y c o u l d a t l e a s t i n p a r t be accounted f o r by t h e d i s t r i b u t i o n o f n e x t - n e a r e s t neighbour i o n s . I n some i n s t a n c e s i t m i g h t be p o s s i b l e t o r e s o l v e t h i s assignment problem b y d e t e r m i n i n g t h e s i g n o f i t w i l l be opposite f o r

Vzz f o r each component, s i n c e i n an i d e a l case

cis and trans c o o r d i n a t i o n s .

The s i g n i s most e a s i l y

measured e i t h e r by u s i n g a s i n g l e c r y s t a l o r b y t h e a p p l i c a t i o n o f a l a r g e magnetic f i e l d (see e.g.

C o l l i n s and T r a v i s , 1967). However, u n t i l a g r e a t e r u n d e r s t a n d i n g

o f t h e importance o f a l l o f t h e f a c t o r s a f f e c t i n g e l e c t r i c f i e l d g r a d i e n t s has been achieved, t h e use o f Mossbauer s p e c t r o s c o p y f o r t h e assignment o f i o n s t o c r y s t a l l o g r a p h i c s i t e s w i l l n o t be e n t i r e l y unambiguous.

4.

CONCLUSIONS Mossbauer s p e c t r o s c o p y can be used t o d e t e r m i n e t h e o x i d a t i o n s t a t e s o f i r o n i n

m i n e r a l s and t o i d e n t i f y t h e presence o f some m i n e r a l s p e c i e s i n samples o f unknown c o m p o s i t i o n , p a r t i c u l a r l y when t h e s e s p e c i e s e x h i b i t magnetic o r d e r i n g . However, s m a l l p a r t i c l e s i z e s and isomorphous s u b s t i t u t i o n s can d r a s t i c a l l y a l t e r t h e Mossbauer s p e c t r a a t a p a r t i c u l a r temperature. I n q u a n t i t a t i v e a n a l y s i s o n l y t h e

134

Fe3+ 1-

-1Ml

12% M1,MZ 43%

c C

a

0

0

5.1

I

F i q . 5.8

I

-2

0 Velocity/

mm 8

2

1

The Mijssbauer spectrum of t h e b i o t i t e i n F i q . 5.7 f i t t e d t o a model t h a t

assumes two doublets f o r Fez+ i n each c r y s t a l l o g r a p h i c s i t e ( f r o m Goodman 1976). d i s t r i b u t i o n o f i r o n can bedeterminedand even then t h i s can o n l y be determined a c c u r a t e l y when s p e c t r a a r e obtained over a range o f temperatures. With pure s i l i c a t e minerals, i t i s p o s s i b l e t o d i s t i n g u i s h i r o n - c o n t a i n i n g s i t e s according t o t h e i r c o o r d i n a t i o n numbers and t h e i r e l e c t r i c f i e l d g r a d i e n t s . The l a t t e r parameter i s determined by t h e c r y s t a l l a t t i c e and n o t j u s t by t h e i n e d i a t e environment o f t h e i r o n so t h a t each c r y s t a l l o g r a p h i c s i t e may have a range of quadrupole s p l i t t i n g s , t h e r e l a t i v e i n t e n s i t y o f which depends upon t h e d i s t r i b u t i o n o f next-nearest-neighbour

ions.

REFERENCES A l d r i d g e , L.P., Bancroft, G.M. , F l e e t , M.E. and Herzberg, C.T., 1978. Omphacite s t u d i e s , 11. Mossbauer s p e c t r a o f C2/c and P2/n omphacites. Am. Mineral., 63: 1107-1 115. 59: Annersten, H. , 1974. Mgssbauer s t u d i e s o f n a t u r a l b i o t i t e s . Am. Mineral 143-1 51 1979. A new method f o r q u a n t i t a t i v e a n a l y s i s o f t h e Mossbauer e f f e c t Bahgat, A.A., Phys. Status S o l i d i ( A ) , 52:K217-220. Bancroft, G.M. , 1973. Mossbauer Spectroscopy: an i n t r o d u c t i o n f o r i n o r g a n i c chemists and geochemists. McGraw-Hill, London. Bancroft, G.M. , 1979. Mossbauer Spectroscopic s t u d i e s o f t h e chemical s t a t e of i r o n i n s i l i c a t e minerals. J. Phys. ( P a r i s ) C o l l o q . C2, 40: 464-471.

.

.,

135 Bancroft, G.M. and Brown, J.R., 975. A Mossbauer s t u d y o f c o e x i s t i n g hornblendes and b i o t i t e s : q u a n t i t a t i v e FeAt/FeZt r a t i o s . Am. Mineral 60: 265-272. B e l o z e r s k i i , G.N. , Kazakov, M.I., Gagarina, E . I . and Khantulev, A.A. , 1978. Use o f Mossbauer spectroscopy f o r s t u d y i n g t h e forms o f i r o n i n f o r e s t s o i l s . S o v i e t S o i l Science, pp. 534-545 Brown, F.F. and P r i t c h a r d , A.M., 1969. The Mossbauer spectrum o f i r o n o r t h o c l a s e . E a r t h Planet. S c i . L e t t s . , 5: 259-260. Burns, R.G., Huggins, F.E. and Drickamer, H.G., 1972. A p p l i c a t i o n s o f h i g h pressure Mossbauer spectroscopy t o mantle mineralogy. 24th I G C Section 14, pp 113-123. Carlson, L. and Schwertmann, U. , 1980. N a t u r a l occurrence o f f e r o x y h i t e (6'-FeOOH). Clays Clay M i n e r a l s , 28: 272-280. Goodman, B.A. and Churchman, G.J., 1978. A p p l i c a t i o n o f Mossbauer Childs, C.W., spectroscopy t o t h e s t u d y o f i r o n oxides i n some r e d and yellow/brown s o i l samples from New Zealand. Proc. I n t e r . Clay Conf. 1978 (Pub. 1979): 555-565. Coey, J.M.0'. , 1975. Clay m i n e r a l s and t h e i r t r a n s f o r m a t i o n s s t u d i e d w i t h n u c l e a r techniques: t h e c o n t r i b u t i o n o f Mijssbauer spectroscopy. I n : 1 s t conference on Mossbauer Spectroscopy, Cracow, Poland. C o l l i n s , R.L., 1978. Q u a n t i t a t i v e Mossbauer a n a l y s i s . Phys. L e t t s . , 66A: 153-154. C o l l i n s , R.L., 1979. Q u a n t i t a t i v e Mossbauer spectroscopy. J . Phys. ( P a r i s ) Colloq. C2, 40: 36-38. C o l l i n s , R.L. and T r a v i s , J.C,., 1967. The e l e c t r i c f i e l d g r a d i e n t tensor. I n : I.J. Gruverman ( E d i t o r ) , Mossbauer E f f e c t Methodology. Plenum, New York, 3: 123-161. Dowty, E. and L i n d s l e y , D.H., 1973. Mossbauer s p e c t r a o f s y n t h e t i c hedenbergitef e r r o s i l i t e pyroxenes. Am. M i n e r a l , 58: 850-868. Ericson, T. and Wappling, R., 1976. Texture e f f e c t s i n 3/2-1/2 Mossbauer spectra. J . Phys. ( P a r i s ) Colloq. C6, 37: 739-722; o r d e r - d i s o r d e r i n cummingtonite, 1972. Mg '-Fe Ghose, S. and Weidner, J.R., (Ms,F~)S ~ i g 022(DH)2: a new geothermometer. E a r t h Planet. Sci. L e t t s . , 16: 346-354. Gol'danskii, V.I., Makarov, E.F,. and Khrapov, V.V., 1963. D i f f e r e n c e i n two peaks o f quadrupole s p l i t t i n g i n Mossbauer spectra. Phys. L e t t s . , 3: 344-346. Golden, D.C., Bowen, L.H., Weed, S.B. and Bigham, J.M. , 1979. kfossbauer s t u d i e s o f s y n t h e t i c and s o i l - o c c u r r i n g a l u m i n i u m - s u b s t i t u t e d g o e t h i t e s . S o i l Sci. SOC. Amer. J . , 43: 802-808. Goldman, D.S., 1979. A r e e v a l u a t i o n o f t h e Mossbauer spectroscopy o f c a l c i c amphiboles. Am. Mineral., 64: 109-118. Goodman, B.A., 1976. The e f f e c t o f l a t t i c e s u b s t i t u t i o n s on t h e d e r i v a t i o n o f q u a n t i t a t i v e s i t e p o p u l a t i o n s f r o m t h e Mossbauer s p e c t r a o f 2 : l l a y e r l a t t i c e s i l i c a t e s . J . Phys. ( P a r i s ) Colloq. C6, 37: 819-823. Goodman, B.A., 1978a. An i n v e s t i g a t i o n by Mdssbauer and EPR spectroscopy o f t h e p o s s i b l e presence o f i r o n - r i c h i m p u r i t y phases i n some m o n t m o r i l l o n i t e s . Clay M i n e r a l s , 13: 351-356. Goodman, B.A., 1978b. The Mossbauer s p e c t r a o f n o n t r o n i t e s : c o n s i d e r a t i o n o f an a l t e r n a t i v e assignment. Clays Clay M i n e r a l s , 26: 176-177. Goodman, B.A., 1980. Mossbauer Spectroscopy. I n : J.W. S t u c k i and W.L. Banwart ( E d i t o r s ) , Advanced Chemical Methods f o r S o i l and Clay M i n e r a l s Research, 0. Reidel, Dordrecht, pp 1-92. Goodman, B.A. and Bain, D.C., 1978. Mossbauer s p e c t r a o f c h l o r i t e s and t h e i r decomposition products. Proc. I n t e r . Clay Conf. 1978 (Pub. 1979): 65-74. Goodman, B.A. and Lewis, D.G., 1981. Mossbauer s p e c t r a o f aluminous g o e t h i t e s (a-FeOOH). J . S o i l Sci., i n press. Goodman, B.A., R u s s e l l , J.D., Fraser, A.R. and Woodhams, F.W.D., 1976. A Mossbauer and i n f r a r e d spectroscopic study o f t h e s t r u c t u r e o f n o n t r o n i t e . Clays Clay M i n e r a l s , 24: 53-59. Goodman, B.A. and I l i l s o n , M.J., 1973. A s t u d y o f t h e weathering o f a b i o t i t e u s i n g t h e Mossbauer e f f e c t . Mineral.. Mag., 39: 448-454. Goodman, B.A. and Wilson, M.J., 1976. A Mossbauer s t u d y o f t h e weathering o f hornblende. Clay Minerals, 11: 153-163. Graham, M.J. and Cohen, M., 1976. A n a l y s i s o f i r o n c o r r o s i o n products u s i n g Mossbauer spectroscopy. Corrosion, 32: 432-438.

.,

136 Haggstrom, L., Wappling, R. and Annersten, H., 1969. Mossbauer s t u d y o f i r o n - r i c h b i o t i t e s . Chem. Phys. L e t t s . , 4: 107-108. H e l l e r - K a l l a i , L., 1980. The use o f Mossbauer spectroscopy o f i r o n i n c l a y mineralogy. Presented a t 4 t h Meeting, European Clay Groups, F r e i s i n g , FRG, t o be published. Janot, C. and G i b e r t , H., 1970. Les c o n s t i t u a n t s du f e r dans c e r t a i n e s b a u x i t e s n a t u r e l l e s e t u d i e e s p a r e f f e t Mossbauer. B u l l . SOC. fr. M i n g r a l . C r i s t a l l o g r . , 93: 213-223. Janot, C., G i b e r t , H., De Gramont, X. and B i a i s , R., 1971. Etude des s u b s t i t u t i o n s A1-Fe dans des roches l a t 6 r i t i q u e s . B u l l . SOC. fr. Min6ral C r i s t a l l o g r . , 94: 36 7 3 80. 1973. C h a r a c t e r i s a t i o n de k a o l i n i t e s Janot, C., G i b e r t , H. and Tobias,..C., f e r r i f e r e s p a r s p e c t r o m e t r i e Mossbauer. B u l l . SOC. fr. M i n e r a l . C r i s t a l l o g r . , 96: 281-289. Keisch,..B., 1976. Analysis o f works o f a r t . I n : R.L. Cohen ( E d i t o r ) , A p p l i c a t i o n s o f Mossbauer Spectroscopy, Academic Press, New York, 1 : 263-286. Kostikas, A., Simopoulos, A. and Gangas, N.H. , 1976. A n a l y s i s o f a r c h a e o l o g i c a l a r t i f i c a t s . 1n:R.L. Cohen ( E d i t o r ) , A p p l i c a t i o n s o f Mossbauer Spectroscopy, Academic Press, New York, 1 : 241-261. Longworth, G., Becker, L.W., Thompson, R., O l d f i e l d , F., Dearing, J.A. and Rummery, T.A., 1979. M'ossbauer e f f e c t and magnetic s t u d i e s o f secondary i r o n oxides i n s o i l s . J. S o i l Sci., 30: 93-110. Ward, J.B., McCann, V.H. and Lewis, D.W., 1979. A Mossbauer McConchie, D.M., i n v e s t i g a t i o n o f g l a u c o n i t e and i t s g e o l o g i c a l s i g n i f i c a n c e . Clays Clay Minerals, 27: 339-348. Mineeva, R.M., 1978. R e l a t i o n s h i p between Mossbauer s p e c t r a and d e f e c t s t r u c t u r e i n b i o t i t e s from e l e c t r i c f i e l d g r a d i e n t c a l c u l a t i o n s . Phys. Chem. M i n e r a l s , 2: 267-277. Murad, E . and Schwertmann, U., 1980. The Mossbauer spectrum o f f e r r i h y d r i t e and i t s r e l a t i o n s h i p t o those o f o t h e r i r o n oxides. Am. Mineral., 65: 1044-1049. Petrera, M., Gonser, U., Hasmann, U., Keune, W. and Lauer, J., 1976. Are monol a y e r s d e t e c t a b l e by conversion e l e c t r o n Mossbauer-spectroscopy (CEMS)? J. Phys. ( P a r i s ) Colloq. C6, 37: 295-296. Richardson, S.M., 1975. A p i n k muscovite w i t h r e v e r s e pleochroism from Archers Post, Kenya. Am. M i n e r a l . 60: 73-78. Rozenson, I . and H e l l e r - K a l l a i , L., 1976a. Reduction and o x i d a t i o n o f Fe3+ i n d i o c t a h e d r a l s m e c t i t e s - 1: Reduction w i t h hydrazine and d i t h i o n i t e . Clays Clay Minerals, 24: 271-282. Rozenson. I . and H e l l e r - K a l l a i , - L., - 1976b. Reduction and o x i d a t i o n o f Fe3+ i n dioctahedral smectites 2: Reduction w i t h sodium s u l p h i d e s o l u t i o n s . Clays Clay Minerals, 24: 283-288. Rozenson, I. and H e l l e r - K a l l a i , L. , 1977. Mossbauer s p e c t r a o f d i o c t a h e d r a l smectites. Clays Clay M i n e r a l s , 25: 94-101. Rozenson, I.and H e l l e r - K a l l a i , L., 1978. tdossbauer s p e c t r a o f g l a u c o n i t e s r e examined. Clays Clay M i n e r a l s , 26: 173-175. Russell, J.D., Goodman, B.A. and Fraser, A.R., 1979. I n f r a r e d and Mossbauer s t u d i e s o f reduced n o n t r o n i t e s . Clays Clay M i n e r a l s , 27: 63-71. Sanz, J., Meyers, J., Vielvoye, L. and Stone, W.E.E., 1978. The l o c a t i o n and c o n t e n t o f i r o n i n n a t u r a l b i o t i t e s and ph1ogopites:a comparison o f several methods. Clay M i n e r a l s , 13: 45-52. Shenoy, G.K., F r e i d t , J.M., M a l e t t a , H. and Ruby, S.L., 1975. Curve f i t t i n g and t h e t r a n s m i s s i o n i n t e g r a l : warnings and suggestions. In: I.J. Gruverman ( E d i t o r ) , Mossbauer E f f e c t Methodology, Plenum, New York, 9: 277-305. Sternheimer, R.M., 1963. S h i e l d i n g and a n t i s h i e l d i n g e f f e c t s f o r v a r i o u s i o n s and atomic systems. Phys. Rev., 146: 140-160. Stevens, J.G., Stevens, V.E. and Gettys, W.L. Mossbauer E f f e c t Reference and Data Journal, Mossbauer E f f e c t Data Center, Univ. N o r t h Carolina, U.S.A. Taylor, G.L., RusQala, A.P. and Keeling, R.O., 1968. Analysis o f i r o n i n l a y e r s i l i c a t e s by Mossbauer Spectroscopy. Clays Clay M i n e r a l s , 16: 381-391. T r i c k e r , M.J., 1977. Iron-57 conversion e l e c t r o n Mossbauer !Pectroscopy. Surf. Defect Prop. S o l i d s , 6: 106-138.

-

-

137 T r i p a t h i , R.P., Chandra, V., Chandra, R. and Lokanathan, S., 1978. A Mossbauer s t u d y o f t h e e f f e c t s o f h e a t i n g b i o t i t e , p h l o g o p i t e and v e r m i c u l i t e . J . I n o r g . Nucl. Chem., 40: 1293-1298. V i r g o , D. and Hafner, S.S., 1969. Fez+, Mg o r d e r - d i s o r d e r i n heated orthopyroxenes. M i n e r a l . SOC. Amer.,.Spec. Pap., 2: 67-81. Wickman, H.H., 1966. Mossbauer paramagnetic h y p e r f i n e s t r u c t u r e . I n : I . J . Gruverman ( E d i t o r ) , Mossbauer E f f e c t Methodology, Plenum, New York, 2: 39-66. Yassoglou, N.J., N o b e l i , C., K o s t i k a s , A.J. and Simopoulos, A.C. .. 1972. Weathering o f m i c a f l a k e s i n two s o i l s i n N o r t h e r n Greece e v a l u a t e d b y Mossbauer and c o n v e n t i o n a l t e c h n i q u e s . S o i l S c i . SOC. Amer. Proc., 36: 520-527.

139 Chapter 6

ELECTRON S P I N RESONANCE STUDIES OF CLAY EINERALS Thomas J . P I N N A V A I A Department o f Chemistry, M i c h i g a n S t a t e U n i v e r s i t y , East Lansing, M i c h i g a n 48824, USA. 6.1

INTRODUCTION I n r e c e n t y e a r s , e l e c t r o n s p i n resonance ( e s r ) s p e c t r o s c o p y has proven t o be

a powerful t o o l i n studies o f c l a y mineral chemistry.

The o r i e n t a t i o n s , dynamics,

and r e a c t i o n s o f a v a r i e t y o f i n t e r c a l a t e d paramagnetic s p e c i e s have been i l l u c i d a t e d b y e s r s p e c t r o s c o p y . The paramagnetic c e n t e r s on t h e basal s u r f a c e s may be s i m p l e h y d r a t e d c a t i o n s (*, C U ( H ~ O2+ ) ~, C U ( H * O ) ~ ~ +VO(H20),2+) , o r they may be metal complexes such as Cu(en)2

.

2+ , Cu(phen)3 2+ . Adsorbed o r g a n i c r a d i c a l s ,

such as t h e p e r y l e n e c a t i o n r a d i c a l o r o r g a n i c m o l e c u l e s c o n t a i n i n g t h e paramagnetic n i t r o x i d e m o i e t y ( = N - O ) , a l s o l e n d themselves t o s t u d y by e s r s p e c t r o s c o p y . The 3+ e s r s p e c t r a o f c e r t a i n t r a n s i t i o n m e t a l i o n s ( p a r t i c u l a r l y Fe ) which s u b s t i t u t e f o r aluminum o r s i l i c o n i n t h e oxygen framework can p r o v i d e u s e f u l i n f o r m a t i o n on t h e n a t u r e o f thermal processes and t h e s t a t e o f o r d e r o r d i s o r d e r o f t h e c l a y s t r u c t u r e b e i n g probed. The i n t e n t o f t h e p r e s e n t paper i s t o p r o v i d e some r e c e n t examples o f t h e k i n d o f i n f o r m a t i o n t h a t can be o b t a i n e d t h r o u g h t h e a p p l i c a t i o n o f e s r t o t h e study o f c l a y minerals.

Several r e v i e w a r t i c l e s on t h e a p p l i c a t i o n s o f e s r

s p e c t r o s c o p y t o c l a y m i n e r a l s have appeared r e c e n t l y , w h i c h complement t h e p r e s e n t work ( H a l l , 1980a, 1980b; HcBride, 1980; P i n n a v a i a , 1980).

Two e a r l i e r r e v i e w s

by Che e t a l . (1974) and by P i n n a v a i a (1976a) a r e a l s o a v a i l a b l e .

Space l i m i t a t i o n s

do n o t a l l o w f o r an adequate t r e a t m e n t o f e s r t h e o r y , b u t s e v e r a l e x c e l l e n t t r e a t i s e s a r e a v a i l a b l e (Wertz and B o l t o n , 1972; Abragam and Bleaney, 1970; Ingram, 1967)

.

HYDRATED EETAL I O N S ON BASAL SURFACES

6.2 6.2.1

Copper( 11)

Clementz e t a l . (1973) addressed t h e q u e s t i o n o f m e t a l i o n o r i e n t a t i o n on t h e i n t e r l a m e l l a r s u r f a c e s o f s m e c t i t e c l a y s c o n t a i n i n g a r e s t r i c t e d number o f w a t e r layers.

The c o p p e r ( I 1 ) i o n was s e l e c t e d as an i d e a l e s r probe, i n p a r t , because

i t has a s i n g l e u n p a i r e d e l e c t r o n w i t h a s p i n S = 1/2.

A l s o , under c o n d i t i o n s

where t h e m e t a l i o n i s s o l v a t e d by one, two o r t h r e e m o l e c u l a r l a y e r s o f w a t e r , t h e i o n i s expected t o possess t e t r a g o n a l symmetry and t o g i v e r i s e t o an a n i s o tropic esr signal.

I n t h e absence o f any i n t e r a c t i o n s between t h e e l c t r o n s p i n

and any n e i g h b o r i n g n u c l e a r s p i n s , t h e s p i n - H a m i l t o n i a n f o r t h e i o n under t e t r a g o n a l

140

symmetry may be w r i t t e n as cos

s,

+ g1 s i n 6 S z ) t h e Bohr magneton (eh/2mc), g I 6

H i s t h e magnetic f i e l d , and

(1 1

I

and g l a r e

spectroscopic s p l i t t i n g f a c t o r s ,

i s the angle between t h e magnetic f i e l d d i r e c t i o n and t h e symmetry a x i s of t h e tetragonal i o n , which i s a r b i t r a r i l y defined along z . Since two g tensors appear i n the Hamiltonian, two resonance components will appear in t h e e s r spectrum, one corresponding t o spin quantization i n a d i r e c t i o n p a r a l l e l t o t h e symmetry a x i s ( g ) and another corresponding t o spin quantizaII t i o n perpendicular t o t h e symmetry a x i s .(g ) . I f we allow f o r coupling between t h e S = 1/2 electron spin and t h e I = 3/2 nuclear spin of t h e copper nucleus, then two more t e r n s must be added t o t h e spin Hamiltonian: 6

1

A / lSZIZ

+

A 1(SXIX+ SYIY)

where A1 I and A

L are

(2)

hyperfine s p l i t t i n g c o n s t a n t s , usually expressed in cm-l o r

in gauss. I n t h e presence of a n applied magnetic f i e l d , t h e r e f o r e , t h e S = 1 / 2 ground s t a t e i s s p l i t by an amount gBH i n t o two energy s t a t e s (corresponding t o quant i z e d o r i e n t a t i o n s of t h e e l e c t r o n spin components i n a d i r e c t i o n p a r a l l e l (!Is= -1/2) or a n t i p a r a l l e l ( M s = 1/2) t o t h e magnetic f i e l d . These two s t a t e s a r e s p l i t f u r t h e r by A h / 2 due t o coupling of the e l e c t r o n spin with t h e four quantized components of t h e I = 3/2 nuclear spin ( M I = +3/2, + 1 / 2 ) . The energy level diagram i s i l l u s t r a t e d i n F i g u r e 6.1. The allowed t r a n s i t i o n s correspond t o AM^ = 0 , nMI = 1 . T h u s , we see t h a t both t h e g I I and g1 resonance components a r e s p l i t i n t o q u a r t e t s due t o hyperfine coupling. 0

From equation 1 we may conclude t h a t when 6 = o , only the g , I resonance component will be observed, a n d when e = 90 , only t h e g component will be observed. I n a random powder sample, however, a l l possible values of 6 occur, and both resonance components will be seen. Figures 6.2A and 6.28 i l l u s t r a t e t h e e s r s p e c t r a of a powder sample of Cu2'-hectorite under conditions where a s i n g l e molecular l a y e r of water occupies t h e i n t e r l a y e r s (dool = 12.4 A ) . As expected, both g II and 4 1 resonance components occur with g 1I = 2.34, A = 0.0165 cm-l ' g1 = 2-08. A i s too small t o be resolved. Figures 6.2C and 6.2D show the esr s p e c t r a o f an 0

L 0

L

oriented film sample of Cu2+-hectorite with t h e magnetic f i e l d d i r e c t i o n oriented I I and L t o t h e s i l i c a t e s h e e t s . Since g i s observed f o r t h e I I o r i e n t a t i o n and 91 I i s observed f o r the o r i e n t a t i o n , we may conclude t h a t t h e symmetry a x i s of the planar C U ( H ~ O ) ~ion ~ + i s oriented a t 90' t o the plane of t h e s i l i cate sheets.

1

2+ .

1

When Cu i s p a r t of a two-water l a y e r system a s in f u l l y hydrated Cu v e r m i c u l l i t e (dool = 14.2 A ) , a n i s o t r o p i c spectra a r e observed f o r oriented film 2+

0

14 1

I f 112)

Figure 6.1.

Energy level diagram for C u 2+

QII

Figure 6 . 2 .

-H

Esr spectra o f Cu2+-hectorite: A, B are for random powders; C , D are for oriented film.

142 1 0.0145 cm- ) , b u t t h e s p e c t r a a r e independent of sample o r i e n t a t i o n in t h e magnetic f i e l d . This means t h a t t h e symmetry samples ( g 1

1

=

2.38, g

=

2.16, A

II

=

~ + i s oriented near 45' t o the s i l i c a t e a x i s of t h e i n t e r c a l a t e d C U ( H ~ O ) ~ion 2t s h e e t s . However, when t h e Cu(H20l6 ion i s doped i n t o t h e three-water l a y e r hydration s t a t e of MgZt-hectorite ( d o o l = 15.0 t h e o r i e n t a t i o n dependent spectra shown i n Fiqure 6 . 3 a r e observed (McBride e t a l . , 1975a). From t h e observed

i),

o r i e n t a t i o n dependence, i t may be concluded t h a t C U ( H ~ O ) ~i ~s +oriented on t h e basal surfaces with t h e symmetry a x i s near 90 sheets.

t o the plane of t h e s i l i c a t e

Figure 6.4 summarizes the d i s t i n c t o r i e n t a t i o n of t h e Cu2+ ions hydrated by one, two, and t h r e e l a y e r s of i n t e r l a m e l l a r water. When the i n t e r l a y e r s of Cu2+0

smectites a r e f u l l y swollen with water ( d o o l = 21 A ) , q u i t e a d i f f e r e n t p i c t u r e emerges from the e s r spectrum. Under these l a t t e r conditions, a s i n g l e i s o t r o p i c l i n e i s observed, s i m i l a r t o t h e resonance found f o r C U ( H ~ O ) ~ i ~n + d i l u t e aqueous s o l u t i o n . The averaging of g l I and g may a r i s e from two very d i f f e r e n t processes: ( 1 ) rapid tumbling of t h e ion may be occurring i n a highly mobile l i q u i d - l i k e i n t e r l a y e r environment o r ( 2 ) a dynamic Jahn-Teller e f f e c t may be occurring in a r i g i d , i c e - l i k e arrangement of water molecules in t h e i n t e r l a y e r regions. The Jahn-Teller dynamic e f f e c t , which involves rapid interchange of t h e t h r e e principal axes of the C U ( H ~ O ) ~ion ~ + through coupling of the v i b r a t i o n modes of t h e aquo l i g a n d s , i s responsible f o r t h e i s o t r o p i c e s r l i n e observed f o r C U ( H ~ O ) in ~ ~frozen + aqueous s o l u t i o n s ( H u d s o n , 1966). As we s h a l l see 2+ and V02+ ions i n s m e c t i t e , t h e ions l a t e r , based on e s r s t u d i e s of hydrated Mn do i n f a c t tumble rapidly in a s o l u t i o n - l i k e environment when t h e i n t e r l a y e r s a r e swollen with m u l t i p l e l a y e r s of water.

1

6.2.2

Vanadyl

, V02+

VO(H20)62+-hectorite in t h e f u l l y wetted s t a t e e x h i b i t s t h e blue c o l o r charact e r i s t i c of t h e ion i n aqueous s o l u t i o n . However, under c e r t a i n conditions of loading and hydration s t a t e , t h e blue color i s l o s t and a tan-brown c o l o r develops, indicating t h a t a surface r e a c t i o n takes place which depends on moisture content. The surface reactions have been investigated in p a r t by Pinnavaia e t a l . (1974) and by HcBride (1979a). The hydrated vanadyl ion has a d 1 e l e c t r o n i c configuration and tetragonal sym2+ metry. I t s e s r p r o p e r t i e s resemble those of C U ( H ~ O ) ~. Under normal conditions, t h e ion e x h i b i t s g and gL resonances with both resonance components being s p l i t i n t o e i g h t hyperfine l i n e s due t o coupling of t h e S = 1 / 2 e l e c t r o n spin w i t h t h e I = 7 / 2 nuclear s p i n . However, when dissolved in water, t h e ion e x h i b i t s only a time-averaged i s o t r o p i c l i n e due t o rapid tumbling which averages t h e g I I and 91 components. Fully wetted V02+-hectorite a l s o gives a n i s o t r o p i c spectrum s i m i l a r

Figure 6.3.

Esr spectra o f Cu2+ doped i n t o Plg*+-hectorite film with H p a r a l l e l ( A ) and perpendicular ( B ) t o s i l i c a t e . sheets (from HcBride e t a1 . , 1975a).

144

Figure 6.4.

Orientations of i n t e r l a y e r aquo copper( 11) ions formed by hydration with one, two, and t h r e e l a y e r s of water. Open c i r c l e s a r e oxygen atoms of t h e s i l i c a t e sheet and 1 igating water molecules (from Pinnavaia, 1976b).

145 t o t h e aqueous s o l u t i o n spectrum. l i k e spectrum i s r e t a i n e d when VO (see F i g u r e 6.5)

2+

McBride (1979) has found t h a t t h e s o l u t i o n i s doped i n t o M g 2 + - h e c t o r i t e a t t h e 50% l e v e l

However, when t h e i o n i s doped i n t o M g 2 + - h e c t o r i t e a t t h e 5%

l e v e l , a tan-brown c o l o r develops and an a n i s o t r o p i c spectrum i s observed w h i c h i s i n d i c a t i v e o f a h i g h l y o r d e r e d , immobile form.

FIcBride has suggested t h a t

t h i s o r d e r e d tan-brown f o r m o f vanadyl i s VO(OH)2(H30)3.

S i n c e t h e spectrum i s

o r i e n t a t i o n independent o f t h e m a g n e t i c f i e l d , t h e symnetry a x i s appears t o be i n c l i n e d n e a r 45

0

t o t h e s i l i c a t e sheets.

A i r - d r i e d V 0 2 + - h e c t o r i t e g i v e s t h e o r i e n t a t i o n - d e p e n d e n t e s r s p e c t r a shown i n F i g u r e 6.6. S i n c e A l

A

I

(704 G) i s observed f o r t h e p e r p e n d i c u l a r o r i e n t a t i o n and

G ) i s observed f o r t h e p a r a l l e l o r i e n t a t i o n , t h e symmetry a x i s , w h i c h i s

1(81

colinear with the

V=O

bond, l i e s p e r p e n d i c u l a r t o t h e s i l i c a t e sheets.

The above s t u d i e s have focused on t h e o r i e n t a t i o n o f m e t a l i o n s on t h e basal surfaces.

E s r s t u d i e s have a l s o proven t o be u s e f u l i n c h a r a c t e r i z i n g mixed

Nat-Cu2+ and m i x e d NR4+-Cu2+ s m e c t i t e systems (McBride, 1976a; N c B r i d e and M o r t l a n d , 1975).

The e s r s p e c t r a o f Cu2+ on reduced charge m o n t m o r i l l o n i t e

and on k a o l i n i t e s u r f a c e s have a l s o been examined ( N c B r i d e and t l o r t l a n d , 1974; McBride, 1976b; Clementz e t a l . , 6.3

1974).

MOBILITY OF INTERLAYER METAL IONS

6.3.1

Manganese( II)

As n o t e d e a r l i e r , Cu2+ i s n o t an i d e a l probe f o r examining i n t e r l a y e r m o b i l i t y because e i t h e r dynamic J a h n - T e l l e r e f f e c t s o r r a p i d t u m b l i n g can l e a d t o a v e r a g i n g Of

91 I and g

effect

components. L does n o t a p p l y and

However, f o r !In2+ and V02+, t h e dynamic J a h n - T e l l e r b o t h i o n s a r e s u i t a b l e probes f o r t h i s purpose, t h e

e s r t h e o r y h a v i n g been w e l l developed ( B u r l a m a c c h i , 1971 ; Burlamacchi e t a l . , 1970, 1973; G a r r e t t and Morgan, 1966; Campbell and Hanna, 1976). 5 The h y d r a t e d $ln(H20)62+ i o n has a h i g h s p i n d e l e c t r o n i c c o n f i g u r a t i o n .

In

most environments, a l l t h r e e g - t e n s o r components a r e equal and i s o t r o p i c s p e c t r a a r e observed.

Because o f i t s i s o t r o p i c n a t u r e , !h2+i s n o t w e l l s u i t e d f o r e s r

s t u d i e s o f m e t a l i o n o r d e r i n g on c l a y s u r f a c e s , b u t t h e e s r l i n e w i d t h s may be readily related t o ion mobility. The s p e c t r a o f Mn2+ i n f u l l y h y d r a t e d forms o f m o n t m o r i l l o n i t e , h e c t o r i t e , v e r m i c u l i t e and n o n t r o n i t e resemble t h e spectrum o f t h e i o n i n homogeneous s o l u t i o n (McBride e t a l . ,

1975b).

I n each case, s i x h y p e r f i n e l i n e s a r e observed due

t o c o u p l i n g o f t h e S = 5 / 2 e l e c t r o n s p i n w i t h t h e I = 5/2 n u c l e a r s p i n .

Each

h y p e r f i n e component c o n s i s t s o f t h r e e superimposed L o r e n t z i a n l i n e s a r i s i n g f r o m f i v e AM,

= 1 transitions.

I n t h e absence o f inhonogeneous l i n e broadening e f f e c t s ,

t h e l i n e w i d t h s (AH) a r e t h e sum o f t w o c o n t r i b u t i o n s AH = AHI

+

AHD

(3)

146

200 GAUSS c------1 H -----t

I

F i g u r e 6.5.

VO*+/ M$'-

Hect.

E s r s p e c t r a o f V02+ i n aqueous s o l u t i o n M, pH = 1.5) and i n a 50:50 (2 x VO2+/Mg2+-Kectorite f i l m ( f r o m McBride, 1979a).

147 where AHI

i s t h e i n t r i n s i c l i n e w i d t h due t o c o l l i s i o n a l r e l a x a t i o n processes and

nHD i s t h e l i n e w i d t h due t o d i p o l a r i n t e r a c t i o n s between n e i g h b o r i n g Mn2+ i o n s ( H i n c k l e y and Morgan, 1966). The AHD t e r m i s c o n c e n t r a t i o n dependent because t h e d i p o l a r i n t e r a c t i o n s a r e p r o p o r t i o n a l t o r-3, where r i s t h e average Hn2+ Mn2+ d i s t a n c e .

11,

In d i l u t e s o l u t i o n ( < 0.01

and d e t e r m i n e d e x c l u s i v e l y by aHI.

r > 55

i), the

e s r l i n e s a r e narrow The l i n e w i d t h s o f f u l l y h y d r a t e d Fln2+ -

h e c t o r i t e a r e a p p r e c i a b l y b r o a d e r t h a n t h o s e o f Nn2+ i n d i l u t e s o l u t i o n , as can be seen by comparing F i g u r e s 6.7a and 6.7b. Furuhata and Kuwata (1969).

A s i m i l a r o b s e r v a t i o n has been made by

D r y i n g t h e m i n e r a l decreases t h e m o b i l i t y o f t h e

i n t e r l a y e r and broadens t h e l i n e s even more ( c f . F i g u r e 6 . 7 ~and 6.7d).

2+

c l e a r t h a t t h e l i n e w i d t h s o f f u l l y s a t u r a t e d En m a i n l y b y d i p o l a r r e l a x a t i o n processes.

It i s

- h e c t o r i t e a r e determined

However, t h e d i p o l a r e f f e c t s may be

removed b y d o p i n g t h e i o n a t t h e 5% l e v e l i n t h e t l g 2 + - h e c t o r i t e . The average 2+ Mn -Mn2+ i n t h e doped m i n e r a l i s 'L 55 v e r s u s 'L 12 i n the f u l l y saturated 2+ m i n e r a l . Theory i n d i c a t e s t h a t T, t h e c o r r e l a t i o n t i m e f o r c o l l i s i o n o f Fn

i

i

i o n w i t h b u l k w a t e r m o l e c u l e s , i s d i r e c t l y p r o p o r t i o n a l t o t h e w i d t h o f t h e ?I1 = -1/2 t r a n s i t i o n

(s, the fourth-highest

i n t e r a c t i o n s a r e absent. h e c t o r i t e i s 28.6 G

E.2 2

f i e l d 1i n e ) p r o v i d e d t h a t d i p o l a r 2+ S i n c e t h e l i n e w i d t h f o r Mn2+ doped i n t o Mg G f o r Mn2+ i n d i l u t e s o l u t i o n , T i s o n l y

i n t h e i n t e r l a y e r o f f u l l y w e t t e d F1g2+-hectorite (dool

= 21

s o l u t i o n , where i t has been e s t i m a t e d t o be 3.2 x 1971).

30% l o n g e r

i n dilute

sec. ( R u b i n s t e i n e t a l . ,

We may conclude, t h e r e f o r e , t h a t t h e i n t e r l a y e r s a r e v e r y s o l u t i o n - l i k e

even when t h e i n t e r l a y e r s a r e o n l y 6.3.2

i) than

%

'i,

12

0

A thick.

Vanadyl i o n

The m o b i l i t y o f h y d r a t e d V02+ doped i n t o M g 2 + - h e c t o r i t e has a l s o been i n v e s I n t h i s system, t h e

t i g a t e d by e s r l i n e b r o a d e n i n g methods (ElcBride, 1979). MI

= 7/2

transition i s proportional t o

T

(Chasteen and Hanna, 1972).

l i n e w i d t h o f f u l l y h y d r a t e d V02+/Mg2+-hectorite d i l u t e solution,

T

i s 35 G

E.

Since t h e

23 G f o r V02+ i n

i s 1.5 t i m e s l a r g e r i n t h e c l a y t h a n i n d i l u t e s o l u t i o n .

S i n c e t h e c o r r e l a t i o n t i m e i s 5 x 16" environment i s 7.5 x

sec.

sec. i n d i l u t e s o l u t i o n ,

Thus, t h e V02+ i o n , l i k e !In2+,

T

f o r the c l a y

tumbles r a p i d l y

i n f u l l y w e t t e d # g 2 + - h e c t o r i t e i n a s o l u t i o n - l i k e environment. When t h e w a t e r i n V02+-tlg2+-hectorite o f t h e V02+ i o n i s decreased.

i s r e p l a c e d by methanol, t h e m o b i l i t y

Though t h e i o n i s n o t c o m p l e t e l y o r i e n t e d on t h e

esr t i m e scale, t h e motion o f t h e i o n i s n o t s u f f i c i e n t l y f a s t t o completely average A l l and

All

i n d i c a t i n g an i n t e r m e d i a t e r a t e o f t u m b l i n g .

A similar

decrease i n m o b i l i t y i s i n d i c a t e d when t h e w a t e r i n Mn2+ s m e c t i t e i s r e p l a c e d by l a r g e r o r g a n i c m o l e c u l e s such as p y r i d i n e (Pafamov e t a l . , Ovcharenko, 1973).

1971; T a r a s e v i c h and

148

n

Ill

VQ2*- Hectorite (air-dry)

F i g u r e 6.6. E s r s p e c t r a o f an a i r - d r y V O * + - h e c t o r i t e f i l m ( f r o m McBride, 1979a).

F i g u r e 6.7.

Mn2+ i n methanol ( A ) and i n h e c t o r i t e f u l l y h y d r a t e d (B), a i r d r i e d (C),

and d r i e d a t 200" (D) ( f r o m I l c B r i d e e t a l . , 1975b).

149

6.4

INTERLAMELLAR METAL COFIPLEXES Esr can be an e x c e p t i o n a l l y p o w e r f u l t o o l f o r o b s e r v i n g t h e f o r m a t i o n o f metal

complexes on t h e i n t e r l a m e l l a r s u r f a c e s o f c l a y s .

The e s r parameters o f t h e com-

p l e x should be d i f f e r e n t from t h o s e o f t h e s i m p l e s o l v a t e d i o n , p r o v i d i n g t h a t r a p i d t u m b l i n g o f t h e complex does n o t average t h e s e parameters.

Studies o f the

o r i e n t a t i o n dependence o f f i l m samples can p r o v i d e i n f o r m a t i o n on t h e o r i e n t a t i o n o f t h e complex. B e r k h e i s e r and F l o r t l a n d (1975) have i n v e s t i g a t e d t h e r e a c t i o n s o f Cu2+m o n t m o r i l l o n i t e w i t h p y r i d i n e . F i g u r e 6.8 i l l u s t r a t e s t h e spectrum o f t h e m i n e r a l When

s o l v a t e d by d i m e t h y l s u l f o x i d e b e f o r e and a f t e r t h e a d d i t i o n o f p y r i d i n e .

t h e m i n e r a l i s s o l v a t e d o n l y by DHSO, an i s o t r o p i c spectrum, = 2.15, i s observed due t o r a p i d t u m b l i n g o f t h e i o n .

However, when p y r i d i n e i s added, t h e spectrum

o f an o r i e n t e d C u ( p ~ ) ~complex ~ + i s clearly indicated with g 1 and A l

I

= 0.0139 cm-l.

1

= 2.24,

9 1 = 2.06

Nagai e t a l . (1974) a l s o observed by e s r spectroscopy

t h e f o r m a t i o n o f Cu2+ complexes on m o n t m o r i l l o n i t e f o l l o w i n g t h e a d s o r p t i o n o f p y r i d i n e and c e r t a i n amino a c i d s . M e t a l complexes c a n be i n t e r c a l a t e d i n s m e c t i t e s by d i r e c t i o n exchange r e a c t i o n ( B e r k h e i s e r and M o r t l a n d , 1977; Velghe e t a l . ,

1977; T r a y n o r e t a l . , 1978).

B e r k h e i s e r and M o r t l a n d (1977) have shown t h a t t h e spectrum ofiCu( p h e n ) 3 2 + - h e c t o r i t e depends on t h e degree o f h y d r a t i o n .

The f u l l y w e t t e d exchange f o r m g i v e s a

As t h e

n e a r l y i s o t r o p i c spectrum, i n d i c a t i n g c o n s i d e r a b l e i n t e r l a y e r m o b i l i t y .

degree o f h y d r a t i o n o f t h e c l a y was decreased, an a n i s o t r o p i c spectrum was observed which i n d i c a t e d t h a t t h e complex became o r i e n t e d on t h e s u r f a c e .

2+

t h e Cu( p h e n ) 3 - h e c t o r i t e t o 200

(gl

I

= 2.240;

gL = 2.058; A ,

I

D

= 0.0172 c m - l ) .

o c c u r r e d on t h e i n t e r l a y e r s u r f a c e s .

.

2+

That i s , t h e e s r d a t a c l e a r l y

showed t h e 1 igand d i s s o c i a t i o n r e a c t i o n Cu(phen)32t Velghe e t a1

-

Heating

gave an e s r spectrum c h a r a c t e r i s t i c o f Cu(phen)2 Cu( phen)22t

+ phen

(1977) i n v e s t i g a t e d t h e n a t u r e o f Cu2+-ethylenediamine complexes

formed on h e c t o r i t e b y i o n exchange r e a c t i o n . c o n t a i n e d m a i n l y Cu(en)'+,

A l t h o u g h t h e exchange s o l u t i o n

t h e e s r spectrum o f t h e c l a y i n d i c a t e d t h e presence

o f two complex s p e c i e s , Cu(en)22+ w i t h g 1I = 2.181,

A,I

w i t h g 1 I = 2.261,

= 0.0204 cm-l,

A,

I

gl =

and Cu(en)'+

A

This observation indicates t h a t ligand r e d i s t r i b u t i o n

I =0.0013.

= 0.0182 cm-l,

'JI =

= 0.0019 cm-'

r e a c t i o n s o c c u r on t h e c l a y s u r f a c e w h i c h f a v o r s t h e Cu(enIz2+ r e l a t i v e t o homogeneous s o l u t i o n .

The a d d i t i o n o f excess en vapor t o Cu(en)22t-saturated 2+ ( g , , = 2.20, A l I = 0.0183 h e c t o r i t e gave an e s r spectrum i n d i c a t i v e o f Cu(en)3 -1 1 2t cm , gL = 2.048, A 0.0007 cm- ) . The spectrum o f a f i l m sample o f Cu(en)3

I=

was o r i e n t a t i o n independent,

i n d i c a t i n g t h a t t h e symmetry a x i s i s i n c l i n e d near

45O t o t h e s i l i c a t e sheets.

The e s r parameters o f Cu(en)

2+

, Cu(en)22+ and

Cu(en)32+ on h e c t o r i t e s u r f a c e s a r e v e r y s i m i l a r t o t h o s e f o r t h e i o n s i n

150

F i g u r e 6.8.

F i g u r e 6.9.

Esr s p e c t r a o f Cu2'-hectorite s o l v a t e d by d i m e t h y l s u l f o x i d e and p y r i d i n e ( f r o m B e r k h e i s e r and M o r t l a n d , 1975).

Na', L i ' and Ca2+ m o n t m o r i l l o n i t e s The Fe3+ s i g n a l s o f K', a t v a r i o u s r e l a t i v e h u m i d i t i e s . Arrows i n d i c a t e t h e weak Fe3+ resonance ( f r o m H c B r i d e e t a1 , 1 9 7 5 ~ ) .

.

151 d i l u t e solution.

U s i n g d i f f e r e n t t h e o r e t i c a l models, Schoonheydt (1978) has con2+ ?I bonding i n Cu -en complexes i s s l i g h t l y

cluded t h a t t h e e x t e n t o f out-of-plane i n c r e a s e d on c l a y s u r f a c e s . 6.5

FRAMEWORK PARAMAGNETIC CENTERS N a t u r a l c l a y s may c o n t a i n a v a r i e t y o f paramagnetic i o n s .

Some o f t h e i o n s

may be p r e s e n t on t h e exchange s i t e s o r i n o c t a h e d r a l o r t e t r a h e d r a l p o s i t i o n s i n t h e oxygen framework.

Other paramagnetic c e n t e r s may be p r e s e n t as a s e p a r a t e

i m p u r i t y phase, such as i r o n o x i d e s (Goodman, 1978).

I m p u r i t y phases can some-

t i m e s be removed b y a s e l e c t i v e e x t r a c t i o n t e c h n i q u e , such as t h e c i t r a t e d i t h i o n i t e method o f Mehra and Jackson (1960) f o r t h e removal o f i r o n o x i d e s . 3t Fe i s b y f a r t h e most abundant e s r o b s e r v a b l e paramagnetic i o n i n n a t u r a l 2t

clays.

O t h e r i o n s such as Mn

magnetic s p e c i e s

and V02' a r e a l s o e s r o b s e r v a b l e , b u t some para-

2+ (a, Fez' and T i

under most c o n d i t i o n s .

a r e non-Kramers s p e c i e s and a r e e s r s i l e n t

Among t h e r e a d i l y a v a i l a b l e n a t u r a l c l a y s , h e c t o r i t e con-

t a i n s one o f t h e l o w e s t c o n c e n t r a t i o n s o f framework Fe3+.

Therefore, d i p o l a r

i n t e r a c t i o n s which broaden t h e l i n e s o f s u r f a c e exchange i o n s a r e m i n i m a l i n t h i s clay.

T h i s i s one r e a s o n why h e c t o r i t e has been most f r e q u e n t l y used i n e s r

s t u d i e s o f s u r f a c e bound s p e c i e s .

I t must be n o t e d , however, t h a t some h e c t o r i t e

samples, depending on e x a c t l o c a t i o n , can c o n t a i n e s p e c i a l l y h i g h c o n c e n t r a t i o n s

o f i r o n o x i d e s w h i c h g i v e r i s e t o a v e r y broad l i n e (AH > 1000 G) c e n t e r e d near g = 2.0. 6.5.1

Smecti t e s

O l i v i e r e t a l . (1975) have examined t h e e s r s p e c t r a o f s e v e r a l s m e c t i t e s .

All

3t samples e x h i b i t e d p r o m i n e n t f e a t u r e s near g = 4.3 w h i c h were a t t r i b u t e d t o Fe i n two d i s t i n c t o c t a h e d r a l s i t e s and two d i f f e r e n t t e t r a h e d r a l s i t e s . resonances have been observed f o r micas and v e r m i c u l i t e (01 i v i e r e t a1 1976b, 1977).

Similar

. , 1976a,

The n o n - e q u i v a l e n t o c t a h e d r a l environments were a t t r i b u t e d t o t h e

two a l t e r n a t i v e ( c i s and t r a n s ) arrangements o f h y d r o x y l groups i n t h e Fe04(0H)2 cavity.

However, McBride e t a l . (1975c, 1975d) have o f f e r e d a n a l t e r n a t i v e

e x p l a n a t i o n f o r n o n - e q u i v a l e n t o c t a h e d r a l Fe3'.

The h i g h f i e l d a n i s o t r o p i c

resonance n e a r g = 4.3 was found t o be s e n s i t i v e t o t h e p o s i t i o n o f t h e exchange c a t i o n i n s m e c t i t e s (McBride e t a1 ., 1975c, 1975d; B e r k h e i s e r and M o r t l a n d , 1975). As can be seen from F i g u r e 6.9,

K+ and Nat exchange i o n s cause s i g n i f i c a n t decreases

i n t h e i n t e n s i t y o f t h e h i g h - f i e l d a n i s o t r o p i c resonance when t h e r e l a t i v e h u m i d i t y i s reduced t o 08, whereas t h e s t r o n g l y h y d r a t e d L i

+

and Ca2+ exchange

i o n s under t h e same c o n d i t i o n s do n o t i n f l u e n c e t h i s resonance l i n e . thermal m i g r a t i o n o f

+ Li

Moreover,

i n t o t h e vacant octahedral p o s i t i o n o f t h e mineral,

a l s o causes a r e d u c t i o n i n s i g n a l i n t e n s i t y w h i c h i s n o t r e s t o r e d upon r e s o l u t i o n o f t h e mineral.

The l o w f i e l d i s o t r o p i c Fe3' component remains u n a f f e c t e d

152 under t h e above c o n d i t i o n s .

These o b s e r v a t i o n s suggest t h a t t h e h i g h f i e l d

a n i s o t r o p i c Fe3+ resonance component i s a s s o c i a t e d w i t h c e n t e r s o f n e g a t i v e charge i n t h e s i l i c a t e framework.

T h e r e f o r e , t h e two n o n - e q u i v a l e n t Fe3+ s i t e s 2+ may a r i s e from o c t a h e d r a l Fe3+ i o n s a d j a c e n t t o c h a r g e - d e f i c i e n t Hg s i t e s and n o n - d e f i c i e n t A13+ s i t e s .

T h i s model a l l o w s a l l Fe3+ i o n s t o occupy o c t a h e d r a l

s i t e s w i t h c i s OH c o n f i g u r a t i o n . 6.5.2

Kaolinites

The e s r s p e c t r a o f k a o l i n i t e s v a r y m a r k e d l y , depending on l o c a l i t y .

However,

a l l n a t u r a l k a o l i n i t e s have two e s r f e a t u r e s i n common w h i c h a r e independent o f i m p u r i t y phases.

They a l l e x h i b i t a group o f l i n e s near g = 4 w h i c h a r e a t t r i b u t a -

b l e t o l a t t i c e Fe3+ and a second s e t o f l i n e s near g = 2 w h i c h a r i s e from l a t t i c e d e f e c t s (Heads and Malden, 1975; Jones e t a l . ,

1974).

Three Fe3+ c e n t e r s have

been d i s t i n g u i s h e d and d e s i g n a t e d c e n t e r s I,I I a and I I b .

Center I g i v e s r i s e

t o an i s o t r o p i c l i n e a t g = 4.2, w h i l e c e n t e r s I I a and I I b g i v e a l i n e near g =

4.9 and two f r e q u e n t l y u n r e s o l v e d l i n e s a t g = 3.7 and 3.5.

Based on c o r r e l a -

t i o n s o f l i n e i n t e n s i t i e s w i t h t h e degree o f c r y s t a l l i n i t y and t h e e f f e c t s of OMSO and o t h e r i n t e r c a l a n t s on s i g n a l i n t e n s i t y , c e n t e r I has been a s s i g n e d t o

o c t a h e d r a l Fe3+ i n a s t r o n g c r y s t a l f i e l d i n l a y e r s w i t h l a y e r s t a c k i n g d i s o r d e r nb such as 3 d i s p l a c e m e n t s o r 120" r o t a t i o n s . Centers I I a and I I b a r e a s s o c i a t e d w i t h o c t a h e d r a l Fe3+ i n s i t e s o f h i g h c r y s t a l l i n i t y and o r d e r e d s t a c k i n g .

The

d i f f e r e n c e between c e n t e r s I I a and I I b may r e s u l t from two d i s t i n c t o r i e n t a t i o n s o f s u r f a c e OH groups a d j a c e n t t o t h e o r d e r e d Fe3+ s i t e s ( G i e s e and D a t t a , 1 9 7 3 ) . The resonances near g = 2.0 u s u a l l y c o n s i s t o f two asymmetric l i n e s w i t h g 2.05 and 9 1 = 2.0 (Angel and H a l l , 1973; Meads and Nalden, 1975). r e s p o n s i b l e f o r t h e s e l i n e s has been d e s i g n a t e d c e n t e r A. p a r a l l e l t o the k a o l i n i t e c-axis.

II

The s p e c i e s

=

The u n i q u e a x i s l i e s

Schwartz e t a l . (1979) have used t h i s observa-

t i o n t o determine t h e o r i e n t a t i o n d i s t r i b u t i o n o f t h e p l a t e l e t s i n a k a o l i n i t e p e l l e t prepared under a x i a l s t r e s s .

As expected, t h e p l a t e l e t s t e n d t o o r i e n t

w i t h t h e s i l i c a t e l a y e r s perpendicular t o t h e s t r e s s d i r e c t i o n . The

A c e n t e r i n k a o l i n i t e can be e l i m i n a t e d by a n n e a l i n g a t 400".

a l s o c o l l a p s e s t h e Fe3+ l i n e s t o a s i n g l e l i n e a t g = 4.2.

Annealing

The A c e n t e r i s

a b s e n t i n Fe3+-doped s y n t h e t i c k a o l i n i t e s , b u t s y n t h e t i c k a o l i n i t e s doped w i t h Mg2+ o f Fez+ and s u b s e q u e n t l y x - i r r a d i a t e d e x h i b i t t h e A c e n t e r resonance (Angel e t al.,

1974, 1976, 1977).

The e f f e c t s of Fe3+ and 11g2+ o n t h e e s r s p e c t r a o f

s y n t h e t i c k a o l i n i t e a r e i l l u s t r a t e d i n F i g u r e 6.10. The A c e n t e r has been a t t r i b u t e d t o a n 0'

c e n t e r bound t o Mg2+(or Fez+) s u b s t i t u t i n g f o r A13+, a l t h o u g h

a t r a p p e d 02- i o n has been suggested as an a l t e r n a t i v e e x p l a n a t i o n (Jones e t a l . , 1 9 7 4 ) . H a l l (1980a) has e s t i m a t e d t h a t replacement o f one A13+ p e r thousand by 2+ Mg2+ ( o r Fe ) i s s u f f i c i e n t t o a c c o u n t f o r t h e c o n c e n t r a t i o n o f A c e n t e r s .

153

I g=4.0

I g = 2.0 Natural kaol ini t e

6 '

Mg doped kaolinite (no signals) Fe3+ doDed kaol inite

D

Mg doped kaolinite X-irradiated

E

Mg doped kaolinite

X-irradiated and annea 1 ed

Fe3+ and Flg doped kaolinite X-irradiated and annea 1 ed

F i g u r e 6.10.

Esr s p e c t r a of synthetic kaolinites (from Angel e t al, 1976).

154 Two o t h e r d e f e c t c e n t e r s (B1 and B 2 ) have been observed i n k a o l i n i t e , b u t t h e e s r s i g n a l s a r e weak and d i f f i c u l t t o r e s o l v e (Angel and H a l l , 1973; Pleads and I l a l d e n , 1975).

These c e n t e r s e x h i b i t g v a l u e s near 2.0,

but they are distinguished

from t h e A c e n t e r s by t h e presence o f h y p e r f i n e s p l i t t i n g due t o c o u p l i n g o f t h e u n p a i r e d s p i n s w i t h t h e n u c l e a r s p i n o f A1 w i t h I = 5 / 2 . c e n t e r s can be g r e a t l y i n c r e a s e d by X - i r r a d i a t i o n .

The c o n c e n t r a t i o n o f B

They a r e s t a b l e t o 200" and

can be r e v e r s i b l y c r e a t e d and d e s t r o y e d by i r r a d i a t i o n and a n n e a l i n g (Angel and H a l l , 1973).

The B c e n t e r s a r e c l e a r l y a s s o c i a t e d w i t h A1 c e n t e r s .

l i k e l y assignments a r e lattice.

+ 0

The most

c e n t e r s w h i c h b r i d g e A l , Si and A1 , A1 p a i r s i n t h e

Many o t h e r e s r a c t i v e s p e c i e s have been observed i n n a t u r a l k a o l i n i t e

samples, i n c l u d i n g adsorbed o r g a n i c f r e e r a d i c a l s i n l o w c o n c e n t r a t i o n ( H a l l e t a1 . , 1974), framework V4+ i o n s ( H a l l ,(1930a), l l n 2 + - c o n t a i n i n g phases (Meads and Halden, 1975), i r o n o x i d e s , and i r o n - r i c h i m p u r i t y phases such as mica (Meads and Malden, 1975; Angel and V i n c e n t , 1978).

ORGANIC RADICALS AND NITROXIDE S P I N PROBES

6.6 6.6.1

Arene Radical C a t i o n s

Esr has been v e r y u s e f u l i n e l u c i d a t i n g e l e c t r o n t r a n s f e r r e a c t i o n s between a r o m a t i c m o l e c u l e s and c e r t a i n t r a n s i t i o n m e t a l i o n s i n t h e i n t e r l a y e r s o f smectites.

The f i r s t r e a c t i o n o f t h i s t y p e was r e p o r t e d by Donor and M o r t l a n d (1969)

f o r Cu2+ and benzene on m o n t m o r i l l o n i t e .

The most s t r i k i n g f e a t u r e o f t h e r e a c -

t i o n was t h e development o f an i n t e n s e l y r e d complex w h i c h e x h i b i t e d anomalous a b s o r p t i o n bands i n t h e i r r e g i o n t h a t i n d i c a t e d t h e a r o m a t i c i t y o f t h e benzene r i n g was l o s t o r g r e a t l y p e r t u r b e d .

F u r t h e r s t u d i e s ( F l o r t l a n d and Pinnavaia;

1971; P i n n a v a i a and M o r t l a n d , 1971) o f t h e r e a c t i o n i n d i c a t e d t h a t t h e development o f t h e r e d "Type 11" benzene s p e c i e s was preceeded b y t h e l o s s o f w a t e r from t h e s i l i c a t e s u r f a c e and p h y s i c a l a d s o r p t i o n o f benzene. The removal o f 2+ w a t e r f r o m t h e c o o r d i n a t i o n sphere o f t h e Eu ions a f f o r d e d a yellow-green edge-bonded form o f c o o r d i n a t e d benzene, w h i c h was d e s i g n a t e d "Type I"benzene. The removal o f s t i l l more i n n e r sphere w a t e r l e d t o t h e f o r m a t i o n o f some C6H6+ a l o n g w i t h t h e r e d Type I 1 s p e c i e s .

F u r t h e r s l o w r e a c t i o n o f t h e Type I 1 s p e c i e s

and/or t h e r a d i c a l c a t i o n e v e n t u a l l y a f f o r d e d polymer, p r o b a b l y p a r a p o l y p h e n y l ( M o r t l a n d and H a l l o r a n , 1976; S t o e s s e l e t a l . ,

1977).

The o v e r a l l r e a c t i o n scheme f o r Cu2+-benzene i s summarized i n F i g u r e 6.11. The Type I s p e c i e s g i v e s a s t r o n g , broad Cu2+ resonance, i n d i c a t i n g t h a t t h e o x i d a t i o n s t a t e o f t h e m e t a l remains unchanged a t t h i s s t a g e o f t h e r e a c t i o n . t i o n o f Cu2+ t o Cu

+ and

t h e f o r m a t i o n o f C6H6

+

Reduc-

and Type I 1 benzene i s accompanied

by t h e replacement o f t h e Cu2+ resonance by a s h a r p i s o t r o p i c resonance near g = 2.0 t h a t may be a s s i g n e d t o C H

l e s s than

%

+

( R u p e r t , 1973; P i n n a v a i a e t a l , 1 9 7 4 ) . However, 62+ + 3% o f t h e i n i t i a l Cu s p i n s a r e r e c o v e r e d as C6H6 . T h i s l a t t e r

o b s e r v a t i o n suggests t h a t most o f t h e s p i n s a r e l o s t t h r o u g h s p i n p a i r i n g i n t h e

155

+cuL;

/ / polymer

F i g u r e 6.11.

R e a c t i o n s of benzene w i t h Cu2+ i n s r n e c t i t e (from P i n n a v a i a , 1 9 7 7 ) .

0 F i g u r e 6.12.

Proposed s t r u c t u r e

for t y p e I 1 benzene.

F i g u r e 6.13.

Structure o f Ternpamine+.

156 Type I 1 species.

The n a t u r e o f t h e Type I 1 s p e c i e s i s s t i l l u n c e r t a i n , b u t a

model based on t h r o u g h space p a i r i n g o f r a d i c a l c a t i o n s has been proposed e a r l i e r ( P i n n a v a i a , 1976b).

A schematic r e p r e s e n t a t i o n o f t h e p a i r i n g model i s g i v e n i n

F i g u r e 6.12. D i s t o r s i o n s i n t h e p a i r e d J a h n - T e l l e r C6H6

+

s p e c i e s may c o n t r i b u t e

t o t h e unusual i r a b s o r p t i o n s i n t h e C=C s t r e t c h i n g r e g i o n . The r e a c t i o n o f a v a r i e t y o f o t h e r a r o m a t i c m o l e c u l e s w i t h Cu2+ on s m e c t i t e s u r f a c e s has

been i n v e s t i g a t e d (Matsunaga, 1972; P i n n a v a i a , 1976b; Fenn e t a l . ,

1973; Van de Poel e t a l . , 1973; Cloos e t a l . , 1973; T r i c k e r e t a l . , 1976). S i n c e 2+ t h e r o l e o f t h e Cu i o n i s t o f u n c t i o n as an o x i d i z i n g a g e n t f o r r a d i c a l c a t i o n f o r m a t i o n , o t h e r o x i d i z i n g agents such a s V02+ and Fe3+ may be used as r e p l a c e 2+ ( P i n n a v a i a e t a l . , 1974).

ments f o r Cu 6.6.2

N i t r o x i d e S p i n Probes

McBride (1976c, 1976d, 1977a, 1977b, 1979b, 1980) s t u d i e d t h e m o b i l i t y and o r i e n t a t i o n o f n i t r o x i d e s p i n probes on s m e c t i t e c l a y s u r f a c e s . form o f 4-amino-Z,2,6,6-

tetramethyl piperidine-N-oxide

e s p e c i a l l y useful i n these studies. i n F i g u r e 6.13.

The p r o t o n a t e d

+

(Ternpami ne ) has been

The s t r u c t u r e o f Tempamine

When t h e s p i n probe i s d i s s o l v e d i n l o w c o n c e n t r a t i o n ( 1 0 -

4

+

i s illustrated

1) i n

solvents o f

l o w v i s c o s i t y , r a p i d t u m b l i n g averages t h e p r i n c i p a l components o f t h e g t e n s o r and t h e h y p e r f i n e c o u p l i n g t e n s o r , A. Thus, a t h r e e - l i n e e s r spectrum i s observed 1 1 + g2,) and A, = (Axx + A + A ) i s observed. I f t h e w i t h go = 3 ( g x x = gYY 3 Yy zz v i s c o s i t y i s l o w and t h e c o r r e l a t i o n t i m e f o r t u m b l i n g i s v e r y s h o r t ( T <~ 2, 10:” sec), t h e t h r e e h y p e r f i n e l i n e s a r e o f equal h e i g h t and w i d t h .

As t h e c o r r e l a - -

t i o n t i m e i n c r e a s e s i n t h e r e g i o n o f m o d e r a t e l y f a s t t u m b l i n g ( T <~ 5 x

lo-’

sec),

t h e l i n e s remain equal i n i n t e g r a l i n t e n s i t y b u t t h e w i d t h s and h e i g h t s o f t h e t h r e e l i n e s b e g i n t o d i f f e r because o f i n c o m p l e t e l y averaged a n i s o t r o p i c terms i n t h e magnetic H a m i l t o n i a n .

The c o r r e l a t i o n t i m e f o r t u m b l i n g may be c a l c u l a t e d

f r o m t h i s l i n e b r o a d e n i n g phenomenon ( S m i t h , 1972; N o r d i o , 1976; Sachs and L a t o r r e , 1974).

As t h e m o b i l i t y o f t h e n i t r o x i d e decreases f u r t h e r i n v i s c o u s f l u i d media,

t h e spectrum becomes more complex.

I n t h e range o f c o r r e l a t i o n t i m e s ,

lop9

sec

5

s e c - l , t h e s o - c a l l e d s l o w m o t i o n a l r e g i o n , t h e shape o f t h e spectrum tends toward two w e l l - r e s o l v e d o u t e r h y p e r f i n e l i n e s and a c e n t r a l o v e r l a p p e d

< T~

region.

The a n a l y s i s o f c o r r e l a t i o n t i m e s i n t h i s t i m e domain i s d i f f i c u l t , b u t

an a p p r o p r i a t e t h e o r y has been developed (Freed, 1976; Hwang e t a l . ,

1975).

S i n c e t h e c o r r e l a t i o n t i m e can be r e l a t e d t o t h e v i s c o s i t y o f t h e medium t h r o u g h Stokes law, Tc

4nqr

3

(4)

=3kT

e s r s t u d i e s o f s p i n probes on c l a y s u r f a c e s m i g h t be expected t o y i e l d t h e microscopic v i s c o s i t y i n t h e c l a y i n t e r l a y e r s .

However, McBride (1976d, 1977a)

157

F i g u r e 6.14.

Esr s p e c t r a o f Ternpamine+ doped a t t h e 1% l e v e l i n t o f u l l y w e t t e d K + - h e c t o r i t e f i l m ( f r o m McBride, 1980).

, 20

F i g u r e 6.15.

#

GAUSS ,

n .

+

Esr s p e c t r a o f Tempamine doped a t t h e 1% l e v e l i n t o K + - h e c t o r i t e d r i e d a t 110" ( f r o m McBride, 1980).

158 has f o u n d t h a t s t r o n g i n t e r a c t i o n s w i t h t h e c l a y s u r f a c e s n o t o n l y reduces t h e r o t a t i o n a l m o b i l i t y , b u t a l s o p a r t i a l l y o r i e n t t h e probe i n t h e i n t e r l a y e r s ; t h a t i s , t h e probe does n o t tumble randomly i n t h e i n t e r l a y e r s even when f u l l y h y d r a t e d . F i g u r e 6.14 i l l u s t r a t e s t h e e s r s p e c t r a o f Ternpamine+ on a f u l l y w e t t e d

+

f i l m sample o f K - h e c t o r i t e w i t h t h e m a g n e t i c f i e l d d i r e c t i o n o r i e n t e d p a r a l l e l and p e r p e n d i c u l a r t o t h e s i l i c a t e sheets. Al

For t h e p e r p e n d i c u l a r o r i e n t a t i o n ,

= 20.5 gauss, and f o r t h e p a r a l l e l o r i e n t a t i o n A l l = 15.2 gauss.

Based on

t h e observed o r i e n t a t i o n dependence, t h e z - a x i s o f t h e probe, w h i c h i s d e f i n e d as b e i n g c o l i n e a r w i t h t h e p o r b i t a l on n i t r o g e n , i s o r i e n t e d w i t h r e s p e c t t o t h e s i l i c a t e sheets a t an a p p a r e n t a n g l e o f 45". F i g u r e 6.15 shows t h e e s r s p e c t r a l p r o p e r t i e s o f Tempatnine+ adsorbed on a f i l m sample o f K + - h e c t o r i t e d r i e d a t 110".

The arrows i n t h e f i g u r e s i n d i c a t e t h e

approximate p o s i t i o n s o f t h e t h r e e resonances f o r t h e p e r p e n d i c u l a r and p a r a l l e l o r i e n t a t i o n o f t h e h e c t o r i t e f i l m r e l a t i v e t o t h e magnetic f i e l d d i r e c t i o n . The s p e c t r a shapes a r e t h o s e expected f o r n i t r o x i d e probe i n t h e s l o w m o t i o n a l region.

C l e a r l y , t h e a l i g n m e n t o f t h e probe i n t h e i n t e r l a y e r s i s g r e a t l y

enhanced by removing w a t e r and c o l l a p s i n g t h e i n t e r l a y e r s .

The z - a x i s o f t h e

probe i s a p p r o x i m a t e l y a t r i g h t angles t o t h e s i l i c a t e s h e e t s . I f t h e l i n e w i d t h s o f Ternpamine'

f o r tumbling i n t h e f u l l y wetted o b t a i n e d (McBride, 1977b).

a r e used t o c a l c u l a t e t h e c o r r e l a t i o n t i m e

+ K -hectorite,

v a l u e s o f 1-3 x

lo-'

sec a r e

The v a l u e s i n t h i s range a r e 20 t o 60 t i m e s l o w e r

than t h e r a t e o f r o t a t i o n i n t h e s o l u t i o n state.

Larger values o f

T~

are found

f o r t h e p e r p e n d i c u l a r o r i e n t a t i o n o f t h e c l a y f i l m s i n t h e magnetic f i e l d com2+

pared t o t h e p a r a l l e l o r i e n t a t i o n , a r e s u l t o f a n i s o t r o p i c r o t a t i o n . hectorites exhibit

+

H -

v a l u e s a b o u t t w i c e as l o n g as M - h e c t o r i t e s , perhaps due 2+ t o t h e more l i m i t e d i n t e r l a m e l l a r volume f o r t h e M -exchange forms. A l s o , T~

+

e t h a n o l s o l v a t e d M - h e c t o r i t e s c o n t a i n i n g s p i n probe e x h i b i t c o r r e l a t i o n t i m e s which a r e a p p r o x i m a t e l y two o r d e r s o f magnitude l o n g e r t h a n t h e h y d r a t e d systems, d e s p i t e t h e r e l a t i v e l y more i s o t r o p i c m o t i o n i n t h e f o r m e r case. RE FE RE WC ES

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Aromatic r a d i c a l c a t i o n f o r m a t i o n on t h e i n t r a c r y s t a l s u r f a c e s o f t r a n s i t i o n metal l a y e r l a t t i c e s i l i c a t e s . J. Phys. Chem., 78: 994-999. Pinnavaia, T.J., 1980. A p p l i c a t i o n s o f e s r spectroscopy t o i n o r g a n i c - c l a y systems. Chapter 8. I n : J.W. S t u c k i and W.L. Banwart ( E d i t o r s ) , Advanced Chemical Methods f o r S o i l and Clay M i n e r a l Research, Reidel P u b l i s h i n g Co., Holland, pp. 391-422. Rubinstein, M., Barum, A. and Lug, Z., 1971. E l e c t r o n i c and n u c l e a r r e l a x a t i o n i n s o l u t i o n s o f t r a n s i t i o n metal i o n s w i t h s p i n = 3/2 and 5/2. Mol. Phys., 20: 67. Rupert, J.P., 1973. E l e c t r o n s p i n resonance s p e c t r a o f i n t e r l a m e l l a r copper( 11)arene complexes on m o n t m o r i l l o n i t e . J . Phys. Chem., 77: 784-790. Sachs, F. and L a t o r r e , R., 1974. Cytoplasmic s o l v e n t s t r u c t u r e of s i n g l e barnacle muscle c e l l s s t u d i e s by e l e c t r o n s p i n resonance. Biophys. J . , 14: 316-326. Schoonheydt, R.A., 1978. A n a l y s i s o f t h e e l e c t r o n paramagnetic resonance spectra o f bis(ethylenediamine)copper(II) on t h e s u r f a c e s o f z e o l i t e s X and Y and o f a Camp Berteau m o n t m o r i l l o n i t e . J . Phys. Chem., 82: 497-498. Schwartz, J.C., Hoffman, B.H., K r i z e k , R.J. and Atmatzides, D.K., 1979. A general prodecure f o r s i m u l a t i n g e p r s p e c t r a o f p a r t i a l l y o r i e n t e d paramagnetic centers. J . Mag. Res., 36: 259-268. Smith, I.C.P., 1972. The s p i n l a b e l method. Chapter 11. I n : H.M. Schwartz, J. R. B o l t o n and D.C. Borg ( E d i t o r s ) , B i o l o g i c a l A p p l i c a t i o n s of E l e c t r o n Spin Resonance, Wiley:Interscience, New York, pp. 483-539. Stoessel, F., Guth, L.J. and Wey, R., 1977. P o l y m e r i z a t i o n o f benzene i n t o p o l y paraphenylene on copper m o n t m o r i l l o n i t e . Clay Miner., 12: 255-259. T r a r s e v i c h , Y . I . and Ovcharenko, F.D., 1973. On t h e mechanism o f i n t e r a c t i o n between n i t r o g e n o u s o r g a n i c substances and m o n t m o r i l l o n i t e surfaces. Proc. I n t e r n . Clay Conf., Madrid, 1972, pp. 627-636. Traynor, M.F., Mortland, II.11. and Pinnavaia, T.J., 1978. I o n exchange and i n t e r s a l a t i o n r e a c t i o n s o f h e c t o r i t e w i t h t r i s - b i p y r i d y l metal complexes. Clays and Clay Miner., 26: 318-326. T r i c k e r , M.J., Tennakoon, D.T.B., Thomas, J.Mv and Graham, S.H., 1975. Novel r e a c t i o n s o f hydrocarbon complexes o f m e t a l - s u b s t i t u t e d sheet s i l i c a t e s : thermal d i m e r i z a t i o n o f t r a n s - s t i l b e n e . Nature Phys. Sci., 253: 110-111. Van De Poel , D. , Cloos, P., Helsen, J. and Hannini, E. , 1973. S p e c i f i c behavior of benzene adsorbed on copper( 11) m o n t m o r i l l o n i t e . B u l l . Groupe Franc. A r g i l e s , 25, 115. Peigneur, P. and Lunsford, Velghe, F . , Schoonheydt, R.A., Uytterhoeven,J.B., J.H., 1977. Spectroscopic c h a r a c t e r i z a t i o n and thermal s t a b i l i t y o f copper( 11) ethylenediamine complexes on s o l i d surfaces 2. M o n t m o r i l l o n i t e . Wertz, J.E. and B o l t o n , J.R., 1972. E l e c t r o n Spin Resonance: Elementary Theory and P r a c t i c a l A p p l i c a t i o n s . Mc-Graw H i l l Book Co., New York.

163 Chapter 7 ULTW V IO L E T P N D V I S I B L E L I G H T SPECTROSCOPY

R o b e r t A . SCHOONHEYDT K a t h o l i e k e U n i v e r s i t e i t Leuven, Centrum v o o r O p p e r v l a k t e s c h e i k u n d e e n C o l l o i d a l e S c h e i k u n d e , D e C r o y l a a n 4 2 , B-3030 Leuven, Belgium. 7.1

INTRODUCTION

The d i c f e r e n t a s p e c t s o f c l a y m i n e r a l

p r o p e r t i e s which can b e

s t u d i e d by u l t r a v i o l e t ( u . v . )

and v i s i b l e ( v i s . ) l i g h t s p e c t r o s c o p y

a r e summarized i n T a b l e 7 . 1 .

I t i s c l e a r t h a t u s e f u l chemical

i n f o r m a t i o n i s a l s o found i n t h e n e a r i n f r a r e d ( n . i . r . )

and t h i s

region i s t h e r e f o r e included i n t h i s review. TABLE 7 . 1

Clay m i n e r a l p r o p e r t i e s s t u d i e d by u . v . - v i s . - n . i . r .

spectroscopy

Lattice p r o p e r t ies

Spectroscopic domains

Surface properties

Spectroscopic domains

Absorption edge

U.V.

Surface imrrobilized transition metal i o n s

u.v.,

vis.,

Pdsorbed chrorophores

u'v'

vis'

Lattice substituted transition metal ions

u.v., v i s . n.i.r.

Vibrational o v e r t o n e s and combination bands

n.i.r.

..

n i r.

A l t h o u g h a s t u d y o f t h e a d s o r p t i o n edge i n i t s e l f i s a n i n t e r e s t i n g r e s e a r c h subject, one i s h i s t o r i c a l l y i n c l i n e d t o work w i t h " w h i t e " s a m p l e s , t h a t i s , s a m p l e s w i t h a b s o r p t i o n e d g e s beyond t h e s p e c t r a l r e g i o n o f i n t e r e s t (X<200nrr o r ~ > 5 0 , 0 0 O ~ r r - ~ F) .o r t h e

m o s t s t u d i e d c l a y rn-inerals, t h e smectites , t h e a b s o r p t i o n edge b e g i n s below 5 0 , 0 0 O ~ m - ~and r e a c h e s i t s maximum above 5 0 , 0 0 O ~ r - ~ . I n t h e s e cases s i g n i f i c a n t s p e c t r o s c o p i c i n f o r m a t i o n can s t i l l b e g a t h e r e d from t h e

U.V.

region.

The change t r a n s f e r and d-d bands o f t r a n s i t i o n n e t a l i o n s c o n t a i n information about t h e f i r s t coordination sphere.

They a r e t h e r e f o r e

164 probes f o r isomorphic s u b s t i t u t i o n .

In t h e i n t e r l a m e l l a r space

t h e d i r e c t c o o r d i n a t i o n o f s u r f a c e oxygens t o t h e t r a n s i t i o n m e t a l i o n and t h e s u r f a c e - t r a n s i t i o n m e t a l complex i n t e r a c t i o n can be s t u d i e d .

I n t h e l a t t e r c a s e t h e s u r f a c e can be c o n s i d e r e d

a s a r i g i d a n i o n i c s o l v e n t i n t h e s e n s e t h a t t h e s u r f a c e oxyqens a r e not coordinated t o t h e t r a n s i t i o n metal i o n . n:: o r II+ 77:: t r a n s i t i o n s of adsorbed chromophoric The nm o l e c u l e s are s u b j e c t t o s u r f a c e i n t e r a c t i o n s .

A d s o r p t i o n on

t h e c l a y s u r f a c e may i n d u c e a g g r e g a t i o n , r e d o x r e a c t i o n s o r a c i d base r e a c t i o n s . F i n a l l y , v i b r a t i o n a l o v e r t o n e s and c o m b i n a t i o n b a n d s n o t o n l y c o n t a i n i n f o r m a t i o n on t h e n a t u r e of t h e a d s o r b e d s p e c i e s b u t a l s o on i t s r o t a t i o n a l m o b i l i t y . The l a t t e r i n f o r m a t i o n h a s b e e n

e x t r a c t e d from t h e v + 6 band o f H 2 0 on s i l i c a b u t i t s e x t e n s i o n t o c l a y m i n e r a l s p o s e s no problem i n p r i n c i p l e ( K l i e r , 1973; K l i e r , Shen and Z e t t l e m o y e r , 1 9 7 3 ) . I t s h o u l d b e i n t e r e s t i n g t o compare t h e r e s u l t s w i t h t h o s e o b t a i n e d by NMR o r n e u t r o n d i f f r a c t i o n . U.v.-vis.-n.i.r.

s p e c t r o s c o p y i s a l s o complementary t o e p r ,

m a g n e t i c s u s c e p t i b i l i t y measurements, Mijssbauer s p e c t r o s c o p y and ESCA, which a r e r e v i e w e d i n o t h e r c h a p t e r s o f t h i s book. 7.2

TECHNIQUES

With t h e d i m e n s i o n s of t h e c l a y p a r t i c l e s c o m p a r a b l e t o t h e w a v e l e n g t h l i t i s i m p o s s i b l e t o d i s t i n g u i s h t h e phenomena o f r e f l e c t i o n , r e f r a c t i o n and d i f f r a c t i o n .

The l i g h t i s s c a t t e r e d .

For c l o s e l y p a c k e d p a r t i c l e s ( d i s t a n c e between p a r t i c l e s l e s s t h a n 2 - 3 p a r t i c l e d i a m e t e r s ) i n t e r f e r e n c e s and p h a s e r e l a t i o n s a r i s e

among t h e s c a t t e r e d l i g h t beams.

Together with absorption of l i g h t

t h e s e s c a t t e r i n g phenomena s t r o n g l y i n h i b i t t r a n s m i s s i o n spectroscopy.

N e v e r t h e l e s s , many q u a l i t a t i v e t r a n s m i s s i o n

measurements on o r i e n t e d f i l m s o r s h e e t s h a v e b e e n p e r f o r m e d .

All

t h e s e s t u d i e s i n v o l v e i n t e n s e b a n d s , e i t h e r due t o t h e i r l a r g e absorption coefficients, or absorbing centers.

to

a

large

concentration

of

For "forbidden" bands of absorbing

c e n t e r s i n r e l a t i v e l y s m a l l c o n c e n t r a t i o n ( t h i s i s m o s t l y t h e case f o r t r a n s i t i o n metal ions)

one c a n u s e e i t h e r d i f f u s e r e f l e c t i o n

s p e c t r o s c o p y (DRS) or p h o t o a c o u s t i c s p e c t r o s c o p y (PAS)

.

Both

t e c h n i q u e s w i l l b e b r i e f l y compared. F o r a d e t a i l e d s t u d y t h e r e a d e r s h o u l d c o n s u l t t h e b a s i c l i t e r a t u r e ( K o r t i i m , 1 9 6 9 ; K e l l e r m a n , 1979; H a r s h b a r g e r and Robin, 1973; Monahan and N o l l e , 1 9 7 7 ) .

165 7.2.1

Diffuse r e f l e c t a n c e spectroscopy

I n DRS t h e l i g h t i n t e n s i t y s c a t t e r e d a t a g i v e n w a v e l e n g t h from an " i n f i n i t e l y t h i c k ' : c l o s e l y - p a c k e d c l a y l a y e r i s compared w i t h t h a t s c a t t e r e d from an i n f i n i t e l y t h i c k l a y e r o f a non-absorbing

(white) reference.

The r a t i o o f t h e l i g h t i n t e n s i t y t o t h a t from t h e r e f e r e n c e i s r e c o r d e d as s c a t t e r e d from t h e c l a y a f u n c t i o n o f t h e wavelength. This c o n s t i t u t e s t h e d i f f u s e r e f l e c t a n c e s p e c t r u m . T h e r e a r e 2 ways t o i l l u m i n a t e s a p p l e and reference. I n t h e f i r s t mode a d i r e c t , d i s p e r s e d l i g h t b e a r impinges ' p e r p e n d i c u l a r l y on t h e sample and on t h e r e f e r e n c e .

The

f i r s t l a y e r o f randomly o r i e n t e d p a r t i c l e s s c a t t e r s t h e l i g h t i n a l l d i r e c t i o n s , i n d e p e n d e n t l y o f t h e a n g l e of i n c i d e n c e ( L a m b e r t ' s l a w ) and e n s u r e s d i f f u s e i l l u m i n a t i o n o f t h e o t h e r p a r t i c l e s .

The

l i g h t s c a t t e r e d from sample and r e f e r e n c e i s c o l l e c t e d i n an i n t e g r a t i o n s p h e r e and d e t e c t e d by a PbS c e l l ( n . i . r . 1 photomultiplier (vis.-u.v.)

or a

on t o p o f t h e i n t e g r a t i o n s p h e r e .

Because t h e i n t e g r a t i o n s p h e r e must s c a t t e r t h e l i g h t and n o t absorb i t , i t is c o a t e d w i t h a p e r f e c t l y w h i t e c o a t i n g ( K o r t i h , 1969; Kellerman, 1 9 7 9 ) .

I n t h i s way a n e g l i g i b l y s m a l l amount of

s p e c u l a r r e f l e c t i o n (on t h e f i r s t l a y e r o f p a r t i c l e s ) i s included. I n t h e s e c o n d mode o f o p e r a t i o n sample and r e f e r e n c e a r e illuminated with d i f f u s e , undispersed r a d i a t i o n , i t s d i f f u s e c h a r a c t e r b e i n g e n s u r e d by t h e i n t e g r a t i o n s p h e r e . Measurements on l u m i n e s c e n t samples p r e s e n t i n p r i n c i p l e no I n mode 1 t h e a b s o r p t i o n s p e c t r u m i s o b t a i n e d ,

difficulties.

whereas i n mode 2 b o t h t h e a b s o r p t i o n and l u m i n e s c e n c e are registered.

By s u b s t r a c t i o n t h e l u m i n e s c e n c e s p e c t r u m is o b t a i n e d .

The e x p e r i m e n t a l s p e c t r u m c o n t a i n s a n a b s o r p t i o n p a r t and a scattering part. A t p r e s e n t t h e r e i s no t h e o r y a b l e t o d e s c r i b e q u a n t i t a t i v e l y t h e phenomenon of " d e p e n d e n t " m u l t i p l e s c a t t e r i n g ,

as it o c c u r s on c l a y powders.

T h e r e are 2 a p p r o a c h e s .

In the

continuum o r phenomenological approach - t h e m o s t p o p u l a r - t h e c h a r a c t e r i s t i c s of t h e I n d i v i d u a l p a r t i c l e s , c o n s t i t u t i n g t h e i n f i n i t e l y t h i c k l a y e r , are n e g l e c t e d .

In the statistical

approach t h e y a r e t a k e n i n t o a c c o u n t and t h e v a l i d i t y of t h e r e s u l t s depends on t h e v a l i d i t y o f t h e model chosen t o d e s c r i b e the particles.

166

The b a s i c e q u a t i o n f o r t h e phenomenological t h e o r i e s i s t h e r a d i a t i o n t r a n s f e r e q u a t i o n (Kortfim, 1 9 6 9 ) :

I i s t h e i n c i d e n t l i g h t i n t e n s i t y o f a g i v e n w a v e l e n g t h and

i t s change w i t h p a t h l e n g t h ;

p

dI

i s t h e d e n s i t y o f t h e medium;

K i s an a t t e n u a t i o n c o e f f i c i e n t c o r r e s p o n d i n g t o t o t a l r a d i a t i o n

l o s s due t o b o t h a b s o r p t i o n and s c a t t e r i n g .

j is a scattering function defined'as :

p ( @ , $ , ~ ' , $ ' ) t,h e p h a s e f u n c t i o n , i s a measure o f t h e i n t e n s i t y of s c a t t e r e d r a d i a t i o n i n t h e s o l i d angle do' = sinO'd@'d$' i f a beam i n t h e d i r e c t i o n ( @ , $ ) s t r i k e s a m a s s e l e m e n t of t h e medium. The one p a r a m e t e r a p p r o x i m a t e s o l u t i o n of e q u a t i o n 1, known as t h e Schuster-Kubelka-Munk o r t h e Kubelka-Munk e q u a t i o n , i s t h e most p o p u l a r ( K o r t k n , 1969) :

Rm =

l i g h t i n t e n s i t y r e f l e c t e d from sample l i g h t i n t e n s i t y r e f l e c t e d from r e f e r e n c e

K i s t h e a p p a r e n t a b s o r p t i o n c o e f f i c i e n t and S t h e a p p a r e n t

scattering coefficient.

E q u a t i o n 3 i s d e r i v e d f o r an i n f i n i t e l y

t h i c k l a y e r of randomly d i s t r i b u t e d p a r t i c l e s , s u b j e c t t o d i f f u s e r a d i a t i o n and i s o t r o p i c s c a t t e r i n g .

I t i s a r e m a r k a b l y good

a p p r o x i m a t i o n i n t h e l i m i t of low c o n c e n t r a t i o n o f a b s o r p t i o n c e n t e r s ( H e c h t , 1 9 7 6 ; K l i e r , 1 9 7 2 ) . F o r s t r o n g l y a b s o r b i n g media s o l u t i o n s of t h e r a d i a t i o n t r a n s f e r e q u a t i o n b a s e d upon a n i s o t r o p i c

scatter g i v e a b e t t e r account of t h e e x p e r i m e n t a l r e f l e c t a n c e (Hecht, 1 9 7 6 ) . For t h e s t a t i s t i c a l t h e o r i e s t h e o r i g i n a l l i t e r a t u r e s h o u l d b e c o n s u l t e d ( K e l l e r m a n , 1 9 7 9 ; Melamed, 1 9 6 3 ) .

167

7.2.2

Photoacoustic Spectroscopy

The e n e r g y o f t h e a b s o r b e d r a d i a t i o n , which i s r a d i a t i o n l e s s r e l e a s e d , h e a t s up t h e g a s e o u s m o l e c u l e s i n t h e immediate environment and a p r e s s u r e i n c r e a s e r e s u l t s .

When t h e i n c i d e n t r a d i a t i o n i s

chopped, p e r i o d i c p r e s s u r e changes o c c u r .

They p r o p a g a t e t h r o u g h t h e

g a s e o u s e n v i r o n m e n t and a r e d e t e c t e d by a microphone. The i n t e n s i t y of t h e microphone r e s p o n s e i s p r o p o r t i o n a l t o t h e amount o f a b s o r b e d radiation.

Thus , when a w a v e l e n g t h r a n g e i s scanned, a "spectrum"

i s o b t a i n e d . T h i s i s a power s p e c t r u m : t h e i n t e n s i t y o f t h e s i g n a l i s n o t o n l y p r o p o r t i o n a l t o t h e number o f p h o t o n s a b s o r b e d b u t a l s o t o t h e i r e n e r g y . The band p r o f i l e i n t h e p h o t o a c o u s t i c s p e c t r u m a p p e a r s d i s t o r t e d due t o t h e f a c t t h a t t h e h i g h f r e q u e n c y s i d e a b s o r b s more power t h a n t h e low f r e q u e n c y s i d e .

For

g a s e o u s samples t h e m o l e c u l e s t h e m s e l v e s c o n s t i t u t e t h e medium f o r t h e p r o p a g a t i o n of t h e p r e s s u r e p u l s e s .

F o r l i q u i d s and s o l i d s a

gaseous environment i s n e c e s s a r y . P o t e n t i a l a d v a n t a g e s of PAS o v e r DRE a r e : ( i ) no problems w i t h s c a t t e r e d and s p e c u l a r l y r e f l e c t e d l i g h t ;

( i i )e a s y measurements

o f l u m i n e s c e n t m a t e r i a l s and ( i i i ) t h e u s e o f s m a l l amounts o f material. The m o s t s e v e r e r e s t r i c t i o n f o r a q u a n t i t a t i v e a p p l i c a t i o n of PAS a t p r e s e n t i s t h e l a c k of a f i r m t h e o r e t i c a l foundation.

The t h e o r y of Rosencwaig and Gersho f o r n o n - s c a t t e r i n g

s o l i d s h a s been r e v i e w e d ( K e l l e r m a n , 1 9 7 9 ) .

F o r powders a

t h e o r y r e l a t i n g t h e PAS s i g n a l i n t e n s i t y t o t h e a b s o r p t i o n c o e f f i c i e n t , i s p u b l i s h e d (Monahan and N o l l e , 1 9 7 7 ) .

It is clear

however t h a t more t h e o r e t i c a l and e x p e r i m e n t a l work i s n e c e s s a r y (Adams, King and K i r k b r i g h t , 1 9 7 6 ) .

For i n s t a n c e , u n c e r t a i n t i e s

a r i s e due t o p a r t i c l e s h a p e s and d i s t r i b u t i o n o f p a r t i c l e s i z e s i n t h e powdered s a m p l e s .

Also, i f t h e c h a r a c t e r i s t i c thermal d i s t a n c e

of t h e f i l l e r g a s i s much l a r g e r t h a n t h e p a r t i c l e d i a r r e t e r , t h e p a r t i c l e s are thermally short-circuited with the r e s u l t t h a t the PAS s i g n a l i n t e n s i t y i s smaller t h a n e x p e c t e d .

i s g i v e n of t h e e f f e c t o f s c a t t e r e d l i g h t .

F i n a l l y , no a c c o u n t

I t h a s b e e n shown

experimentally t h a t s c a t t e r e d l i g h t contributes s i g n i f i c a n t l y t o t h e PAS s i g n a l a t l o w o p t i c a l d e n s i t i e s o r / a n d h i g h r e f l e c t i v i t i e s .

P

f r a c t i o n o f t h e s c a t t e r e d l i g h t may r e a c h t h e c e l l w a l l s , b e p a r t i a l l y a b s o r b e d t h e r e , t h u s p r o d u c i n g a background s i g n a l d e p e n d e n t on t h e o p t i c a l p r o p e r t i e s o f sample and c e l l w a l l s .

A

s u i t a b l e c e l l d e s i g n w i t h t r a n s p a r e n t windows e l i m i n a t e s t h i s e f f e c t (McClelland and K n i s e l e y , 1 9 7 6 ) . About t h e e f f e c t o f i n t e r p a r t i c l e s c a t t e r i n g n o t h i n g i s known.

168 Experimental c o n s i d e r a t i o n s The c o n d i t i o n s o f i s o t r o p i c s c a t t e r i n g and d i f f u s e i l l u m i n a t i o n ,

7.2.3

r e q u i r e d b y e q u a t i o n 3 , are most c l o s e l y m e t when t h e medium c o n s i s t s o f d e n s i l y p a c k e d , randomly shaped p a r t i c l e s whose s i z e s a r e comparable o r smaller t h a n t h e w a v e l e n g t h o f t h e l i g h t . Clay l a y e r s o f

%

5mm t h i c k a r e s u f f i c i e n t f o r t h e i n f i n i t e

thickness criterion.

For work i n vacuum, a d s o r p t i o n s t u d i e s

o r c a t a l y t i c e x p e r i m e n t s , s p e c i a l l y d e s i g n e d c e l l s are n e c e s s a r y w i t h s i l i c a windows w i t h e x t r e m e l y low OH and H20 c o n t e n t s . I t i s a d v i s a b l e t o s i e v e t h e c l a y and t o work w i t h i d e n t i c a l

fractions.

The s c a t t e r i n g p r o p e r t i e s a r e s t r o n g l y i n f l u e n c e d

by a d s o r b e d l a y e r s on t h e e x t e r n a l s u r f a c e s . Thus, t h e H20 c o m b i n a t i o n and o v e r t o n e b a n d s i n t e n s i t i e s are n o t l i n e a r l y d e p e n d e n t on t h e amount a d s o r b e d from c o m p l e t e d e h y d r a t i o n up t o f u l l s a t u r a t i o n b e c a u s e of a d s o r p t i o n on t h e e x t e r n a l s u r f a c e i n t h e l a t t e r case. The p h o t o a c o u s t i c S p e c t r o m e t e r r e p o n d s t o t h e t o t a l amount o f l i g h t a d s o r b e d i n t h e sample w i t h i n a d e p t h r o u g h l y e q u a l t o t h e thermal d i f f u s i o n length.

I f a l l t h e l i g h t i s absorbed within t h a t

d e p t h t h e sample i s p h o t o a c o u s t i c a l l y "opaque". t h i s (EG

& G PAR

Pn example i l l u s t r a t e s

6001 Photoacoustic s p e c t r o m e t e r ) .

At a

m o d u l a t i o n f r e q u e n c y of 40Hz t h e t h e r m a l d i f f u s i o n l e n g t h i n an aqueous s o l u t i o n i s of t h e o r d e r o f 30pm.

The p h o t o a c o u s t i c d e t e c t i o n

p r o c e s s w i l l o n l y y i e l d a measure o f t h e l i g h t a b s o r p t i o n w i t h i n t h e 30pm d i f f u s i o n l e n g t h . O p t i c a l a b s o r p t i o n c o e f f i c i e n t s g r e a t e r t h a n I.

3 0 0 0 ~ m - c~a n n o t b e d i s t i n g u i s h e d i n t h a t s i t u a t i o n : t h e y w i l l

a p p e a r e q u a l l y opaque t o t h e p h o t o a c o u s t i c d e t e c t i o n p r o c e s s . S o l u t i o n s t o t h i s problem a r e (i) t o work w i t h s a m p l e s t h i n n e r t h a n t h e reciprocal absorption c o e f f i c i e n t o r (ii)to increase t h e modulation frequency.

Both methods have t h e i r odds and optimum

c o n d i t i o n s have t o b e chosen d e p e n d e n t on t h e problem u n d e r i n v e s t i g a tion.

169

I n any c a s e f o r two-dimensional

s o l i d s o r m a t e r i a l s w i t h weak

surface adsorption p r o p e r t i e s , adsorbed gases (transparent i n t h e s p e c t r a l r e g i o n o f i n t e r e s t ) do n o t p l a y a s i g n i f i c a n t r o l e i n t h e p r o d u c t i o n o f t h e PAS s i g n a l .

A l s o , t h e r m a l e x p a n s i o n and c o n t r a c -

t i o n o f t h e s o l i d and any t h e r m a l l y i n d u c e d v i b r a t i o n a r e t o o s m a l l t o p r o d u c e a d e t e c t a b l e PAS s i g n a l . INTERLAMELLAR T R A N S I T I O N METAL I O N COMPLEXES

7.3

T h e r e a r e 3 t y p e s of t r a n s i t i o n s i n t h e s p e c t r a : d-d, t r a n s f e r 'and i n t r a l i q a n d b a n d s . molecules w i t h n

d

T::

or

charge

The l a t t e r a r e o n l y o b s e r v e d when

7 1 4 ~ : :

t r a n s i t i o n s a r e coordinated.

The

spectra w i l l be discussed with reference t o t h e solution spectra. T h i s a l l o w s a d i s c u s s i o n of t h e e f f e c t s o f t h e b i d i r e n s i o n a l c l a y s u r f a c e on t h e geometry and on t h e e l e c t r o n i c p r o p e r t i e s o f t h e a d s o r b e d complexes. T h i s s e c t i o n w i l l b e d i v i d e d i n 3 p a r t s , depending on t h e n a t u r e o f t h e l i g a n d s : H 2 0 and l a t t i c e oxygens, a l i p h a t i c p o l y a m i n e s and a r o m a t i c m o l e c u l e s . 7.3.1

Water and l a t t i c e oxygens Cu (11)-, Co (11)- and N i (11)- c l a y s have t h e o c t a h e d r a l

H 0-saturated 2

[M(H20) 12+ complexes i n t h e i n t e r l a m e l l a r s p a c e .

The d-d band

s p e c t r u m o f t h e s e hexaquo-complexes i s - w i t h i n t h e l i m i t s o f experiment-

a l accuracy-similar tothe aqueous s o l u t i o n s p e c t r a (Kdnig, 1 9 7 1 ; L e v e r , 1968; T a r a s e v i c h and S i v a l o v , 1975a, 197513, 1975c, 1 9 7 7 ) . 2+

s y s t e m s [ C U ( € ! ~ O ) ~ I *c+o l l a p s e s t o t h e p l a n a r [ C U ( H ~ O ) ~ ],

I n air-dry the 2

o t h e r s remain o c t a h e d r a l ( T a r a s e v i c h and S i v a l o v , 1975a; Velghe , Schoonheydt and U y t t e r h o e v e n , 1 9 7 7 ) .

T h e s e r e s u l t s a r e c o n f i r r e d by

e p r ( C l e m e n t z , P i n n a v a i a and M o r t l a n d , 1 9 7 3 ) . That Cu(I1) p r e f e r s t h e t e t r a g o n a l l y d i s t o r t e d coordination ( D 4 h symmetry) i s well-known

i n i n o r g a n i c c h e m i s t r y (Hathaway and B i l l i n g , 1 9 7 0 ) and a t t r i b u t e d t o t h e J a h n - T e l l e r e f f e c t . For t h i s p l a n a r complex t h e c o n t r i b u t i o n o f l a t t i c e oxygens t o t h e l i q a n d f i e l d i s n e g l i g i b l y s m a l l ( V e l q h e , Schoonheydt and U y t t e r h o e v e n , 1 9 7 7 ) . 2+ I t i s t h e r e f o r e n o t e x a c t t o w r i t e [ C U ( H ~ O ) ~ ( O ~(OQ ) ~ ]= l a t t i c e oxygen) as s u g g e s t e d i n t h e l i t e r a t u r e ( T a r a s e v i c h and S i v a l o v , 1 9 7 5 a ) . I n s t r o n g l y d e h y d r a t i n g c o n d i t i o n s t h e s p e c t r a d r a s t i c a l l y change. The s c a t t e r i n g power of t h e m a t e r i a l i s a l t e r e d and t h e background h a s i n c r e a s e d s t r o n g l y : t h e s p e c t r a become i l l - r e s o l v e d . a r e only determined approximately. decide

on

Band maxima

I t i s i n most cases i m p o s s i b l e t o

t h e number of components u n d e r one band e n v e l o p e .

The

170

a s s i g n m e n t s p r o p o s e d a r e t h e r e f o r e i d e a l i z e d and t e n t a t i v e

( T a r a s e v i c h and S i v a l o v , 1975a, 1975b , 1 9 7 5 ~ ;Velghe, Schoonheydt and U y t t e r h o e v e n , 1 9 7 7 ) . R e s u l t s a r e summarized i n T a b l e 2. They c o r r e s p o n d t o t h e f o l l o w i n g i d e a l i z e d d e h y d r a t i o n s e q u e n c e s ( T a r a s e v i c h , 1975) :

Especially t h e intermediate 5-coordinate species a r e d i f f i c u l t t o v i s u a l i z e i n terms o f t h e i d e a l i z e d CllV ( s q u a r e pyramid) or D 3h ( t r i g o n a l b i p y r a m i d ) s t r u c t u r e s . The i n t e r p r e t a t i o n i s a l s o made d i f f i c u l t by t h e p o s s i b l e p r e s e n c e o f d-d bands i n t h e n . i . r .

o u t s i d e t h e s p e c t r o m e t e r r a n g e , e s p e c i a l l y f o r Co(11) ( T a r a s e v i c h

and S i v a l o v , 197533). The o b s e r v a t i o n o f [Cu(O ) 12+ ( T a r a s e v i c h and S i v a l o v , 1975a) w i t h a d-d band a t 14800cm-‘ is i n t e r e s t i n g . I n d e h y d r a t e d 2+ s y n t h e t i c z e o l i t e s A , X o r Y [ C U ( O ~ ) ~ ]has i t s band maximum a t 1 0 8 0 0 ~ m -w ~ i t h a s h o u l d e r around 1 5 0 0 0 ~ m - ~ .The d i f f e r e n c e (1480010800) i n d i c a t e s t h a t C u ( I 1 ) i s more s t r o n g l y c o o r d i n a t e d t o t h e I n t h e l a t t e r case s u r f a c e oxygens of c l a y s t h a n t h o s e o f z e o l i t e s . s p i n - o r b i t c o u p l i n g and J a h n - T e l l e r e f f e c t s must b e i n c l u d e d i n t h e l i g a n d f i e l d c a l c u l a t i o n t o e x p l a i n t h e s p e c t r u m ( S t r o n e and K l i e r , 1980).

Better resolved s p e c t r a a r e necessary f o r clays, before

m e a n i n g f u l l i g a n d f i e l d p a r a m e t e r s can b e d e r i v e d .

171 TABLE 7.2 S p e c t r a l c h a r a c t e r i s t i c s of t h e a q u o complexes Band p o s i t i o n s ( c m - l )

P s s i gnn-en t s

8550 7800

13300

6320(1)7250(1)8130

8400 (

19400 21500sh 18050

22000

15750 17260 18760

i390

(Ok)

1

'+

25600

14500

2 5000

8770

15050

17000 24800 ( 2 )

14400(

1055 0 (

[Co ( H 2 0 )

[Co(OR)3H2012+

15150 154 50 ( 2 )

13 8 0 0 T 2

lOOOOsh

[CO(H~O)~]~+

[CU (H20) 1

12650 1 3$00 1 3100 (2

[cu (

15200 ( )

2+

~ ~ 12+ 0 )

1 4 e00 (1) n o t o b s e r v e d ( 2 ) V e l g h e , S c h o o n h e y d t and U y t t e r h o e v e n ,

1977; t h e o t h e r s p e c t r a a r e f r o m T a r a s e v i c h and Givalov, 1 9 7 5 a , 1 9 7 5 b , 1 9 7 5 c .

7.3.2

A l i p h a t i c polyamines

The complexes o f t r a n s i t i o n m e t a l i o n s w i t h p o l y a m i n e s on c l a y s h a v e b e e n i n v e s t i g a t e d as m o l e c u l a r p r o p s (Knudsen and McPtee,

1972;

T h i e l m a n n and McFtee, 19751, as s o i l c o n d i t i o n e r s and as waste water c l e a n e r s ( C r e m e r s , P e i g n e u r a n d Maes, 1979)

are

consequences of t h e e x t r a - s t a b i l i t y

.

The l a t t e r a p p l i c a t i o n s

g a i n e d b y t h e s e complexes

on t h e s u r f a c e w i t h r e s p e c t t o t h e i r s t a b i l i t y i n a q u e o u s s o l u t i o n

(Maes and C r e m e r s , 1979; S i l l e n a n d M a r t e l l , 1 9 6 4 ) . ( = e t h y l e n e d i a m i n e ) and C u ( I 1 ) t h e dOOl d i s t a n c e

For en

of 1.2751111-

( V e l g h e , S c h o o n h e y d t , U y t t e r h o e v e n , P e i g n e u r a n d L u n s f o r d , 1 9 7 7 , Maes and C r e m e r s , 1979) i s i n d i c a t i v e f o r a p l a n a r complex C u ( e n )2+ 2 ( i d e a l i z e d symmetry = D 4 h )

i n t h e interlamellar space.

This suggests

t h e removal of t h e l o o s e l y bound a x i a l w a t e r m o l e c u l e s upon adsorption :

172

+

[ C u ( e n ) (H20 2]2+

+

2 N a -clay

[Cu(en)2]-clay

+

2Na+

+

2H20

(7)

-

The d-d band s y s t e m o f [ C u ( e n ) 2 1 - c l a y i s i n t h e r a n g e 1 9 4 0 0

a s compared t o 1 8 2 0 0 ~ m - i~n a q u e o u s s o l u t i o n ( V e l a h e ,

20200cm-1

S c h o o n h e y d t , U y t t e r h o e v e n , P e i g n e u r and L u n s f o r d , 1 9 7 7 ) . F i g . 7 . 1 shows how t h e i n c r e a s e d t e t r a g o n a l d i s t o r t i o n , due t o t h e r e p l a c e m e n t o f t h e a x i a l w a t e r m o l e c u l e s by t h e s u r f a c e o x y g e n s , a c c o u n t s f o r t h e hypsochromic s h i f t .

The e x p e r i n e n t a l band p r o f i l e s

a r e s l i g h t l y a s y m m e t r i c t o w a r d s l o w e r wavenurrbers, b e c a u s e o f t h e t r a n s i t i o n s 2B-2B

19

t h e main band 19' (Maes, S c h o o n h e y d t , C r e m e r s and U y t t e r h o e v e n , 1980) 29

= lODq and

2B -2P 19

b e i n g 2B -2E 19 9 The c o m p l e t e r e s o l u t i o n o f t h e s e t r a n s i t i o n s by G a u s s i a n c u r v e

.

a n a l y s i s i s shown i n F i g . 7 . 2 t o g e t h e r w i t h t h e c o r r e s p c n d i n g c r y s t a l f i e l d s t a b i l i z a t i o n e n e r g i e s (C.F.F.E.).

There is a

p o s i t i v e c o r r e l a t i o n between lODq, t h e i n - p l a n e l i g a n d f i e l d s t r e n g t h , t h e C.F.S.E.,

and

of t h e mineral.

the

average

negative

charge

density

P similar c o r r e l a t i o n w a s found between t h e a v e r a g e

n e g a t i v e c h a r g e d e n s i t y and t h e f r e e e n e r g y change o f e x c h a n g e , AGO (Maes and C r e m e r s ,

1979).

I n f a c t , t h e v a l u e s of A G O are

- w i t h i n e x p e r i m e n t a l e r r o r - e q u a l t o A(C.F.S.E.1 surface

-

C.F.S.E.

i n aqueous s o l u t i o n .

= C.F.F.E.

on

T h i s s u g g e s t s t h a t AGO i s

g o v e r n e d by e n e r g y terms. The h e t e r o g e n e o u s c h a r g e d i s t r i b u t i o n o f t h e r r i n e r a l s ( F t u l and M o r t i e r , 1 9 7 4 ) i s e x p e r i m e n t a l l y shown by t h e h y p s o c h r o m i c s h i f t of t h e d-d band maximum w i t h d e c r e a s i n g e x c h a n g e l e v e l s (Maes, f c h o o n h e y d t , C r e m e r s and U y t t e r h o e v e n , 1 9 8 0 ) .

Thus, m i c r o c r y s t a l s

w i t h t h e h i g h e s t c h a r g e d e n s i t y a r e p r e f e r e n t i a l l y exchanged i n c o n d i t i o n s o f p a r t i a l exchange.

I n these crystals the strongest

e l e c t r o s t a t i c i n t e r a c t i o n surface-complex e x i s t s .

Both e f f e c t s ,

removal o f a x i a l H 2 0 and e l e c t r o s t a t i c i n t e r a c t i o n , are c o r r e l a t e d a s t h e f o r m e r o c c u r s more e a s i l y on t h e m i n e r a l s w i t h t h e h i g h e s t a v e r a g e n e g a t i v e c h a r g e d e n s i t y (Maes and Creners, 1 9 7 9 ) .

That t h e

a x i a l H 2 0 removal i s a c o n s e q u e n c e o f t h e e l e c t r o s t a t i c i n t e r a c t i o n s i s a t p r e s e n t a u s e f u l w o r k i n g h y p o t h e s i s f o r which no d i r e c t

experimental evidence e x i s t s .

173

9 d :CU

d

8

:

Ni

*Oh

dxy : b2g

/ ~ D s -5Dt increasing tetragonal

distortion

Fig. 7.1 d-orbital splitting under D4h symmetry

* CI

P

8

0

0

m

E

0 -

-

-

-

-

-

214 21 20.1

217 24 17.4

In

?

-

207 14 14

198 193 CFSE(kJmoi')

5

12

A CFSE A Go (kJrnol-') 2+

Fig. 7.2 d-d transitions, C.F.S.E. and AGO (exchange) of Cu(en)2 on hectorite (hect.), Otay and Camp Berteau montmorillonites (Otay, Camp B.); 0.75 C.B. and 0.65 C.B. = Camp Berteau with respectively 75 % and 6 5 % of the original C.E.C. The Cu-N bonding of [Cu(en)2]2+ on the surface is essentially the same as in solution (Schoonheydt, 1978). This and the surface not being a ligand -or a weak one- suggests that the

174 c l a y s u r f a c e i s a r i g i d a n i o n i c s o l v e n t t o w a r d s [ C ~ ( e n ) ~ ] ~I n+d.e e d , t h e LMCT ( l i g a n d - t o - m e t a l i n t h e range 40700 and 43860cm-1

-

c h a r g e t r a n s f e r ) band on t h e c l a y s i s 4 1 9 0 0 ~ m - ~ and a t a b o u t 4 0 0 5 0 ~ m - f~o r CF30F

i n water.

Fs a result the optical electroneuativity

o f e n i n C u ( e n ) ; + on c l a y s i s 1.32

+

0.04

a u a i n s t 1.18 i n aqueous

s o l u t i o n (Maes, S c h o o n h e y d t , C r e m e r s and U y t t e r h o e v e n , 1 9 8 0 ) . P h y s i c a l l y , t h i s i n c r e a s e means t h a t e n on c l a y s i s s l i u h t l y more b a s i c t h a n i n aqueous s o l u t i o n .

Such a b e h a v i o r r e r i n d s u s o f t h e

f a c t t h a t t h e f r e e e n e r g i e s of exchange of N a

+

f o r alkylammonium

i o n s f o l l o w t h e g a s p h a s e b a s i c i t y s e q u e n c e (Maes, Y a r i j n e n and Cremers, 1977).

Thus, t h e " s c r e e n i n g " power o f t h e s o l v e n t

" h y d r a t e d c l a y " i s less t h a n t h a t o f t h e s o l v e n t "water", b o t h f o r 2+ alkylammonium i o n s and f o r C u ( e n ) 2

'2

A l l t h e s e r e s u l t s f o r Cu ( e n )

.

.

a p p l y t o N i (11)

The b i s - c o r r p l e x i s exchanged w i t h removal o f t h e 2 a x i a l w a t e r m o l e c u l e s and [Ni ( e n ) 2 ] 2 + on t h e c l a y i s d i a m a g n e t i c ( V e l g h e , S c h o o n h e y d t and

Uytterhoeven,

1979).

reference t o Fig. 7.2.

The d i a m a g n e t i s m c a n b e u n d e r s t o o d w i t h The removal of 2 a x i a l F 2 0 m o l e c u l e s i n c r e a s e s

t h e t e t r a g o n a l d i s t o r t i o n t o s u c h an e x t e n t t h a t t h e e n e r g y d i f f e r e n c e between d 2 2 and d Z 2 o r d exceeds t h e s p i n p a i r i n g energy. x -Y XY -1 contains the c h a r a c t e r i s t i c band of N i ( e n ) 2 - c l a y a t 21500crr

The

.

f o l l o w i n g 3 t r a n s i t i o n s 'A- -1~ 'A B'and 'A-lE 29' 19 1g 19 g 19 T h i s c a n b e s e e n from t h e e n e r g y d i a q r a r o f F i g . 7 . 3 , c o n s t r u c t e d w i t h t h e e n e r g y matrices o f a p p e n d i x 7 . 7 .

Frorr a f i t o f t h e

observed spectrum w i t h t h i s l i g a n d f i e l d c a l c u l a t i o n , i t f o l l o w s t h a t R a c a h ' s i n t e r e l e c t r o n i c r e p u l s i o n p a r a m e t e r i s B = 788 l O D q = 22800

1 1 2 00cm-I

-

-

-1

995~117 and

2 4 0 0 0 ~ m - ~ . The l a t t e r i s much l a r g e r t h a n t h e 2+ ( L e v e r , 1968)

f o r t h e p a r a m a g n e t i c [ N i ( e n ) ( F 2 0 )2 1

.

Two e f f e c t s c o n t r i b u t e t o t h i s d i f f e r e n c e : t h e d i f f e r e n c e i n t h e e f f e c t i v e n u c l e a r c h a r g e on N i and t h e d i f f e r e n c e i n t h e N i - N distance.

bond

I f t h e f o r m e r i s s u p p o s e d t o b e c o n s t a n t , a maximurr v a l u e

f o r t h e l a t t e r c a n b e c a l c u l a t e d from ( L e v e r , 1968) : 10

zeff

lODq = where Z e f f distance.

e2 r4

6a5

i s t h e e f f e c t i v e n u c l e a r c h a r g e and a t h e N i - N bond = 0.2124nm f o r t h e p a r a m a g n e t i c complex

With N i - N

( M a r t i n , S p e r a t i and Busch, 1 9 7 7 ) o n e o b t a i n s f o r t h e d i a m a g n e t i c complex a v a l u e of 0.183nm.

175 A,

A,

=

30B

A,

=

208

= 348

-EI 8C

60

40

e

_..--:

m

2 0 s2/ .... ................. -.>-: ___.--............. _ic. .........

---

/. *+

0

Fig.7.3 Energy level diagrams for Ni(en)22+ on clays Octahedral complexes do not undergo significant band shifts on the surface. Mono-complexes such as [Cu(en) (H20)412+ and [Ni(en)(H20) 12+ disproportionate on the surface into bis-corplexes and aquo-complexes (Velghe, Schoonheydt, Lunsford, 1977; Velghe, Schoonheydt, Uytterhoeven, Peigneur and Lunsford, 1977; Velghe, Schoonheydt and Uytterhoeven, 1979). The stepwise adsorption of gaseous en on hydrated clays proceeds over the bis-complex : [M(H20)612+ + 2en

+ [M(en)2]2+ +

6H20

V(en),2+ (M=Ni, Cu) (9)

With ligands such as dien(diethy1enetriamine) and tetren(tetraethylenepentamine) an important amount of protonated 1igand.s is exchanged together with the complexes (Schoonheydt, Velghe, Baerts and Uytterhoeven, 1979). Dependkng on the water content the following equilibria are observed :

176

tetrenH

+

2dienH +

-HZO'

+ [Ni(tetren)(H20) ] 2 + s 2 0 t e t r e n

+

[ N i ( d i e n I 2 l2 +

-+H20

2dien

-H20

+

+

[I

> t r e n H )13+(10)

Pla

diamagnetic

[ N i ( d i e n H ) 2 ]4+

(11)

p l a n a r , diamagne

S i m i l a r r e a c t i o n s can b e w r i t t e n f o r Cu. P 1 1 t h e s e d a t a d e m o n s t r a t e t h e l a r g e a f f i n i t y of t h e b i d i

ion-

a 1 s u r f a c e o f c l a y s f o r " b i d i m e n s i o n a l " ( = p l a n a r ) .complexes. 7.3.3

Aromatic m o l e c u l e s

I n t h e e a r l y 7 0 ' s t h e most p o p u l a r s y s t e m w a s Cu2+

+

benzene,

forming on t h e s u r f a c e g r e e n , t y p e I , complexes and r e d , t y p e 11, complexes : g r e e n r t y p e 1971, Mortland, 1973).

+H 0

r e d , t y p e I1 ( P i n n a v a i a and -3 A u ( I I 1 ) i s a l s o able t o form t y p e s

+ 2+ N a , Ca , F13+,

3+

Mortland, I and 2+ 3+

I1 complexes (Vandepoel, 1 9 7 5 ) . Cr , Fe , Fe , 2+ + 2+ Mn2+, Zn , Ag , N i 2 + and Co o n l y form t y p e I complexes. A r o m a t i c l i g a n d s c a n b e d i v i d e d i n 2 g r o u p s a c c o r d i n g t o t h e i r a b i l i t y t o form

2+

.

The l a t t e r group c o n t a i n s t y p e I o r b o t h t y p e s o f complexes w i t h Cu benzene, d i p h e n y l , n a p h t a l e n e , a n t h r a c e n e and a n i s o l e , t h e f o r m e r t o l u e n e , x y l e n e s , p h e n o l , hexamethylbenzene and 1, 2 , 4 , 5 tetrachlorobenzene. complex. complexes.

-

Vandepoel e t a l . (1973) a l s o found a brown

T a b l e 7 . 3 summarizes t h e s p e c t r a l c h a r a c t e r i s t i c s of t h e

The a s s i g n m e n t s g i v e n i n T a b l e 7.3 a r e t h o s e of Vande

P o e l e t a l . (1973) and must b e r e g a r d e d as working h y p o t h e s e s . p r e s e n t t h e r e i s n o f i r m e x p l a n a t i o n of t h e s e s p e c t r a . Complexes w i t h 2 ,2 ' - b i p y r i d i n e

( b i p ) and 1 , l o - p h e n a n t h r o l i n e

P t

(phen)

a r e exchanged w i t h e x t r e m e l y h i g h s e l e c t i v i t i e s and w i t h p r e c i p i t a t i o n o f s a l t s i n t h e i n t e r l a m e l l a r s p a c e (Schoonheydt, P e l g r i m s , Heroes and U y t t e r h o e v e n , 1 9 7 7 ; B e r k h e i s e r and M o r t l a n d , 1 9 7 7 ) .

Epacinns are

a b o u t 1.8nm and even a f t e r c a l c i n a t i o n i n t e r e s t i n g a d s o r p t i v e p r o p e r -

t i e s a r e e v i d e n t ( T r a y n o r , M o r t l a n d and P i n n a v a i a , 1978; L o e p p e r t , The s p e c t r u m of [ R u ( b i p ) 12+ i s M o r t l a n d and P i n n a v a i a , 1 9 7 9 )

.

dominated by a ~ C band T a r o u n d 460nm and a n i n t r a l i w n d band around 280nm ( F e r g i s o n and H a r r i s , 1966; L Y t l e and

~ + n "

177 1 9 6 9 ; Crosby and K l a s s e n , 1 9 6 5 ) . TABLE 7 . 3

A b s o r p t i o n b a n d s and t e n t a t i v e a s s i g n m e n t s f o r Cu2+-benzene complexes Species

assignment

benzene 255 (39200)

TI

-+

71

+TI::

comp1e x

iT::

physisorbed benzene 284 (35200) type I (green) 390 (25640) 686 (14577)

charge t r a n s f e r d-d

H2°\ H20’

t y p e I1 ( r e d ) 510 (19608)

charge t r a n s f e r

900-5900

d-p

(11000-1700)

brown 355 (28169) 825 ( 1 2 1 2 1 )

charge t r a n s f e r

cu

;+/OH2 \

OH2

P H

d-d

On t h e c l a y t h e l a t t e r i s r e d u c e d i n i n t e n s i t y and a s h o u l d e r a p p e a r s around 320nm ( K r e n s k e , Abdo, Van Damme, Cruz and F r i p i a t , 1 9 8 0 ) . The f o l l o w i n g h y p o t h e t i c a l r e a c t i o n was p r o p o s e d t o a c c o u n t f o r t h i s new band :

R e a c t i o n ( 1 2 ) r e s u l t s i n a l o s s of aromatic c h a r a c t e r o f t h e ligand. A b l u e s h i f t o f t h e n+n’: band i s e x p e c t e d ( P l b e r t , 1 9 6 7 ) . T h i s is o p p o s i t e t o t h e experimental o b s e r v a t i o n . however, n o a l t e r n a t i v e i s a v a i l a b l e .

A t present,

178

The LMCT band i s a t 463nm on t h e h y d r a t e d h e c t o r i t e , a t 455nm on t h e d e h y d r a t e d c l a y and a t 452nm i n aqueous s o l u t i o n ( K r e n s k e , ?&do Van Damme, Cruz and F r i p i a t , 1 9 8 0 ) . Two e f f e c t s g o v e r n t h e s e s h i f t s : ( i ) t h e change o f t h e s o l v e n t "H20" t o t h e s o l v e n t " c l a y " and ( i i ) d i s t o r t i o n of t h e complex i n t h e i n t e r l a m e l l a r s p a c e .

The

l a t t e r i s e a s y t o imagine f o r 2 r e a s o n s : t h e " k e y i n u i n " phenomenon and t h e i n t e r a c t i o n of a f l a t s u r f a c e w i t h a corrplex o f d i f f e r e n t geometry.

The e x a c t n a t u r e o f t h e d i s t o r t i o n i s unknown a t p r e s e n t .

However d i s t o r t i o n s have b e e n invoked t o e x p l a i n s i r i l a r band s h i f t s ( F e l i x , F e r g u s o n , Giidel and L u d i , 1 9 8 0 ) .

'32

The l u m i n e s c e n c e p r o p e r t i e s o f R u ( b i p ) H 0-dependent. 2

t o c a . 15 %

on h e c t o r i t e a r e a l s o

The quantum y i e l d i n c r e a s e s by a b o u t 2 from ca. 3% H20 H20 ( 0 . 0 4 -+ 0 . 0 9 ) . Cr(bip):+ on h e c t o r i t e , i r r a d i a t e d

i n i t s d-d band, h a s a quantum y i e l d d e c r e a s i n g from ca. 0.08 t o c a . 0.002 w i t h i n c r e a s i n g w a t e r c o n t e n t ( K r e n s k e , Pbdo, Van Dame, Cruz, and F r i p i a t , 1 9 8 0 ) . F o r b o t h complexes t h e l u m i n e s c e n c e i s 3+ 3+ quenched i n a d i f f u s i o n - c o n t r o l l e d way by 02. Q u e n c h i n g by Eu , C r o r Fe3+ i s more e f f e c t i v e w i t h t h e s e c a t i o n s s u b s t i t u t e d i n t h e o c t a h e d r a l l a y e r s o f t h e l a t t i c e t h a n when t h e y a r e i n i o n exchange positions. These r e s u l t s open new r e s e a r c h a r e a s b u t many problems must b e s o l v e d b e f o r e t h e s p e c t r a o f t h e s e complexes a r e u n d e r s t o o d and r e l a t e d t o t h e i r p e c u l i a r i o n exchange b e h a v i o r .

The i m p o r t a n c e o f

a complete u n d e r s t a n d i n g of t h e s p e c t r o s c o p i c p r o p e r t i e s o f t h e s e monolayer complexes r e a c h e s f u r t h e r t h a n t h a t . I n d e e d , t h e photoc h e m i c a l s u b s t i t u t i o n o f 2H20 m o l e c u l e s on Cr(bip):+ on t h e s u r f a c e i s i n c i s - p o s i t i o n s , whereas t h e r e a c t i o n i n s o l u t i o n g i v e s trans-products

( C r u z , Pbdo, Van Damme and F r i p i a t , 1 9 8 0 ) .

This is

an example o f a s t e r e o s p e c i f i c , p h o t o c h e r r i c a l s u b s t i t u t i o n r e a c t i o n , t h e s t e r e o s p e c i f i c i t y b e i n g i n d u c e d by t h e s u r f a c e . Monolayer

'32

a r r a n g e m e n t s of Ru ( b i p ) d i s s o c i a t i o n of H 2 0 .

have been p r o p o s e d f o r p h o t o c a t a l y t i c

I t i s expected t h a t s t e r e o s p e c i f i c r e a c t i o n s

can b e performed on t h e c l a y s u r f a c e r o c c u p i e d by c a t i o n i c corrplexes with c a t a l y t i c a c t i v i t y . 7.4

LATTICE TRFNSITION METAL IONS

U.v-vis.-n.i.r. s p e c t r o s c o p y o f t r a n s i t i o n metal i o n s i n l a t t i c e s i t e s i s mainly c o n c e r n e d w i t h Fe. The d-d bands o f Fe a r e f o r b i d d e n and u s u a l l y n o t s e e n a t t h e low Fe c o n c e n t r a t i o n s of k a o l i n i t e and smectite materials.

a t 240-5nm and a t 190-5nmr due t o an 0

They have 2 weak bands

+Fe(II1)

charae t r a n s f e r

179 transitions

( K a r i c h k o f f and B a i l e y , 1 9 7 3 ) , w i t h F e ( I I 1 ) i n o c t a h e d r a l

sites s u b s t i t u t i n g f o r A l ( I I 1 ) . For suspensions t h e i n t e n s i t y of t h e b a n d a t 240nm i s d e p e n d e n t on t h e Fe c o n t e n t and on t h e t a c t o i d s i z e of t h e m i n e r a l s (Chen, Shaked and B a n i n , 1 9 7 9 ) . I n n o n t r o n i t e n e a r l y a l l F l ( I I 1 ) i s s u b s t i t u t e d by F e ( I I 1 ) and t h e 2 b a n d s a r e f o u n d a t 260nm and a t 200nm. F e ( I I 1 ) s u b s t i t u t i n g f o r M g ( I 1 ) g i v e s the c h a r g e t r a n s f e r b a n d i n t h e r e g i o n 245-250nm w h i l e i t i s a t 215nm f o r t e t r a h e d r a l F e ( I I 1 ) ( K a r i c h k o f f and B a i l e y , 1 9 7 3 ) . of t h e bands i s - p a r t i a l l y bond l e n g t h s : FeIV-O

<

The p o s i t i o n

a t l e a s t - d e t e r p i n e d by t h e F e ( I I 1 ) - 0

FeV1(Al)-O

<

For nontrcrnite t h e

FeV1(Mg)-O.

s t r o n g l y i n t e r a c t i n g n e i g h b o u r i n g Fe i o n s may c r e a t e e x c i t o n s t a t e s , e x p l a i n i n g t h e r e d s h i f t t o 260nm ( K a r i c h k o f f a n d B a i l e y , 1 9 7 3 ) .

In

any case t h e b a n d s o f i m p u r i t y Fe o x i d e a r e r e p o r t e d a t 4 5 0 and 530nm, d i a g n o s t i c a l l y d i f f e r e n t from t h e l a t t i c e b a n d s

(Bart, Furriesci,

C a r i a t i , Micene and Gessa, 1 9 8 0 ) . The s p e c t r u m o f t h e F e - r i c h n o n t r o n i t e shows a l s o d-d t r a n s i t i o n s . They a r e compared t o t h o s e o f V e r m i c u l i t e and m u s c o v i t e i n T a b l e 7.4 ( K a r i c h k o f f and B a i l e y ,

1973)

.

TABLE 7.4 d-d

t r a n s i t i o n s (nm) o f F e ( I I 1 ) i n o c t a h e d r a l s i t e s i n c l a y s

transition

nontronite

vermiculite

muscovite

540

523 , 503

575 , 6 0 0

579

445

4 60

441

6Al (S)

+ 4T 2 ( G I

520

6Pl(S)

+4 T 1 ( G )

-

6 A 1 ( S ) -b 4 P l ,

4E ( G )

'A1(s)

+4 T2(D)

384

4 00

379

6Al(S)

+4 E ( D )

367

-

363

The e n e r g y i n c r e a s e of t h e 6Pl ( S )

+4 P1,

t r a n s i t i o n s i n t h e series v e r m i c u l i t e

-

<

4 E ( G ) a n d 6Pl ( S )

+4 E ( D )

n o n t r o n i t e < muscovite

f o l l o w s t h e decrease o f t h e F e ( I I 1 ) - 0 bond l e n g t h s . When Fe3+ i n 2+ n o n t r o n i t e i s r e d u c e d t o Fe t h e t y p i c a l d-d b a n d s d e c r e a s e and a 2+ Fe Fe3+ charge t r a n s f e r t r a n s i t i o n a p p e a r s a r o u n d 760nr@, moving

t o 720nm w i t h h i g h r e d u c t i o n d e g r e e ( P n d e r s o n and F t u c k i , 1 9 7 8 ) .

180

The band splittings observed for vermiculite and muscovite are ascribed to the Jahn-Teller effect (Karichkoff and Bailey, 1973). The doublet at llOOnm and 400nm of Fe(II1) in micas is also caused by the Jahn-Teller effect (Faye, 1968) , but there is some discussion about the origin of the band intensity (Smith, 1977). Indeed, for chlorite and biotite they increase with decreasing temperature, a behavior not expected for symmetry allowed or vibronically coupled transitions. 7.5 ORGANIC MOLECULES

Spectroscopy of adsorbed organic molecules allows surface acidity measurements and -for coloured bases- identification of clays and their application in the manufacture of pressure-sensitive, carbon-free paper (Thompson, 1977; Lim, 1975). More recent interest is found in the adsorption and degradation of pesticides and in the luminescence, photochemical and photocatalytic properties of surface-immobilized "monolayers" of these rrolecules. The chemistry of these molecules on the surface is well understood (Theng, 1974). The recent spectroscopic work is mainly concerned with assignment of the observed bands. Thus, the usefulness of spectroscopy for surface acidity measurements with H a m e t t dye indicators has been demonstrated (Bailey and Karichkoff, 1973). Tarasevich and coworkers studied the acidity of dehydrated H- and P1-montmorillonites, cation-exchanged kaolinite and palygorskite (Telichkun, Tarasevich and Goncharuk, 1976; Telichkun and Tarasevich, 1973; Tarasevich and Fedorova, 1979). The indicators used are p-dimethylaminoazobenzene ( D W B , pKa = 3.3) , dicinnamalacetone ( D C P , pKa = -3.0) , benzo-acetophenone (BAPh, pKa = -5.61, triphenylcarbinol (g3COH, pKa = -6.6) and diphenylcarbinol (d2CHOP, pKa = -13.3). Their characteristic bands are smmarized in Table 7.5. Very strong Brijnsted and Lewis acidities are found on P- and P1-clays Radicals Z 3 C o are thought to be generated through (pKa = -6.6). interaction with Fe3+. With the butylamine titration technique no acidities stronger than pKa = -3.7 were found (Frenkel, 1974). This is another illustration of the difficulties encountered by comparinu different methods of surface acidity measurements (Jacobs, 1977).

181

TABLE 7.5 C h a r a c t e r i s t i c bands of i n d i c a t o r s on c l a y m i n e r a l s DCF

DMPAB

X (nm)

a s signmen t

240-300

benzene r i n g

350

TI

4 00

chromophore

490-5 5 30

on e x c h a n g e a b l e P 1 p r o t o n a t ed

525 325

=yH-N='=>=NMe2

a

A (nm)

+

a N H = N e & H M e 2

450-480

chromophore on s u r f a c e

560

on A 1 3 + a t exchange s i t e s

6 50

on p13+ a t e x t e r n a l s u r f a c e

assignment

+ TI::

3+

BaPh A

(nn)

assignment

+ 71::

305

TI

4 30

protonated

490-5

on e x c h a n g e a b l e P 1 3+ on l a t t i c e F 1

5 50 @,

3+

CHOH

(nm)

X (nm)

assignment

A

230, 285

p h y s i c a l l y adsorbed

415, 450

430, 445 460

g3 C+ g3 c+

310, 530

Co

assignment

g2

CH+

on A I ~ +

Dye m o l e c u l e s a r e exchanued on t h e s u r f a c e or s y n t h e s i z e d i n s i t u ( S i f f e r t , 19-78).

On t h e s u r f a c e t h e y u n d e r a o a c i d - b a s e r e a c t i o n s ,

r e d o x r e a c t i o n s , c o m p l e x a t i o n w i t h t r a n s i t i o n m e t a l i o n s and associations.

The b e s t known example i s b e n z i d i n e (Theng, 1 9 7 4 1 ,

which c a n undergo e l e c t r o n t r a n s f e r t o l a t t i c e Fe3+ by s h o r t r a n g e i n t e r a c t i o n between r - e l e c t r o n s of b e n z i d i n e and s u r f a c e oxygens ( Y a r i v , 1 9 7 6 ) . These s t u d i e s have b e e n e x t e n d e d t o more c o m p l i c a t e d m o l e c u l e s and a summary o f t h e s p e c t r o s c o p i c d a t a i s g i v e n i n T a b l e 7.6.

182

TABLE 7 . 6 S p e c t r o s c o p y o f d y e s on c l a y s solution

X (nm) Na-V

Na-M

Ni-M

FeM

Safranine T S+

528

504-10

504-510

SH2+

579

550

555

SHZ+

6 30

592

546 ( b r o a d )

methylene b l u e BM+

665

665

(BM+)

595-615

570

BN+

640

640

(BN+)

5 8 0 - 6 10

560-590

625

640-5

4 30

445-50

435-55

435-60

-

N i l blue

b r i l l i a n t green vB+

ChrysoIdine CR+

V = vermiculite, M = Montmorillonite I t i s shown t h a t d i m e r i z e d c a t i o n s a b s o r b a t s h o r t e r w a v e l e n g t h s

t h a n t h e c o r r e s p o n d i n g monomers and a f u r t h e r b l u e s h i f t o f

i s o b s e r v e d f o r t h e d i m e r s on t h e s u r f a c e . found i n t h e l i t e r a t u r e f o r t h e monomers.

2.

20n~

N o such u n i f o r m i t y i s

Thus, t h e bands o f

rhodamine B and c h r y s o i d i n e do n o t undergo s i g n i f i c a n t f r e q u e n c y s h i f t s a f t e r a d s o r p t i o n , w h i l e t h o s e of b r i l l i a n t g r e e n , t h e p e s t i c i d e chloramben and t h e S o r e t band o f p o r p h y r i n s show r e d s h i f t s ( G h o s a l and Mukherjee, 1 9 7 3 : B e r k h e i s e r and A h l r i c h s , 1 9 7 6 ; Van Dame, C r e s p i n , O b r e c h t , Cruz and F r i p i a t , 1 9 7 8 ; S a e h r , L e Dred and F o f f n e r , 1 9 7 8 ) . The c h a r a c t e r i s t i c bands of s a f r a n i n e and j a n u s g r e e n are r e p o r t e d t o undergo b l u e s h i f t s (Van D a m e , C r e s p i n , Cruz and F r i p i a t , 1 9 7 7 ; Ghosal and Mukherjee, 1 9 7 3 ) . for

+ S

on Na-M

An e x t r e m e case o f b l u e s h i f t was found

(Van D a m e , C r e s p i n , Cruz and F r i p i a t , 1 9 7 7 ) : t h e

183 band maximum of 528nm was a t 468nm i n s u s p e n s i o n .

T h i s was a s c r i b e d

t o t h e d e g r e e o f d i s p e r s i o n : i n o t h e r words t o l i a h t s c a t t e r i n g . L i g h t s c a t t e r i n g i s w a v e l e n g t h d e p e n d e n t and f o r g r a i n s i z e s s i m i l a r t o t h e w a v e l e n g t h t h e s c a t t e r i n g c o e f f i c i e n t i s - t o a f i r s t approximat i o n - p r o p o r t i o n a l t o t h e wavelength. contrary t o the observation.

Calculation gives a red s h i f t ,

A l s o the reported red s h i f t s are

u s u a l l y l a r q e r t h a n e x p e c t e d on t h e b a s i s o f l i g h t s c a t t e r i n g a l o n e Many o t h e r f a c t o r s can c o n t r i b u t e .

(Kortfim, 1 9 6 9 ) .

They a r e : ( i )

s t e r i c e f f e c t s i n d u c e d by t h e r i g i d b i d i m e n s i o n a l s u r f a c e : ( i i ) s o l v e n t e f f e c t s , a s t h e h y d r a t e d c l a y h a s a p o l a r i t y d i f f e r e n t from t h e t r a n s i t i o n s underao s o l v e n t from which t h e dye i s a d s o r b e d , n+n:: t r a n s i t i o n s undergo b l u e s h i f t s , when t h e p o l a r i t y i n c r e a s e s : T + I T : : r e d s h i f t s (Rao, 1 9 7 5 ) . O t h e r f a c t o r s a r e P-bondina t o s u r f a c e oxygens, r e d i s t r i b u t i o n o f p o s i t i v e c h a r g e o v e r t h e m o l e c u l e when a d s o r b e d ( S a e h r , L e Dred and H o f f n e r , 1978) a n d , f o r p o r p h y r i n s , i n c r e a s e d r e s o n a n c e i n t e r a c t i o n between p h e n y l o r p y r i d y l s u b s t i t u e n t s and t h e p o r p h y r i n n u c l e u s (Van Damme, C r e s p i n , O b r e c h t , Cruz, F r i p i a t , 1978). I t w i l l be very d i f f i c u l t t o individualize t h e d i f f e r e n t c o n t r i b u t i o n s b e c a u s e t h e y always a c t s i m u l t a n e o u s l y . 7.6

CONCLUSIONS I n t h e a r e a of t r a n s i t i o n m e t a l i o n s i n t h e i n t e r l a r r e l l a r s p a c e

o n l y complexes o f h i g h o x i d a t i o n s t a t e metals ( h a r d a c i d s ) w i t h h a r d b a s e s have been i n v e s t i g a t e d i n d e t a i l .

Three c o n c l u s i o n s can b e

drawn : ( i )p l a n a r complexes a r e p r e f e r e n t i a l l y a d s o r b e d on t h e "bidimensional" surface.

T h i s a l l o w s t h e e a s y s y n t h e s i s of

complexes on t h e s u r f a c e , which are d i f f i c u l t t o s y n t h e s i z e i n c l a s s i c a l inorganic chemistry:

( i i ) t h e s u r f a c e oxygens do n o t

e n t e r i n t o t h e f i r s t c o o r d i n a t i o n s p h e r e t o any s i g n i f i c a n t extent,

( i i i ) a d e t a i l e d l i g a n d f i e l d a n a l y s i s of t h e s p e c t r a i s

necessary t o quantify t h e information leading to t h e 2 previous conclusions.

How

f a r t h e s e conclusions can b e extended t o

complexes o f low o x i d a t i o n s t a t e m e t a l s ( s o f t a c i d s ) w i t h e l e c t r o n r i c h l i g a n d ( s o f t b a s e s ) , remains t o be i n v e s t i u a t e d . An e x c i t i n g new r e s e a r c h a r e a h a s been opened : p h o t o p h y s i c s and p h o t o c h e m i s t r y o f monolayer complexes.

The p r e l i m i n a r y r e s u l t s look

v e r y p r o m i s i n g a s r e a c t i o n s can b e c o n t r o l l e d s t e r e o s p e c i f i c a l l y on t h e s u r f a c e . An e x t e n s i o n t o p h o t o c a t a l y t i c work i s e x p e c t e d . The e l e c t r o n i c i n t e r a c t i o n s o f o r g a n i c c a t i o n s w i t h t h e s u r f a c e are p o o r l y understood.

T h e i r ( p h o t o ) c h e m i s t r y and ( p h o t o ) c a t a l y s i s on t h e

184

surface w i l l t h e r e f o r e be very d i f f i c u l t .

A

detailed spectroscopic

a n a l y s i s o f model s y s t e m s , e v e n t u a l l y s u p p o r t e d by quantum m e c h a n i c a l calculations, is necessary t o i n i t i a t e t h e s e s t u d i e s . F i n a l l y , t h e r e i s a l a c k of q u a n t i t a t i v e s p e c t r o s c o p i c s t u d i e s . T h i s i s an e x t r e m e l y d i f f i c u l t t o p i c b e c a u s e of t h e h e t e r o p e n e o u s n a t u r e of t h e s y s t e m s b u t some a t t e m p t s t o w a r d s s i m p l e model s y s t e m s should be c a r r i e d out.

7.7 EPPENDIX S t a t e f u n c t i o n s and e n e r g y m a t r i c e s f o r N i ( I 1 ) i n D q h symmetry 7.7.1 The 5 3d o r b i t a l s are r e p r e s e n t e d a s u = d,2, v = d X 2 - y2, I = d 5 = d and n = dxz. Only t h e u p p e r h a l f o f t h e symmetric XY' Y= m a t r i c e s i s shown. F o r d e t a i l s of t h e c o n s t r u c t i o n o f s t a t e f u n c t i o n s and c a l c u l a t i o n o f e n e r g y m a t r i c e s I r e f e r t o " M u l t i p l e t s o f t r a n s i t i o n - m e t a l i o n s i n c r y s t a l s " by S . Sugano, Y . Tanabe and H . Karrinmra, Academic P r e s s , N e w York, 1970, 331 pp. A, B and C a r e R a c a h ' s i n t e r e l e c t r o n i c r e p u l s i o n p a r a m e t e r s . E i s a common p a r a m e t e r and

C i s assumed e q u a l t o 4B. A l l A2 and A3 a r e

a r b i t r a r i l y t a k e n 0. d e f i n e d i n f i g u r e 7.1.

1 --[I5 2

A+4B+3C-E

4B+C 2Al+A+4B +3C-E

+ + 5 \+In n I1

C 4B+C

fi (3B+C)

2 A2+E+4B +3C-E

E = A +F+4B+2C 2

A2+A+4B-E

-6B 2A3+A-5B-E

185

- 2 6 B 2A 3+A+B+2C-E

A1+A+2C-E

A 3+A+B+ZC-E

A3+A-5B-E

-G/2 B

-3/2 B A 2+ A 3 +P+B+ZC-E

-5?/2

3B

-3fi

A 2+A 3+A-5B-E

- 3 0 B

A

2A3+A+B+2C-E

+A

B +P+ B-E

2fl B Al+A2+A+2C-E

B

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Hathaway, B . J . and B i l l i n g , D . E . , 1 9 7 0 . The e l e c t r o n i c p r o p e r t i e s and s t e r e o c h e m i s t r y o f mononuclear complexes o f t h e c o p p e r ( I 1 ) ion. C o o r d i n . Chem. Rev., 5 : 143-207. Hecht, H . G . , 1 9 7 6 . Comparison o f c o n t i n u u m models i n q u a n t i t a t i v e d i f f u s e r e f l e c t a n c e s p e c t r o m e t r y . Fn. Chew., 48: 1775-1779. C a r b o n i o g e n i c A c t i v i t y of Z e o l i t e s . J a c o b s , P.A., 1 9 7 9 . E l s e v i e r , Amsterdam, 253 p p . K a r i c k h o f f , S.W. and B a i l e y , G . W . , 1973. O p t i c a l a b s o r p t i o n s p e c t r a of c l a y m i n e r a l s . C l a y s and C l a y Min., 2 1 : 59-70. K e l l e r m a n , R . , 1979. D i f f u s e R e f l e c t a n c e and P h o t o - a c o u s t i c Spectroscopies. I n : W.N. D e l g a s s , G.L. F a l l e r , R. K e l l e r m a n and J . H . L u n s f o r d ( E d i t o r s ) , S p e c t r o s c o p y i n F e t e r o g e n e o u s C a t a l y s i s , Academic P r e s s , N e w York, p p . 86-131. K l i e r , K., 1972. A b s o r p t i o n and s c a t t e r i n q i n p l a n e p a r a l l e l t u r b i d m e d i a . J . O p t . SOC. A m e r i c a , 6 2 : 882-885. K l i e r , K . , 1973. R o t a t i o n a l c o r r e l a t i o n m o t i o n i n l i q u i d and a d s o r b e d w a t e r . J . Chem. P h y s . , 5 8 : 737-743. K l i e r , K., S h e n , J . H . and Z e t t l e m o y e r , P . C . , 1973. Water on s i l i c a I. P a r t i a l l y hydrophobic s i l i c a s . and s i l i c a t e s u r f a c e s . J . P h y s . Chem., 7 7 : 1458-1465. C a l c u l a t i o n and a c c u r a c y Kijnig, E . , 1 9 7 1 . The n e p h e l a u x e t i c e f f e c t . t h i n erelec r o n i c r e p u l s i o n p a r a m e t e r s . I . Cubic h i g h - s p i n S t r u c t u r e and Bonding, 9 : 175-212. d5, df and d b s y s t e m s . KortEm, G . , 1 9 6 9 . R e f l e c t a n c e S p e c t r o s c o p y . P r i n c i p l e s , Methods, Fpplications. S p r i n g e r , B e r l i n , 366 p p . Knudsen, M . I . , J r . and M c A t e e , J . L . , J r . , 1 9 7 2 . P study of thermal d e c o m p o s i t i o n o f tris(ethylenediamine)Cobalt(III)chloride : d i l u t i o n e f f e c t s . Thermochimica A c t a , 4 : 4 1 1 - 4 2 0 . K r e n s k e , D . , Abdo, S . , Van Dawme, H . , C r u z , M . and F r i p i a t , J . J . , 1980. P h o t o c h e m i c a l and p h o t o c a t a l y t i c p r o p e r t i e s o f a d s o r b e d I . Luminescence q u e n c h i n g o f t r i s o r g a n o m e t a l l i c compounds. ( 2 , 2 ' - b i p y r i d i n e ) r u t h e n i u m ( I 1 ) and c h r o m i u m ( I I 1 ) i n c l a y membranes. J . P h y s . Chem., 84: 2447-2457. 1968. I n o r g a n i c E l e c t r o n i c S p e c t r o s c o p y . E l s e v i e r , L e v e r , A.B.P., Amsterdam, 4 2 0 p p . Lim, J . C . , 1975. B r i g h t n e s s of c r y s t a l l i n e l a y e r e d s i l i c a t e minerals. U.S. P a t e n t N o 3 , 8 9 9 , 3 4 3 . L o e p p e r t J r . , R . H . , M o r t l a n d , M.M. and P i n n a v a i a , T . J . , 1 9 7 9 . S y n t h e s i s and p r o p e r t i e s of h e a t - s t a b l e expanded smectite and vermiculite. C l a y s and C l a y Min., 2 7 : 2 0 1 - 2 O e . L y t l e , F.E. and H e r c u l e s , D . M . , 1 9 6 9 . The l u m i n e s c e n c e o f t r i s ( 2 , 2 ' - b i p y r i d i n e ) ruthenium(1I)dichloride. J . Am. Chem. SOC., 9 1 : 253-257. Maes, A . and C r e m e r s , A . , 1 9 7 9 . S t a b i l i t y of m e t a l uncharcred l i g a n d complexes i n i o n e x c h a n g e r s . 4 . H y d r a t i o n e f f e c t s and s t a b i l i t y c h a n g e s o f c o p p e r - e t h y l e n e d i a m i n e complexes i n m o n t m o r i l l o n i t e . J . C . S . F a r a d a y I , 75: 513-524. Maes, A . , M a r i j n e n , P . and C r e m e r s , A . , 1977. The i o n e x c h a n g e a d s o r p t i o n of alkylammonium i o n s : a n a l t e r n a t i v e view. Clays and C l a y Min., 25: 309-310. Maes, A . , S c h o o n h e y d t , R.F. , C r e m e r s , P. and U y t t e r h o e v e n , J . B . , 1980. S p e c t r o s c o p y o f C u ( e n ) $ + on c l a y s u r f a c e s . S u r f a c e and J . P h y s . Chem., 84: 2795-2799. charge-density e f f e c t s . M a r t i n , L . T . , S p e r a t i , C.R. and Busch, D . H . , 1 9 7 7 . The s p e c t r o c h e m i c a l p r o p e r t i e s of t e t r a g o n a l complexes o f h i a h J . Am. Chem. spin n l c k e l ( I 1 ) containing macrocyclic ligands. S O C . , 99: 2968-2981. M c c l e l l a n d , J . F . and K n i s e l e y , R.N. , 1976. S c a t t e r e d l i g h t e f f e c t s i n p h o t o - a c o u s t i c s p e c t r o s c o p y . Appl. O p t i c s , 15: 2967-2968.

z',

188 Melamed, N . G . , 1 9 6 3 . O p t i c a l p r o p e r t i e s o f powders. I. O p t i c a l a b s o r p t i o n c o e f f i c i e n t s and t h e a b s o l u t e v a l u e o f t h e d i f f u s e reflectance. 11. P r o p e r t i e s of l u m i n e s c e n t powders. J. Appl. P h y s . , 34: 560-570. Monahan J r . , E.M. a n d N o l l e , P . W . , 1977. Q u a n t i t a t i v e s t u d y o f a p h o t o - a c o u s t i c s y s t e m f o r powdered samples. J . P p p l . P h y s . , 48: 3519-3523. M o r t l a n d , M.M., 1973. C l a y - a r e n e c o m p l e x e s and p r o c e s s of p r o d u c i n g same. U S P a t e n t N o 3 , 7 7 8 , 4 5 7 . P i n n a v a i a , T . J . and Mortland, M.M., 1971. I n t e r l a m e l l a r metal complexes on l a y e r s i l i c a t e s . I . C o p p e r ( I 1 ) - a r e n e complexes on m o n t m o r i l l o n i t e . J . P h y s . Chem., 7 5 : 3957-3962. Rao, C . N . R . , 1975. U l t r a - v i o l e t and v i s i b l e s p e c t r o s c o p y . Chemical A p p l i c a t i o n s . B u t t e r w o r t h s , London, 242 p p : S a e h r , D . , Le D r e d , R . a n d H o f f n e r D . , 1978. C o n t r i b u t i o n a l'gtude des interactions vermiculite-colorants cationiques. C l a y Min., 13: 411-426. S c h o o n h e y d t , R . A . , 1978. A n a l y s i s o f t h e e l e c t r o n p a r a m a g n e t i c r e s o n a n c e s p e c t r a o f bis(ethylehediamine)copper(II) on t h e s u r f a c e o f z e o l i t e s X a n d Y a n d o f a Camp B e r t e a u m o n t m o r i l l o n i t e . J. P h y s . Chem., 82: 497-498. S c h o o n h e y d t , R.A. , P e l g r i m s , J . , Heroes, Y . a n d U y t t e r h o e v e h J . B . , 1977. C h a r a c t e r i z a t i o n o f t r i s ( 2 , 2 I - b i p y r i d y l ) r u t h e n i u m ( I 1 ) on hectorite. C l a y Min. , 1 3 : 435-438. S c h o o n h e y d t , R.A. , V e l g h e , F. , B a e r t s R . and U y t t e r h o e v e n , J . B . , 1 9 7 9 . Complexes of diethylenetriamine(dien) a n d t e t r a e t h y l e n e p e n t a m i n e ( t e t r e n ) w i t h C u ( I 1 ) and N i ( I 1 ) on h e c t o r i t e . Clays and C l a y Min., 27: 269-278. S c h o o n h e y d t , R . A . , V e l g h e , F. a n U y t t e r h o e v e n J . B . , 1979. (x = 1, 2 , 3 ; e n = e t h y l e n e C h a r a c t e r i z a t i o n of [Ni(en),]'+ d i a m i n e ) on t h e s u r f a c e o f m o n t m o r i l l o n i t e s . I n o r g . Chem., 18: 1842-1847. S i f f e r t , B., 1978. P r B p a r a t i o n e t d t u d e s p e c t r o m g t r i q u e de c o m p l e x e s s i l i c a t e s p h y l l i t e u x c o l o r a n t s a z o T q u e s . C l a y M i n . , 1 3 : 147-166. S i l l e n , G.L. and M a r t e l l , A . E . , 1 9 6 4 . S t a b i l i t y C o n s t a n t s o f Metal I o n Complexes. The C h e m i c a l S o c i e t y , London, 754 p p . S m i t h , G . , 1977. L o w - t e m p e r a t u r e o p t i c a l s t u d i e s o f metal-metal charge-transfer transitions i n various minerals. C a n a d i a n Min., 15: 500-507. S t r o m e , D . H . and K l i e r , K . , 1980. The e f f e c t o f oxygen on photoluminescence and resonance e n e r g y t r a n s f e r i n c o p p e r ( 1 ) Y zeolite. A.C.S. Symp. S e r . N o 135: 155-176. S t u l , M.S. and Mortier, W.J., 1974. The h e t e r o g e n e i t y o f t h e charge density i n montmorillonites. C l a y s a n d C l a y Min., 2 2 : 391-396. T a r a s e v i c h , Y u . I . , 1975. The i n v e s t i g a t i o n o f c o o r d i n a t i o n compounds o n t h e s u r f a c e o f l a y e r s i l i c a t e s . P r o c . I n t . Conf. C o l l o i d and S u r f a c e S c i . , 1: 27-31. Tarasevich, Yu.1. and S i v a l o v , E . G . , 1 9 7 5 a . E l e c t r o n i c s p e c t r a of aquo-cations o f d i v a l e n t c o p p e r , s o r b e d by m o n t m o r i l l o n i t e . K o l l o i d . Z h . , 37: 814-817. T a r a s e v i c h , Yu.1. and S i v a l o v , E . G . , 1975b. The e l e c t r o n i c s p e c t r a o f aquo-cations o f d i v a l e n t cobalt, s o r b e d by m o n t m o r i l l o n i t e . T e o r . Eksp. Khim., 11: 4 1 0 - 1 5 . T a r a s e v i c h , Yu.1. and S i v a l o v , E . G . , 1 9 7 5 c . E l e c t r o n i c s p e c t r a o f a q u o - c a t i o n s o f b i v a l e n t n i c k e l a d s o r b e d on m o n t n o r i l l o n i t e . Ukr. Khim. Zh., 4 1 : 354-356 T a r a s e v i c h , Yu.1. S i v a l o v , E . G . , 1977. S p e c t r a l s t u d y of t h e e n v i r o n m e n t of i o n s i n k a o l i n i t e . Uks. Khim. Zh., 43: 367370.

3+

189 T a r a s e v i c h , Y u . 1 . and F e d o r o v a , V.P., 1 9 7 9 . E l e c t r o n i c s p e c t r a o f K o l l o i d Zh., a r y l c a r b i n o l s s o r b e d on a r g i l l a c e o u s m i n e r a l s . 4 1 : 812-814. T e l i c h k u n , V.P., T a r a s e v i c h , Yu.1. and Goncharuk, V . V . , 1 9 7 6 . E l e c t r o n s p e c t r a o f H a m m e t t d y e i n d i c a t o r s s o r b e d by m o n t m o r i l l o n i t e . T e o r . E k s . Khim., 1 3 : 131-134. T e l i c h k u n , V.P. and T a r a s e v i c h , Y u . I . , 1977. E l e c t r o n i c s p e c t s a o f a r y l c a r b i n o l s a d s o r b e d b y k a o l i n i t e s . Teor. E k s . Khim., 14: 401-404. Theng, B . K . G . , 1 9 7 4 . The C h e m i s t r y o f C l a y - O r g a n i c R e a c t i o n s . A d a m H i l g e r , London, 343 p p . T h i e l m a n n , V . J . and M c A t e e , J . L . , J r . , 1975. G a s c h r o m a t o g r a p h i c b e h a v i o r o f metal-tris(ethy1enediamine)complex c a t i o n - e x c h a n g e d montmorillonites. J . C h r o m a t o g r a p h y , 105: 115-123. Thompson, F.D., 1977. C o l o r d e v e l o p i n g c o a t i n g c o m p o s i t i o n s c o n t a i n i n g r e a c t i v e p i g m e n t s p a r t i c u l a r l y f o r m a n i f o l d copy UP P a t e n t 4 , 0 2 2 , 7 3 5 . paper. T r a y n o r , M.F., M o r t l a n d , M.M. a n d P i n n a v a i a , T . J . , 1978. Ion e x c h a n g e and i n t e r s a l a t i o n r e a c t i o n s of h e c t o r i t e w i t h trisb i p y r i d y l metal c o m p l e x e s . C l a y s a n d C l a y Y i n . , 26: 318-326. Van Dame, H . , C r e s p i n , M . , Cruz4 M . I . and F r i i a t , J . J . , 1977. A d s o r p t i o n o f s a f r a n i n e b y N a , N i 2 + a n d Fey+ m o n t m o r i l l o n i t e s . C l a y s a n d C l a y Min., 25: 19-25. Van Dame, H . , C r e s p i n , M . , O b r e c h t , F . , C r u z , M . I . and F r i p i a t , J . J . , 1978. F c i d - b a s e a n d c o m p l e x a t i o n b e h a v i o r of p o r p h y r i n s on t h e i n t r a c r y s t a l s u r f a c e o f s w e l l i n g c l a y s : meso-tetrap h e n y l p o r p h y s i n a n d m e s o - t e t r a ( 4 - p y r i d y l ) p o r p h y r i n o n rnontmorJ . C o l l o i d I n t e r f a c e S c i . , 66: 43-54. illonites. Van d e P o e l , D . , 1975. F d s o r p t i o n d e C O I P ~ O S ~ Sa r o m a t i q u e s s u r l a montmorillonite saturge en Cu(I1). Universitd Catholique d e L o u v a i n , Ph.D. Van d e P o e l , D . , C l o o s , P . , H e l s e n , J . a n d J a n n i n i , E . , 1973. Comportement p a r t i c u l i e r d u b e n z e n e a d s o r b & s u r l a m o n t m o r i l l o n i t e cuivrique. B u l l . Groupe F r a n q a i s F r g i l e s , 25: 115-126. V e l g h e , F. , S c h o o n h e y d t , R.A. a n d U y t t e r h o e v e n , J . B . , 1977. The c o o r d i n a t i o n of h y d r a t e d C u ( I 1 ) - and N i (11)- i o n s on montmorillonite surface. C l a y s and C l a y Min., 25: 375-380. V e l g h e , F. , S c h o o n h e y d t , R.A. , U y t t e r h o e v e n , J . B . , P e i g n e u r , P. a n d 1977. S p e c t r o s c o p i c c h a r a c t e r i z a t i o n and t h e r m a l Lunsford, J . H . , s t a b i l i t y of c o p p e r ( I 1 ) e t h y l e n e d i a m i n e complexes on s o l i d surfaces. 2. Montmorillonite. J . P h y s . Chem., 81: 1187-1194. Yariv, S . , Lahav, N . a n d L a c h e r , M . , 1 9 7 6 . On t h e mechanism of s t a i n i n g m o n t m o r i l l o n i t e by b e n z i d i n e . C l a y s and C l a y M i n . , 2 4 : 51-52.

191 Chapter 8

APPLICATION OF FAR INFRARED SPECTROSCOPY TO THE STUDY OF CLAY MINERALS AND ZEOLITES

Jose J . FRIPIAT C.N.R.S. - Centre de Recherche s u r l e s S o l i d e s a O r g a n i s a t i o n C r i s t a l l i n e I m p a r f a i t e , l b , Rue de l a F 6 r o l l e r i e , 45045 Orleans Cedex, France.

8.1

INTRODUCTION I n f r a r e d spectroscopy i n t h e range o f 250-4000 wave number r e p r e s e n t s an e s t a -

b l i s h e d t o o l w i d e l y used by c l a y s c i e n t i s t s f o r t h e s t u d y o f t h e m i n e r a l framework and o f molecules adsorbed on i t s s u r f a c e . I n 1974, V.C. sponsorship o f t h e B r i t i s h

Farmer e d i t e d , under t h e

M i n e r a l o g i c a l Soci t y , an e x h a u s t i v e r e v i e w on these

subjects. However, as i t w i l l be shown l a t e r below 250 cm-’

f o r v a r ous reasons, t h e s p e c t r a l domain

has n o t r e c e i v e d much a t t e n t i o n

The aim o f t h i s r e v i e w i s t o s t i -

m u l a t e t h e i n t e r e s t o f c l a y s c i e n t i s t s f o r f a r i n f r a r e d spectroscopy. As compared t o t h e o t h e r t o p i c s t r e a t e d i n t h i s book where numerous r e c e n t papers a r e c r i t i c a l l y reviewed t h e p r e s e n t c h a p t e r aims : ( i ) t o d e s c r i b e b r i e f l y t h e p r i n c i p l e o f t h e i n t e r f e r o m e t e r used t o r e c o r d s p e c t r a i n t h e range o f 10-300 cml; ( i i ) t o summarize t h e t h e o r e t i c a l background which p e r m i t s t o o b t a i n i n f o r m a t i o n f r o m these s p e c t r a and f i n a l l y ( i i i ) t o r e v i e w papers d e a l i n g w i t h c l a y m i n e r a l s and z e o l i t e s t h a t c o n t a i n e x p e r i m e n t a l r e s u l t s which i l l u s t r a t e t h e u s e f u l n e s s o f s p e c t r a l s t u d i e s i n the f a r i n f r a r e d region. The r a p i d p r o g r e s s o f f a r i n f r a r e d s p e c t r o s c o p y found i t s o r i g i n i n t h e a v a i l a b i l i t y o f F o u r i e r t r a n s f o r m s p e c t r o m e t e r s which became p o s s i b l e because o f t h e r e d u c t i o n o f t h e c o s t o f f a s t e l e c t r o n i c minicomputers w i t h l a r g e s t o r a g e c a p a c i t y . I t i s o n l y about t e n y e a r s t h a t t h i s new i n s t r u m e n t a t i o n has been c o m m e r c i a l l y

a v a i l a b l e and t h a t t h e s o l i d s t a t e p h y s i c i s t s and chemists have t a k e n advantage o f t h i s new r e s e a r c h t o o l

.

A r e f e r e n c e book on t h e F o u r i e r t r a n s f o r m s p e c t r o s c o p y has been p u b l i s h e d by B e l l (1972). As i t w i l l be shown l a t e r , t h e F o u r i e r t r a n s f o r m s p e c t r o s c o p y has s e v e r a l i n s t r u m e n t a l advantages o v e r t h e c l a s s i c a l I R s p e c t r o s c o p y which uses d i s p e r s i v e ( p r i s m s o r g r a t i n g s ) i n s t r u m e n t s . F o r c l a y s s c i e n t i s t s , t h e main s c i e n t i f i c i n t e r e s t i s i n t h e s t u d y o f v i b r a t i o n s o f exchangeable c a t i o n s i n t h e i n t e r l a y e r space o r i n t h e z e o l i t e cages and o f some l a t t i c e v i b r a t i o n s o c c u r i n g below 300 cm-l. I f t h e t i m e used f o r r e c o r d i n g s p e c t r a becomes c r u c i a l t h e n t h e use o f F o u r i e r

192 t r a n s f o r m i n t e r f e r o m e t e r a l s o o f f e r s t h e advantage o f b e i n g a v e r y f a s t t e c h n i q u e

if t h e r e g i o n between, say,400 t o 1500 cm-l, shows v i b r a t i o n a l bands which p e r m i t i d e n t i f i c a t i o n o f the mineral.

8.2

FOURIER TRANSFORM SCANNING INTERFEROMETRY The e q u i v a l e n c e o f s p e c t r a l i n f o r m a t i o n i n t h e t i m e domain and i n t h e f r e q u e n c y

domain r e p r e s e n t s t h e b a s i c p r i n c i p l e o f t h e F o u r i e r t r a n s f o r m I R s p e c t r o s c o p y . The i n t e r f e r o m e t e r encodes t h e s p e c t r a l i n f o r m a t i o n i n t h e t i m e domain and by F o u r i e r t r a n s f o r m a t i o n o f t h e i n t e r f e r o g r a m , one o b t a i n s t h e c o n v e n t i o n a l I R spect r u m i n t h e frequency domain.

Fig.8.1.

Schematic r e p r e s e n t a t i o n o f an i n t e r f e r o m e t e r w o r k i n g b y t r a n s m i s s i o n

The o p t i c a l p a r t o f a modern i n s t r u m e n t i s shown i n F i g . 8.1. The i n f r a r e d beam i s focused on t h e p l a n e o f a semi t r a n s p a r e n t m i r r o r c a l l e d a beam s p l i t t e r ( 6 s ) .

A f r a c t i o n o f t h e beam i s r e f l e c t e d on m i r r o r My, whereas a f r a c t i o n i s r e f l e c t e d by m i r r o r M2. From m i r r o r s M1 and Me t h e beams a r e r e f l e c t e d by a d o u b l e m i r r o r MM o s c i l l a t i n g on an a i r cushion. T h i s m o t i o n generates a d i f f e r e n c e i n t h e o p t i c a l p a t h o f t h e two beams w h i c h i n t e r f e r e s on t h e beam s p l i t t e r . About 50% o f t h e energy source reaches m i r r o r M3. L e t 6 be t h e d i f f e r e n c e i n t h e o p t i c a l pathway o f t h e two beams f o r a g i v e n p o s i t i o n of t h e m i r r o r and w be t h e f r e q u e n c y o f t h e photon. The s p e c t r a l s i g n a l a f t e r i n t e r f e r e n c e i s given by :

193

S ( W , ~ ) = A2(w)

[l

+

cos ( 2

IT w

6)l

(1)

A b e i n g t h e a m p l i t u d e o f two c o h e r e n t waves w i t h t h e phase d i f f e r e n c e 2

IT w 6.

Since MM o s c i l l a t e s a t c o n s t a n t speed v, 6 = 2 v t and t h e o p t i c a l i n t e n s i t y r e a c h i n g m i r r o r M3 i s

I

I(t) =

W=m

L)) =m

A2(.)

dw

+

cos(4

A2(.)

IT w

v t ) dw

0

W=O

The i n t e r f e r o g r a m c o n t a i n i n g a l l s p e c t r a l components i s w =m

F(t) =

J

A2(w)

cos(4

II w

v t ) dw

(3)

0

and t h e F o u r i e r t r a n s f o r m e d o f F ( t ) i s t h e s p e c t r a l d i s t r i b u t i o n o f t h e i n t e n s i t y , S(W)

L/2v F(v) exp(- i 4

IT w

v t) dt

(4)

-L/2v where L i s t h e maximum d i s p l a c e m e n t o f t h e m i r r o r . L/2v i s t h e scanning t i m e . Thus F ( v ) r e p r e s e n t s t h e I R spectrum i n t h e t i m e domain whereas S ( W ) corresponds t o t h e I R spectrum i n t h e f r e q u e n c y domain. I n summary, because o f t h e sequence o f c o n s t r u c t i v e and d e s t r u c t i v e i n t e r f e rences, any o p t i c a l energy a t a g i v e n wavelength ( p h o t o n ) which e n t e r s t h e i n t e r f e r o m e t e r i s t r a n s f o r m e d i n t o a s i n u s o i d o f g i v e n p e r i o d and a m p l i t u d e . The sample i s t h u s i r r a d i a t e d b y a f l u x o f s i n u s o i d a l waves w h i c h c o n t a i n components w i t h f r e q u e n c i e s c o r r e s p o n d i n g t o c h a r a c t e r i s t i c v i b r a t i o n f r e q u e n c i e s o f t h e sample. The i n t e r f e r o g r a m r e a c h i n g t h e d e t e c t o r ( a p y r o e l e c t r i c d e v i c e ) c o n t a i n s t h e s p e c t r a l i n f o r m a t i o n on t h e v i b r a t i o n a l modes o f t h e sample encoded i n t h e t i m e domain. T h i s i n f o r m a t i o n i s r e s t i t u t e d as t h e c o n v e n t i o n a l f r e q u e n c y domain spectrum a f t e r F o u r i e r transformation. The i n f r a r e d sources a r e e i t h e r a mercury lamp e m i t t i n g between 10 and 300 cm-’ o r a G l o b a r f o r t h e 200-4000 cm-’

domain.

The n a t u r e o f t h e beam s p l i t t e r depends upon t h e i n v e s t i g a t e d s p e c t r a l domain. I t c o n s i s t s o f a germanium f i l m d e p o s i t e d o n t o a KBr t h i n s l a b between 400 and

4000 cm-1, a m e t a l l i c g r a t i n g between 50-2000 cm-l and f i l m s o f m y l a r between

10-700 cm-’.

The m o t i o n o f m i r r o r MM i s m o n i t o r e d by an a u x i l i a r y i n t e r f e r o m e t e r

194 using

a

He-Ne l a s e r whereas t h e change o f t h e sources, o f t h e a p e r t u r e s , o f

t h e beam s p l i t t e r s and o f t h e o p t i c a l f i l t e r s d r e c o n t r o l l e d by t h e computer. Thus a complete r e c o r d o f t h e s p e c t r a can be d i g i t a l l y c o n t r o l l e d and m o n i t o r e d by t h e computer. A F o u r i e r t r a n s f o r m s p e c t r o m e t e r has two c1o;ely

r e 1 a t e d advantages w i t h r e s p e c t

t o a c l a s s i c a l d i s p e r s i v e i n s t r u m e n t . The f i r s t advantage, t h e s o - c a l l e d m u l t i p l e x o r F e l l g e t advantage, i s r e l a t e d t o t h e way one r e c o r d s

an

I R spectrum i n a FT

i n t e r f e r o m e t e r . I n a c o n v e n t i o n a l d i s p e r s i v e i n s t r u m e n t one scans t h r o u g h t h e s p e c t r a l range o f i n t e r e s t and observes each s p e c t r a l r e g i o n f o r a v e r y l i m i t e d t i m e depending on t h e t o t a l t i m e o f r e c o r d i n g t h e spectrum. By c o n t r a s t , i n a FT i n t e r f e r o m e t e r one observes t h e whole s p e c t r a l range a l l t h e t i m e . O b v i o u s l y , one can r e p e a t t h e number o f s p e c t r a l scans, o f say 1 sec, and t h u s accumulate a l a r g e number o f s p e c t r a . T h i s r e s u l t s i n a m a j o r improvement i n t h e s i g n a l t o t o t h e ope-

n o i s e r a t i o when one compares t h e o p e r a t i o n o f a FT i n t e r f e r o m e t e r r a t i o n o f a conventional d i s p e r s i v e I R spectrometer.

The second advantage, t h e s o - c a l l e d t h r o u g h p u t o r J a c q u i n o t advantage i s r e l a t e d t o t h e amount o f energy a v a i l a b l e a t t h e d e t e c t o r o f t h e i n t e r f e r o m e t e r . The amount o f energy i s much h i g h e r a t comparable r e s o l u t i o n . I n a d i s p e r s i v e i n s t r u m e n t , i f x i s t h e s p e c t r a l domain d i s p e r s e d by t h e g r a t i n g and dx i s t h e dx output s l i t width, i s t h e amount o f energy a c t u a l l y a v a i l a b l e . I n t h e h i g h wavelength r e g i o n , t h e l o w amount o f energy p e r p h o t o n would r e q u i r e s l i t w i d t h s i n c o m p a t i b l e w i t h a r e a s o n a b l e r e s o l u t i o n whereas i n an i n t e r f e r o m e t e r as sketched i n F i g . 8.1, a b o u t 50% o f t h e source energy i s used w i t h o u t t h e o r e t i c a l 1 i m i t a t i o n o f t h e r e s o l u t i on. The l i m i t a t i o n o f t h e F o u r i e r t r a n s f o r m i n s t r u m e n t l a y s i n t h e h i g h frequency r e g i o n ( s a y 3000-4000 cm-1) because o f t h e v a s t amount o f s p e c t r a l i n f o r m a t i o n cont a i n e d i n t h e i n t e r f e r o g r a m . I n t h i s case a d i s p e r s i v e i n s t r u m e n t becomes more c o m p e t i t i v e , s p e c i a l l y i f a c c u r a t e band shapes a r e needed. As usual t h e two t e c h n i q u e s a r e t h u s complementary,but f o r i n v e s t i g a t i o n of v i b r a t i o n a l s p e c t r a between 300 and 10 cm-1, t h e F o u r i e r t r a n s f o r m i n t e r f e r o m e t e r i s unique. 8.3

SAMPLE PREPARATION

P.s i t c o u l d be expected f r o m u s u a l p r a c t i c e i n spectroscopy, t h e s i m p l e s t way t o use a F o u r i e r t r a n s f o r m s p e c t r o m e t e r i s by t r a n s m i s s i o n t h r o u g h t h e sample.

A v e r y i n t e r e s t i n g r e v i e w o f t h e use o f r e f l e c t a n c e i n t e r f e r o m e t r y w i l l be p u b l i s h e d by F. G e r v a i s ( 1 9 8 1 ) . W i t h c l a y m i n e r a l s , a u t o - c o h e r e n t f i l m s o b t a i n e d by s e d i m e n t a t i o n f r o m suspens i o n s , a s t h o s e m o s t l y used i n c l a s s i c a l I R s p e c t r o s c o p y s t u d i e s , a r e

perfectly

s u i t a b l e . Mica f l a k e s produced by c l e a v a g e can a l s o be used. Wafers made f r o m

195 f i n e l y ground powders can be e q u a l l y adequate. I t i s d i f f i c u l t t o e s t a b l i s h a l i m i t t o t h e absorbance c h a r a c t e r i s t i c s o f samp l e s which p e r m i t t o use a t r a n s m i s s i o n i n t e r f e r o m e t e r . T o t a l absorbance n o t exceed i n g lo2 cm-1 can be r e g a r d e d as an a p p r o x i m a t e e s t i m a t e . I f t h i n a u t o - c o h e r e n t f i l m s o r w a f e r s cannot be o b t a i n e d , p e l l e t s made by d i s -

p e r s i n g t h e f i n e l y ground s o l i d i n t o a p o l y e t h y l e n e powder and by c o l d p r e s s i n g a r e w e l l adapted t o r e c o r d s p e c t r a below 300 cm-l. The s e d i m e n t a t i o n o f t h e s o l i d p a r t i c l e s on p o l y e t h y l e n e f i l m u s u a l l y produces i n t e r f e r e n c e f r i n g e s which may o v e r l a p w i t h t h e a b s o r p t i o n bands.

I n summary,the v a r i o u s ways t o p r e p a r e samples f o r f a r i n f r a r e d s t u d i e s a r e n o t d i f f e r e n t f r o m t h o s e used i n t h e h i g h e r wavelength r e g i o n s , e x c e p t t h a t p e l l e t s a r e made f r o m p o l y e t h y l e n e . W i t h a u t o - c o h e r e n t f i l m s w i t h a p r e f e r r e d o r i e n t a t i o n o f m i c r o c r y s t a l s around one a x i s ( t h e C a x i s i n l a y e r l a t t i c e m i n e r a l s f o r example), d i c h r o i c e f f e c t s can be observed j u s t as i t i s sometimes t h e case i n t h e OH s t r e t c h i n g r e g i o n . As t h e sample compartment i n a F o u r i e r t r a n s f o r m s p e c t r o m e t e r i s c o n t i n u o u s l y purged by a stream o f d r y n i t r o g e n , and because o f t h e l o w p h o t o n i c energy,there a r e severe l i m i t a t i o n s f o r t h e use o f c e l l s and sample h o l d e r s . T h e r e f o r e t h e c o n t r o l o f t h e w a t e r c o n t e n t o f t h e f i l m cannot be achieved v e r y a c c u r a t e l y . It i s o n l y by i n c r e a s i n q t h e t e m p e r a t u r e o f t h e f u r n a c e ( F i g . 8.1) and by r e -

p r o d u c i n g s i m i l a r c o n d i t i o n s o u t s i d e t h e s p e c t r o m e t e r t h a t t h e sample h y d r a t i o n can be e v a l u a t e d . The use o f c e l l s f i t t e d w i t h windows i n w h i c h t h e sample can be p r e t r e a t e d and conditioned,reduces

so much t h e energy a v a i l a b l e , t h a t no d e v i c e s which

c o u l d meet t h e d e s i r a b l e r e q u i r e m e n t s have so f a r been d e s c r i b e d .

8.4

THEORETICAL ASPECTS. I n a system c o n t a i n i n g N atoms,the number o f i n t e r n a l normal modes o f v i b r a -

tions i s (3 N

-

6):

The group t h e o r y f o r a known symmetry o f t h e m a t e r i a l p r e d i c t s

t h e number o f o p t i c a l ( e i t h e r Raman o r I n f r a r e d ) a c t i v e modes. F o r m i c r o c r y s t a l s w i t h l a r g e u n i t c e l l s such as found i n c l a y m i n e r a l s and even i n z e o l i t e s , t h e d e s c r i p t i o n and,a eht , ihoi @ .Lr

c a l c u l a t i o n o f t h e frequen-

c i e s o f t h e s e modes i s p r a c t i c a l l y u n t r a c t a b l e and anyway f a r f r o m t h e scope o f t h i s paper. The p h y s i c a l r e q u i r e m e n t f o r t h e s e modes t o be i n f r a r e d a c t i v e i s t o produce a change o f t h e d i p o l e moment. The u s u a l p r a c t i c e f o r i n t e r p r e t i n g t h e i n f r a r e d s p e c t r a o f complex m a t e r i a l s c o n s i s t s o f t h e i s o l a t i o n o f m o l e c u l a r b u i l d i n g u n i t s t o which c h a r a c t e r i s t i c bands can be assigned. F o r example,onecan a s s i g n t h e i n f r a r e d a c t i v e v3 and v modes 4 o f t e t r a h e d r a w i t h T d symmetry o r t h e s t r e t c h i n g and bending v i b r a t i o n s o f t h e OH groups.

The i n t e r a c t i o n o f i n f r a r e d r a d i a t i o n w i t h c r y s t a l s and t h e r e l a t i o n s h i p s b e t -

196 ween symmetry and c r y s t a l v i b r a t i o n s have been c l e a r l y summarized i n Farmer's book (1974) by A. Hadni (pp. 27-49), by V.C.

Farmer and A.N.

Lazarev (pp. 51-67).

I n t h e same monograph Lazarev a l s o wrote an i n t r o d u c t o r y c h a p t e r t o t h e dynamics o f c r y s t a l l a t t i c e s , (pp. 69-85). The r e a d e r , i n t e r e s t e d i n t h e t h e o r e t i c a l aspects, would f i n d i n these references t h e i n f o r m a t i o n r p l e v a n t t o t h e issues discussed here. I n p a r t i c u l a r , Lazarev l i s t e d t h e p o s s i b l e e r r o r s o f t h e molecular a p p r o x i mation i n normal c o o r d i n a t e c a l c u l a t i o n s and t h e f o r c e constants e s t i m a t i o n s o f complex anions i n c r y s t a l s , as f o l l o w s : " ( i ) t h e use o f an i d e a l i z e d c o n f i g u r a t i o n o f t h e complex anion, ( i i ) t h e n e g l e c t o f t h e mechanical c o u p l i n g between low frequency i n t e r n a l deformations o f t h e complex anion and l a t t i c e v i b r a t i o n s and ( i i i ) t h e n e g l e c t o f d i r e c t mechanical c o u p l i n g o f t h e s t r e t c h i n g v i b r a t i o n s o f t h e complex anion w i t h l a t t i c e v i b r a t i o n s " . Items ( i i ) and ( i i i ) a r e o f course o f importance f o r a s s i g n i n g s p e c t r a f e a t u res i n t h e f a r i n f r a r e d domain, as i l l u s t r a t e d by t h e same a u t h o r who compared t h e c a l c u l a t e d wave numbers o f o p t i c a l v i b r a t i o n s o f Sc2 S i 2 O7 ( t h o r t w e i t i t e ) and those o f an i d e a l i z e d c r y s t a l composed o f Sc3+ and r i g i d Si207 groupings.

I n view o f t h e d i f f i c u l t y o f t h e t a s k o f a s s i g n i n g a p r i o r i t h e bands i n t h e f a r i n f r a r e d range,the method proposed by Lazarev (1972) and c a l l e d " t h e method o f quasi i s o t o p i c s u b s t i t u t i o n " i s probably t h e b e s t adapted t o t h e s i t u a t i o n we a r e d e a l i n g w i t h here. B a s i c a l l y t h i s method i s based on t h e use o f t h e s p e c t r a o f s o l i d s o l u t i o n s between compounds Mn Xm Op and t h e isomorphous compound Mn X ' n Op, X and X I , being elements o f t h e same group o f t h e p e r i o d i c system. I n s u b s t i t u t i n g X by X ' t h e frequency s h i f t s a r e governed p r i m a r i l y by t h e change o f masses. For instance, i n c l a y m i n e r a l s o r i n z e o l i t e s , t h e s u b s t i t u t i o n o f an exchangeable c a t i o n by another o f t h e same valency should produce t h i s type o f s p e c t r a l modifications without a f f e c t i n g strongly the l a t t i c e vibrations. This i s e x a c t l y t h e type o f i n f o r m a t i o n c l a y s c i e n t i s t s can l o o k f o r . I f a d e t a i l e d v i b r a t i o n a l a n a l y s i s was p o s s i b l e , o t h e r p h y s i c a l c h a r a c t e r i s t i c s c o u l d e v e n t u a l l y be computed. For example, t h e depth o f t h e p o t e n t i a l w e l l s i n which c a t i o n s a r e l o c a t e d c o u l d be evaluated and t h e n a t u r e o f t h e b i n d i n g f o r c e b e t ween t h e exchangeable c a t i o n s and t h e s u r f a c e c o u l d be e l u c i d a t e d . I t i s a l s o p o s s i b l e t h a t isomorphic s u b s t i t u t i o n s i n t h e t e t r a h e d r a l o r octa-

hedral u n i t s produce some c h a r a c t e r i s t i c bands i n t h e f a r i n f r a r e d domain. I f we consider, i n t h e l i m i t , isomorphic s u b s t i t u e n t s as i m p u r i t i e s , i n i o n i c c r y s t a l s , i m p u r i t y - i n d u c e d a b s o r p t i o n bandscould be more d i f f i c u l t t o d i s c o v e r because o f considerable a b s o r p t i o n a t low frequencies a t room temperature. The c o o l i n g o f t h e sample t o h e l i u m temperature should help,but i n t r a n s m i s s i o n spectroscopy t h i s i s n o t easy because o f i n s t r u m e n t a l l i m i t a t i o n s e x p l a i n e d above.

197 8.5

APPLICATION OF FAR INFRARED SPECTROSCOPY TO CLAY MINERALS I s h i i and coworkers (1969)

have been among t h e f i r s t researchers t o study

a b s o r p t i o n s p e c t r a o f n a t u r a l and s y n t h e t i c micas from 1300 t o 60 cm-l. The normal c o o r d i n a t e t r e a t m e n t was a p p l i e d t o some i d e a l i z e d s t r u c t u r e o f micas and Si205 l a y e r s . On t h e b a s i s o f frequency c a l c u l a t i o n s t h e observed a b s o r p t i o n bands were assigned t o Si205 and octahedral l a y e r s . The p o t e n t i a l f u n c t i o n U used by these authors i s expressed as

u

=

u.1 n t e r l ayer

where ' i n t r a l a y e r

+

(5)

'i n t r a l ayer concerns t h e A12(Si205)2(0H)2 o r Mg3(Si205)2(OH)2 l a y e r s

i s t h e i n t e r a c t i o n p o t e n t i a l between t h e p o s i t i v e K+ c a t i o n and 'interlayer and t h e neighbouring t w e l v e oxygen atoms. I n t h e r e g i o n o f i n t e r e s t here t h e Au and Bu i n f r a r e d a c t i v e modes should appear a t t h e f o l l o w i n g frequencies :

1 M dioctahedral

Au

211 cm-'

Au

178 cm-l 1 M t r i o c t a h e d r a l

Bu

190 cm-l 1 M d i o c t a h e d r a l

Bu

159 cm-l 1 M t r i o c t a h e d r a l

I f these c a l c u l a t e d values a r e compared t o t h e observed frequencies shown i n Table 8.1,

a reasonably good agreement i s found f o r p h l o g o p i t e whereas f o r t h e

o t h e r m i n e r a l s t h e frequency s h i f t s and t h e number o f bands do n o t p e r m i t r e l i a b l e assignments. Probably t h e symmetry o f t h e s i l i c a network i s t o o i d e a l i z e d s i n c e i t i s w e l l e s t a b l i s h e d t h a t t h e "hexagonal c a v i t y " i s i n f a c t d i t r i g o n a l . The l o w e s t frequency o f t h e v i b r a t i o n i n v o l v i n g an in-phase motion o f t h e octahedral A13+ and t h e a d j a c e n t oxygen sheet which accounts f o r t h e absorption i n t h e 200 cm-1 r e g i o n should a l s o be s t r o n g l y i n f l u e n c e d by d i s t o r t i o n o f t h e octahedral s h e l l . F - i n a l l y , t h e most i n t e r e s t i n g i n f o r m a t i o n i s t h a t g i v e n by t h e l o w e s t frequency band. According t o t h e model proposed by I s h i i e t a l . ,

the

modes o f v i b r a t i o n o f t h e potassium atom a r e those shown i n F i g . 8.2. I f t h e mica f l a k e s a r e o r i e n t e d p e r p e n d i c u l a r t o t h e beam, o n l y t h e v i b r a t i o n a l modes p a r a l l e l t o t h e a b p l a n e should appear. Indeed t h e p r e d i c t e d frequencies a r e in

t h e experimental range o f t h e l o w e s t frequency band, t h e i n t e r a c t i o n force

constant b e i n g somewhat s m a l l e r f o r p h l o g o p i t e than f o r muscovite. According t o Farmer (Farmer, 1974, p.358),

t h e corresponding o u t - o f - p l a n e v i b r a t i o n s i n these

micas l i e a t 144 cm-1 and 154 cm-l, r e s p e c t i v e l y . These modes should grow i n i n t e n s i t y by t i l t i n g t h e f l a k e around t h e C a x i s . F a r i n f r a r e d spectroscopy has been proposed as a "simple" method f o r d i f f e r e n t i a t i n g v a r i o u s micas and c l a y m i n e r a l s by S. Larson e t a l . (1972). I t i s i n t e r e s t i n g t o p o i n t o u t t h a t these authors have used c l a y s molten i n petroleum j e l l y supported on a p o l y e t h y l e n e f i l m . Again t h e s p e c t r a f o r micas were o b t a i n e d w i t h

198 t h e beam p e r p e n d i c u l a r t o t h e t h i n f l a k e s . TABLE 8.1 Frequencies (cm-l) observed by I s h i i e t a l . (1969) f o r v a r i o u s l a y e r l a t t i c e minerals

.

Trioctahedral P o l y l it h i o n i t e (synthetic)

200(sh),

170 ( m ) ,

Lepidol it e (Czechoslovakia) L i fluorphlogopite (synthetic) Phlogopi t e (Japan) Talc

100 (m)

120 ( s h ) 120 (m)

95

(5)

160 ( s ) ,

150 (m)

105 ( s )

170 (sh),

155 ( s )

80 ( s )

260 ( n ) ,

230 (sh),

180 (w),

150 (m)

263*(s)

192*(m)

168*(m)

143*(m)

205 ( s )

160

215 (m),

-

Dioctahedral Muscovite Pyrophi 11it e

*

108 ( s ) 110 (5)

(5)

Frequencies r e p o r t e d by V.C. Farmer (Farmer, 1974, p. 351)

00 108cm-l

1

95cm-

00 0 fi

Fig. 8.2. Modes o f v i b r a t i o n o f t h e potassium atom between t h e s i l i c a t : : o f muscovite. C a l c u l a t e d frequencies a r e shown ( I s h i i e t a1 . ) .

layers

199 H a l l o y s i t e , k a o l i n i t e and d i c k i t e show p a i r s o f bands o f d i f f e r e n t shapes i n t h e 250 cm-l and 330 cm-I r e g i o n b u t i n absence o f an assignment and because o f considerable s c a t t e r i n g l o s s , i t i s n o t c e r t a i n t h a t these f e a t u r e s a r e a c t u a l l y representative o f characteristic vibrations. For muscovite, Larson e t a l . d e t e c t e d two i n t e n s e bands a t 105 and 161 cm-’, r e s p e c t i v e l y . The f i r s t one corresponds o b v i o u s l y t o t h e K+ i n t e r l a y e r v i b r a t i o n whereas t h e second one i s a t t h e c e n t e r o f t h e broad band w i t h 3 peaks observed by Farmer (see Table 1). I n l e p i d o l i t e t h e K+ v i b r a t i o n band s h i f t s t o 100 cm-l and i n b i o t i t e i t i s observed a t . 8 3 cm-’.

For t h e l a t t e r a broad band i s a l s o observed a t 142 cm-l.

I n t h e s t u d i e s summarized so f a r , t h e i n t e r l a y e r c a t i o n was always potassium. The p o s s i b i l i t y o f g e t t i n g auto-coherent f i l m s o f c a t i o n exchanged smectites off e r s an i n t e r e s t i n g challenge f o r a s s i g n i n g bands t o t h e i n t e r l a y e r c a t i o n v i b r a t i o n s . A f i r s t attempt i n t h a t d i r e c t i o n was made i n o u r l a b o r a t o r y by B. Chourabi (1980). Ammonium, Potassium, Cesium and Baryum Black Jack Mine B e i d e l l i t e and Camp Berteau M o n t m o r i l l o n i t e gave i d e n t i c a l bands a t t h e f o l l o w i n g frequencies. TABLE 8.2 I n t e r l a y e r c a t i o n frequencies (cm-l) observed f o r t h e Black Jack Mine B e i d e l l i t e and Camp Berteau M o n t m o r i l l o n i t e .

N H ~ 150

K+ 100

and

cs+

Ba2’

55

80

85

These bands were-observed w i t h t h e f i l m p e r p e n d i c u l a r t o t h e beam and they should r e p r e s e n t t h e r e f o r e t h e i n - p l a n e v i b r a t i o n o f t h e c a t i o n i f t h e d i s o r i e n t a t i o n degree o f t h e m i c r o c r y s t a l s i n t h e f i l m i s n o t t o o l a r g e . When t h e frequencies o f Table 8.2 were p l o t t e d a g a i n s t c a t i o n valency and M i t s

where Z i s t h e

molecular mass,an almost p e r f e c t s t r a i g h t l i n e passing

through t h e o r i g i n was o b t a i n e d i f t h e 55 cm-l frequency band observed f o r t h e Cs samples was o n l y considered ( F i g . 8.3). The t h e o r e t i c a l s i g n i f i c a n c e o f t h i s s t r a i g h t l i n e w i l l be discussed l a t e r . I n a d d i t i o n t o t h e c a t i o n v i b r a t i o n band, another band was observed f o r b o t h m i n e r a l s a t about 200 cm-l. T h i s band i s s i m i l a r t o t h a t observed by I s h i i f o r p y r o p h y l l i t e and thus t h e assignment proposed by I s h i i can be t h e same f o r d i o c t a h e d r a l smectites. As shown f o r t h e a l c a l i exchanged Black Jack Mine samples i n Fig. 8.4,

an a d d i t i o n a l band appears a t 255

cm-l. T h i s band i s absent i n t h e Camp Berteau M o n t m o r i l l o n i t e b u t i t i s observed very d i s t i n c t l y i n Muscovite. Since t h e l a t t i c e charge i n B e i d e l l i t e o r i g i n a t e s

F i g . 8.3. I n - p l a n e i n t e r l a y e r c a t i o n f r e q u e n c y observed f o r d i o c t a h e d r a l smect i t e s B l a c k Jack Mine b e i d e l l i t e and Camp B e r t e a u m o n t m o r i l l o n i t e E. (Chourabi, 1980). f r o m S i 4 + , A13+ s u b s t i t u t i o n s i n t h e t e t r a h e d r a l l a y e r (as i n m u s c o v i t e ) , t h i s mode c o u l d be r e l a t e d t o some p e r t u r b a t i o n i n t h e m o t i o n o f o c t a h e d r a l A13+ w i t h r e s p e c t t o t h e t e t r a h e d r a l oxygens i n f l u e n c e d by t h e s e s u b s t i t u t i o n s . A t t h e 6 t h I n t e r n a t i o n a l C l a y Conference i n O x f o r d , C. Roth (1978) r e p o r t e d f o r a CS'

s m e c t i t e a d o u b l e t s i m i l a r t o t h a t shown i n F i g . 8.4. These p r e l i m i n a r y o b s e r v a t i o n s prompted A l c o v e r , Gatineau and G e r v a i s (1981)

t o pursue a more e x t e n s i v e s t u d y o f c a t i o n exchanged v e r m i c u l i t e s . L l a n o vermic u l i t e was chosen f o r t h a t work because o f t h e p o s s i b i l i t y o f g e t t i n g good a u t o c o h e r e n t f i l m s i n which t h e m i c r o c r y s t a l s have a r a t h e r h i g h degree o f o r i e n t a t i o n . I n a d d i t i o n , t h e s p e c t r a were r e c o r d e d w i t h t h e beam e i t h e r p e r p e n d i c u l a r o r t i l t e d by 45°C w i t h r e s p e c t t o t h e C c r y s t a l a x i s . The r e s u l t s f o r d e h y d r a t e d samples ( h e a t e d t o 2 5 O O C ) a r e shown i n T a b l e 8.3.

I n a d d i t i o n , a s t r o n g band a t

160 cml i s observed i n a l l cases as i n p h l o g o p i t e ( s e e T a b l e 8.1). As f a r as t h e v i b r a t i o n s o f t h e i n t e r l a y e r c a t i o n a r e concerned, t h e r e i s a s h a r p d i s c o n t i n u i t y i n t h e s p e c t r a l f e a t u r e s between c a t i o n s w h i c h can w i g g l e i n t h e d i t r i g o n a l c a v i ties, the

r a d i u s o f which i s a b o u t 1.2

w,

and t h o s e f i t t i n g c l o s e l y i n t o t h e

a v a i l a b l e space. F o r t h e l a t t e r t h e i n - p l a n e v i b r a t i o n band can be s e p a r a t e d f r o m

201

F i g . 8.4. F a r i n f r a r e d t r a n s m i s s i o n s p e c t r a observed f o r t h e a l c a l i exchanged B l a c k Jack Mine b e i d e l l i t e (Chourabi, 1980). t h e o u t - o f - p l a n e v i b r a t i o n band by o b s e r v i n g t h e dichro'ic e f f e c t . One s h o u l d p o i n t o u t t h a t f o r t h e Cs'

sample t h e o u t - o f - p l a n e band i s a t about t h e same f r e -

quencyas t h e h i g h e r f r e q u e n c y band observed f o r s m e c t i t e s ( T a b l e 8.2). F o r c a t i o n s w i t h r a d i i s m a l l e r t h a n 1.2

150 cm-'

1, no

d i c h r o i c e f f e c t i s a c t u a l l y n o t i c e a b l e below

: one s i n g l e s t r u c t u r e l e s s b r o a d band between 50 and 90 cm-I appears

f o r a l l o f them. P r o b a b l y t h e s e c a t i o n s have s e v e r a l e q u i l i b r i u m p o s i t i o n s i n t h e d i t r i g o n a l c a v i t i e s and t h e r e i s a m i x t u r e o f o v e r l a p p i n g m o d e s w h i c h g i v e r i s e t o t h e broad adsorption. F i g . 8.5,which in-plane

(V

N

m.W i t h i n

s h o u l d be compared w i t h F i g . 8.3, shows t h e v a r i a t i o n o f t h e

) and o u t - o f - p l a n e (u,)

t h e l i m i t s o f t h e e x p e r i m e n t a l u n c e r t a i n t y as shown by v e r t i c a l

bars, t h e v a r i a t i o n o f ses w i t h

v i b r a t i o n frequencies w i t h respect t o

V,

i s l i n e a r whereas u

m.C o n s i d e r i n g

N

e x h i b i t s a c u r v a t u r e which i n c r e a -

t h a t an anharmonic o s c i l l a t o r can be d e s c r i b e d by a

Morse f u n c t i o n

U = A [1 - exp (TI x)12

(6)

202

- - - - II- - -

0.05

-1-

I

---

0.10

0.15

F i g . 8.5. I n - p l a n e v , and o u t - o f - p l a n e v c a t i o n v i b r a t i o n f r e q u e n c i e s o b s e r ved f o r L l a n o v e r m i c u f i t e ( A l c o v e r e t a l . 1481). TABLE 8.3 I n t e r l a y e r c a t i o n v i b r a t i o n frequencies observed by A l c o v e r e t a l . (1981) f o r c a t i o n exchanged L l a n o v e r m i c u l i t e s . Cation CS+

Rb+ K+ Ba2+ Sr2+

Ca2+ Nat Lit Mg2+

NB

*

ionicoradius (A)

1.65 1.47 1.33 1.34 1.15

In-plane f r e quency (cm-1)

59 66 80 68 80

O u t - o f - p l ane f r e q u e n c y (cm-1)

80 105 145 105 110

*

Broad band between 50-90 cm-1

0.68 0.66

no d i c h r o i c e f f e c t o b s e r v a b l e e x c e p t i n t h e 160-200 cm-l r e g i o n .

: u n c e r t a i n because o f o v e r l a p p i n g w i t h t h e 160 cm-l l a t t i c e band.

1

203

where

q

i s f u n c t i o n o f t h e c u r v a t u r e o f U a t t h e minimum, A i s t h e energy r e q u i -

r e d t o b r i n g t h e c a t i o n f r o m i t s e q u i l i b r i u m p o s i t i o n t o i n f i n i t y and x t h e v i b r a t i o n a m p l i t u d e , t h e v i b r a t i o n f r e q u e n c y can be c a l c u l a t e d as i n d i c a t e d by Landau e t a l . ( 1 9 6 6 ) . I t f o l l o w s

I f A = Z $ e, where Ze i s t h e c a t i o n charge and J, t h e e l e c t r i c a l p o t e n t i a l a t t h e c a t i o n s i t e , v i s a t f i r s t approximation a l i n e a r f u n c t i o n o f v a t i o n t h a t 'vl

>

v//

m.The

obser-

means t h a t t h e c u r v a t u r e o f t h e p o t e n t i a l energy f u n c t i o n i s

more pronounced f o r t h e p e r p e n d i c u l a r t h a n f o r t h e p a r a l l e l modes. T h i s may be expected i f e q u i p o t e n t i a l s u r f a c e s a r e normal t o t h e C a x i s . A c c o r d i n g t o equat i o n ( 7 ) t h e anharmonicity f o r the v v

I

v/,

//

mode s h o u l d be h i g h e r t h a n t h a t f o r t h e

mode i n v e r m i c u l i t e , w h e r e a s t h e a n h a r m o n i c i t y f a c t o r s h o u l d be l e s s f o r t h e i n d i o c t a h e d r a l s m e c t i t e s w i t h a l o w e r l a t t i c e charge. These f i n d i n g s d e s e r v e - f u r t h e r d e t a i l e d s t u d i e s . The p r o g r e s s i v e h y d r a t i o n o f

c a t i o n exchanged v e r m i c u l i t e has no deep e f f e c t on t h e f r e q u e n c i e s r e p o r t e d i n T a b l e 8 . 3 . A t a f i r s t g l a n c e t h i s i s unexpected b u t i n f a c t t h e f r e q u e n c y o f t h e d i f f u s i o n a l jumps,which i s i n c r e a s e d by t h e h y d r a t i o n , i s s t i l l s e v e r a l o r d e r s o f magnitude l e s s t h a n t h a t o f t h e v i b r a t i o n . However t h i s e f f e c t o f h y d r a t i o n s h o u l d be c a r e f u l l y studied,as

t h e d a t a o b t a i n e d f o r z e o l i t e s w i l l show.

The i n t e r e s t i n g o b s e r v a t i o n o f a d o u b l e t a t 55 and 85 cm-l f o r t h e Cs'

smecti-

t e s and o f a s i n g l e band f o r Cs+ v e r m i c u l i t e s ( t h e Benhavis v e r m i c u l i t e behaves s i m i l a r l y as t h e L l a n o sample) may be t e n t a t i v e l y e x p l a i n e d as f o l l o w s . Since t h e o u t - o f - p l a n e v i b r a t i o n i n v e r m i c u l i t e appears a t a b o u t 80 cm-',

by assuming a

degree o f d i s o r i e n t a t i o n i n Cs+ s m e c t i t e s much h i g h e r t h a n i n t h e c o r r e s p o n d i n g vermiculites the p r o b a b i l i t y o f f i n d i n g a noticeable f r a c t i o n o f microcrystals t i l t e d w i t h r e s p e c t t o t h e normal t o t h e f i l m , c o u l d be h i g h enough i n s m e c t i t e s t o make t h e o u t - o f - p l a n e comparing S.E.M. e t al.,

v i b r a t i o n o b s e r v a b l e . T h i s h y p o t h e s i s i s supported by

photographs o f L i '

1974). The Li'

smectite

aggregates a r e observed f o r t h e Cs'

8.6

and Cs'

s a t u r a t e d Wyoming b e n t o n i t e ( F r i p i a t

i s formed by p l a t y aggregates,whereas

granular

sample.

APPLICATION OF FAR INFRARED SPECTROSCOPY TO ZEOLITES T h i s c h a p t e r has been i n t r o d u c e d f o r t h e f o l l o w i n g reasons. I n z e o l i t e s t h e

l a t t i c e charge b a l a n c i n g c a t i o n s a r e l o c a t e d on s i t e s w i t h d i f f e r e n t environments and t h e c a t i o n i c p o p u l a t i o n s o f t h e s e s i t e s depend upon t h e h y d r a t i o n degree. T h e r e f o r e , a s t u d y o f t h e charge b a l a n c i n g c a t i o n s v i b r a t i o n s i n z e o l i t e s p r o v i des i n f o r m a t i o n

on t h e e f f e c t o f environment. T h i s paragraph w i l l deal o n l y w i t h

t h e s e c a t i o n i c v i b r a t i o n bands.

204

I.A. B r o d s k i i e t a l . (1971) were t h e f i r s t t o s t u d y t h e a b s o r p t i o n s p e c t r a o f c a t i o n exchanged Li',

Na',

Rb',

Cs'

s y n t h e t i c X z e o l i t e s a f t e r v a r i o u s dehydra-

t i o n t r e a t m e n t s . The a b s o r p t i o n bands which

were i n s e n s i t i v e t o t h e n a t u r e of

t h e c a t i o n o r t o h y d r a t i o n were c o n s i d e r e d as l a t t i c e bands.

F i g . 8.6 P r o j e c t i o n o f t e t r a h e d r a l l y a r r a n g e d s o d a l i t e u n i t s and c a t i o n i c s i t e s i n X and Y m o l e c u l a r s i e v e s . The h i g h

requency bands were a s s i g n e d t o c a t i o n s i n t h e S I and S I 1 s i t e s ,

namely those l o c a t e d i n t h e six-membered r i n g s windows opening i n t o t h e supercage ( S i t e I ) and t h e s m a l l c a v i t y ( s i t e I ) , see F i g . 8.6. Those i n s i d e t h e two b r i d g i n g f o u r r i n g s ( S i t e 111) i n t e r a c t more weakly w i t h t h e framework t h a n t h e c a t i o n s i n t h e S I and S I1 s i t e s (whose b i n d i n g e n e r g i e s a r e a l m o s t i d e n t i c a l ) and c o n s e q u e n t l y t h e i r v i b r a t i o n bands a r e a t l o w e r frequency. L a t e r on B r o d s k i i e t a l . (1974) extended t h e i r s t u d y t o z e o l i t e s w i t h d i f f e r e n t S i / A 1 r a t i o s , c o v e r i n g t h e range i n c o m p o s i t i o n between t h e X and Y members

205 o f t h e n e a r - f a u j a s i t e f a m i l y . This r a t i o i n f l u e n c e s t h e c a t i o n i c p o p u l a t i o n s i n d i f f e r e n t s i t e s . F i r s t o f a l l , as t h e Si/A1 r a t i o increases, according t o c r y s t a l l o g r a p h i c s t u d i e s t h e S I 1 1 p o p u l a t i o n decreases. The v i b r a t i o n o f charge balanc i n g c a t i o n s i n E z e o l i t e ( a s y n t h e t i c z e o l i t e w i t h c h a b a z i t e s t r u c t u r e ) was s t u d i e d by t h e same group i n 1977 ( B r o d s k i i e t a l . ,

1977). F i n a l l y i n 1980, B r o d s k i i

e t a l . presented a t t h e z e o l i t e conference a summary o f t h e i r f i n d i n g s . They a r e summarized i n Table 8.4 and they w i l l be examined l a t e r on. P a r a l l e l s t u d i e s were c a r r i e d o u t by B u t l e r e t a l . (1977) f o r s y n t h e t i c X and Y z e o l i t e s . T h e i r r e s u l t s a r e summarized i n Table 8.5 f o r t h e dehydrated s t a t e .

TABLE 8.4 I n t e r i o n i c v i b r a t i o n s (cm-l) o f c a t i o n s i n t h e framework o f X, Y and E z e o l i t e s f o r d i f f e r e n t c r y s t a l l o g r a p h i c p o s i t i o n s (dehydrated s t a t e ) , a f t e r B r o d s k i i e t a l . (1980). Site X Na' K+

S i t e 11, E

S

I' and 11" Y

E

66

88

90

188 185 155 (sh) 160

54

90

98

160

135

135

55

57

65

Rb'

40

83

cs+

33

82

Ca2+

Y

I

155

105

Ba2+

275

195

140

84

Sr2+

200

55

150 110

107

I n general t h e assignments were made according t o these main c r i t e r i a :

1. S e n s i t i v i t y t o i o n i c exchange 2. Frequency s h i f t s obeying a fVy2 3. I n t e n s i t y depending upon t h e s i t e s p o p u l a t i o n , suggested by t h e c r y s t a l l o g r a phic studies. The s i t e 11, which i s t h e most h i g h l y populated i n d r y monovalent Y z e o l i t e , should g i v e r i s e t o t h e most i n t e n s e band. A s i m i l a r band a t s l i g h t y h i g h e r f r e quency i s expected i n t h e corresponding X z e o l i t e s due t o t h e i r h i g h e r l a t t i c e charge. Since t h e s t r e n g t h o f t h e t h r e e f o l d

bonding t o a s i x - r i n g a t s i t e I' i s near-

l y i n d i s t i n g u i s h a b l e f r o m t h a t a t s i t e 11, t h e two v i b r a t i o n a l bands must o v e r l a p

considerably, as suggested by B r o d s k i i

et

a l . However t h e use of p o t e n t i a l ener-

gy c a l c u l a t i o n f o r t h e band assignment must be used w i t h c a u t i o n because t h e s t e r i c s t r a i n s must a l s o be taken i n t o account. For instance, t h e s t r u c t u r a l s t u d i e s i n d i c a t e t h a t t h e cation-oxygen d i s t a n c e a t s i t e I i s g r e a t e r than

at

206 s i t e I 1 f o r Na'

Therefore, i n s p i t e o f t h e f a c t t h a t t h e p o t e n t i a l ener-

and K'.

gy c a l c u l a t i o n s i n d i c a t e t h a t s i t e I i s a t a h i g h e r energy than s i t e 11, t h e s i t e I frequency should l i e lower than t h e s i t e I 1 frequency.

TABLE 8.5 Assignment o f t h e c a t i o n v i b r a t i o n s i n X and Y dehydrated z e o l i t e s , according t o B u t l e r e t a l . (1977). Site I11 v(cm-1) Intensity

"(cm-1)

Site I Intensity

Li+Y

Site I 1 v(cm-1) Intensity 380

sh

Na'Y

167

m sh

180

S

K'Y

107

m sh

133

S

CS+Y

30

wsh

ACJ+Y

50

W

S

m

190

ms

Na'X

67

wm

K'X

58

m

156

S

Rb+X

48

wm

108

ms

cs+x

39

wm

86

in

227

m

189

m ms

150

rn

137

S

107

S

287

wm

273

m

160

256

For d i v a l e n t c a t i o n s ,

m sh

62 82

s i t e I i s t h e most s t a b l e i n d r y z e o l i t e s and t h e

assignments i n Table 8.5 o f t h e bands observed f o r Ca2+Y and Ba2'Y

agree w i t h

t h i s i d e a . I n d i v a l e n t c a t i o n z e o l i t e s t h e band assigned t o s i t e I 1 encompasses n o t o n l y t h e c a t i o n s on I ' b u t a l s o 1 1 ' . The p a r t i c u l a r i n t e r e s t o f t h e work o f B u t l e r e t a l . i s t h e i r o b s e r v a t i o n o f t h e e f f e c t o f adsorbed molecules on t h e c a t i o n frequencies. The charge d e l o c a l i s a t i o n r e s u l t i n g from c a t i o n s o l v a t i o n i s expected t o reduce t h e v i b r a t i o n a l f r e quencies f o r n o n - s t e r i c a l l y c o n s t r a i n e d c a t i o n s . Indeed w i t h K'Xand

K+Y t h e v i b r a t i o n bands assigned t o s i t e I 1 s h i f t upon hydra-

t i o n from 156 t o 122 cm-' and from 133 t o 114 cm-l, r e s p e c t i v e l y . The s i t e I 1 1 v i b r a t i o n s observed i n X z e o l i t e s s h i f t n e g l i g i b l y i n frequency as t h e h y d r a t i o n increases b u t t h e i n t e n s i t y drops very r a p i d l y . Far i n f r a r e d a b s o r p t i o n by e x t e r n a l v i b r a t i o n s o f sorbed water has been s t u d i e d by M o e l l e r e t a l . (1971) b u t t h i s s p e c i f i c s t u d y i s beyond t h e scope o f t h i s

207

review . B r o d s k i i e t a l . (1980) p o i n t e d o u t p e r t i n e n t l y t h a t t h e assumption,that

the

c a t i o n v i b r a t i o n s a r e so weakly c o u p l e d w i t h framework v i b r a t i o n s t h a t t h e y can be c o n s i d e r e d i n d e p e n d e n t l y , i s

n o t always j u s t i f i e d .

F o r i n s t a n c e , t h e f r e q u e n c y assigned by B u t l e r e t a l . ( T a b l e 8.5) t o L i + on s i t e I 1 o f Y z e o l i t e d o e s n ' t s h i f t i n Na z e o l i t e s p a r t i a l l y s u b s t i t u t e d by 6 L i and 7 L i + c a t i o n s and t h e r e f o r e , t h e 380 cm-l band i s p r o b a b l y a l a t t i c e mode p e r t u r b e d by t h e c a t i o n . C o u p l i n g w i t h framework v i b r a t i o n s may a l s o r i s e q u e s t i o n s about assignments f o r l i g h t d i v a l e n t c a t i o n s , suchs as Mg2+

I

2

Iv

200

--_1--L-_I,J

@m 0 Butler et al. 1 @ Brodskii etal. I - - 1--1-- 1 - - 1

I

- - -I I I I

-

-17

1

F i g . 8.7 C a t i o n i c v i b r a t i o n f r e q u e n c i e s assigned t o s i t e I 1 i n X and Y z e o l i t e s and t o s i t e I 1 1 i n X z e o l i t e p l o t t e d vs

m.

F o r s i t e s 11, 1 1 ' and I ' on t h e one hand,and f o r s i t e I 1 1 on t h e o t h e r hand, t h e r e i s a r e a s o n a b l e agreement between t h e d a t a o f B u t l e r e t a l . (1977) and o f

208 B r o d s k i i e t a l . (1980). I n a d d i t i o n t h e f r e q u e n c i e s observed f o r t h e s e bands obey p r e t t y w e l l l i n e a r r e l a t i o n s h i p s w i t h r e s p e c t t o 8.7.

m,as

shown i n F i g .

I t i s w o r t h w i l e t o p o i n t o u t t h a t t h e s l o p e o f t h e l i n e a r r e l a t i o n s h i p ob-

t a i n e d f o r t h e c a t i o n s i n s i t e I 1 i s a b o u t t h e same as t h a t f o r vI i n v e r m i c u l i t e s ( F i g . 8.5). The f r e q u e n c i e s observed f o r c a t i o n s i n s i t e I 1 1 a r e more s i m i l a r t o t h o s e observed f o r v , ~

.

The f r e q u e n c i e s g i v e n f o r s i t e

I in

X z e o l i t e by B r o d s k i i e t a l . and B u t l e r

e t a l . do n o t appear t o f o l l o w such a s i m p l e r e l a t i o n s h i p and i n a d d i t i o n b o t h s e r i e s o f measurements a r e i n disagreement. These d i s c r e p a n c i e s c o u l d be due t o i n t e r a c t i o n s w i t h l a t t i c e v i b r a t i o n s f o r t h o s e c a t i o n s which a r e i n s i d e t h e hexagonal p r i s m s , l i n k i n g cubooctahedra.

8.7

CONCLUSION

I n f o c u s i n g o u r a t t e n t i o n on t h e v i b r a t i o n a l motions o f t h e charge b a l a n c i n g c a t i o n s i n c l a y s and z e o l i t e s , i t appears c l e a r l y t h a t ( i ) t h e symmetry a t t h e c a t i o n s i t e , ( i i ) t h e l a t t i c e charge and ( i i i ) e n v i r o n m e n t a l c o n s t r a i n t s , i n f l u ence s t r o n g l y t h e f r e q u e n c i e s a t which t h e v i b r a t i o n a l bands appear. What t y p e o f p h y s i c a l i n f o r m a t i o n can be o b t a i n e d f r o m t h e knowledge o f t h e s e v i b r a t i o n frequencies ?

As a l r e a d y shown by e q u a t i o n s ( 6 ) and ( 7 ) , i n f o r m a t i o n on t h e p o t e n t i a l w e l l s i n which c a t i o n s a r e l o c a t e d c o u l d be o b t a i n e d f r o m t h e s e f r e q u e n c i e s . B u t l e r e t a l . (1977) have t r i e d t h a t t y p e o f c a l c u l a t i o n u s i n g an a n a l o g y w i t h t h e models of i o n i c t r a n s p o r t i n s u p e r i o n i c c o n d u c t o r s . By thermal e x c i t a t i o n , t h e c a t i o n can be b r o u g h t i n t o energy s t a t e s above an energy gap

E~

so t h a t t h e y propagate t h r o u g h t h e l a t t i c e w i t h a v e l o c i t y

a near f r e e p a t h

$,

V, and

t o a n o t h e r l o c a l i z e d c a t i o n i c s i t e . By r e l a t i n g t h e e q u a t i o n

f o r i o n i c c o n d u c t i v i t y t o a c o n v e n t i o n a l hopping model, c 0 i s equal t o t h e a c t i v a t i o n energy, E , r e q u i r e d t o produce an i o n i c hop.

E = 1/2 M v 2 a: a.

i s t h e h o p p i n g d i s t a n c e between an o c c u p i e d and a v a c a n t s i t e . The most p r o b a b l e hopping d i s t a n c e i n Y z e o l i t e i s t h a t between s i t e s I 1 and

111 : i t v a r i e s w i t h t h e s i z e ( a n d t h u s t h e mass M) o f t h e c a t i o n i n t h e same way as t h e f o r c e c o n s t a n t and t h u s i t t e n d s t o l o w e r E. U s i n g t h i s s i n g l e r e l a t i o n s h i p , B u t l e r e t a l . f o u n d a c t i v a t i o n e n e r g i e s w h i c h were i n r e a s o n a b l e agreement w i t h t h o s e o b t a i n e d by measurements o f i o n i c c o n d u c t i v i t i e s . I n a somewhat analogous way, C a l v e t (1972) has a t t e m p t e d t o c a l c u l a t e t h e frequency o f v i b r a t i o n o f a c a t i o n w i t h respect t o i n t e r l a y e r c l a y surfaces from t h e s e l f - d i f f u s i o n c o e f f i c i e n t measurements i n a d i r e c t i o n p a r a l l e l t o t h e a b

209 plane. F o r K',

Rb'

and Cs'

m o n t m o r i l l o n i t e s t h e f r e q u e n c i e s were 105, 54 and

44 cm-l r e s p e c t i v e l y . The K'

and Cs'

r e s u l t s a r e n o t f a r from t h e i n - p l a n e v i -

b r a t i o n frequencies r e p o r t e d i n Table 8.2.

However, o u r i n t u i t i o n t e l l s us t h a t

t h e expected c o r r e l a t i o n should be w i t h t h e o u t - o f - p l a n e v i b r a t i o n . There i s no doubt t h a t c a l c u l a t i o n s along these l i n e s should l e a d t o i m p o r t a n t i n f o r m a t i o n , e s p e c i a l l y i f t h e h y d r a t i o n e f f e c t s were c a r e f u l l y c o n t r o l l e d . The comparison made between t h e v i b r a t i o n f r e q u e n c i e s observed f o r vermicul it e s and z e o l i t e s i s i n t e r e s t i n g because i t r e v e a l s s i m i l a r i t i e s when c a t i o n s a r e v i b r a t i n g i n six-membered r i n g s e.g. f o r z e o l i t e

s i t e s I 1 and vI i n v e r m i c u l i -

t e s . Because: d i c h r o f c e f f e c t s cannot be observed f o r z e o l i t e s , t h e p o s s i b i l i t y o f a s s i g n i n g some o f t h e observed v i b r a t i o n frequencies t o modes p a r a l l e l t o planes c o n t a i n i n g r i n g s has n o t been taken i n t o c o n s i d e r a t i o n by t h e authors who have s t u d i e d t h a t t y p e o f m a t e r i a l s . T h i s q u e s t i o n should o b v i o u s l y deserve more a t tention. I n t h e f u t u r e t h e f a r i n f r a r e d s t u d i e s a p p l i e d t o z e d l i t e s and t o c l a y miner a l s should be d i r e c t e d towards improving o u r understanding o f t h e e f f e c t s of c a t i o n s o l v a t a t i o n and o r complexation, and on t h e l a t t i c e v i b r a t i o n a l frequencies

i n these m a t e r i a l s . REFERENCES Alcover, J.F., Gatineau, L. and Gervais, F., 1981. Far i n f r a r e d study o f t h e v i b r a t i o n s i n v e r m i c u l i t e , i n p r e p a r a t i o n . T h i s paper was presented as a p o s t e r a t t h e European Clay Conference i n Munich, Sept. 1980. B e l l , R.J., 1972. I n t r o d u c t o r y F o u r i e r t r a n s f o r m spectroscopy. Academic Press. B r o d s k i i , I . A . , Zhdanov, S.P. and Stanevich, A.E., 1971. Far i n f r a r e d spectra o f c a t i o n - s u b s t i t u t e d t y p e X z e o l i t e s ( f a u j a s i t e s ) and t h e i r changes on dehyd r a t i o n . Opt. & Spektrosk. (USSR) p.58-62. Opt. & Spectros. (USA) 30: 30-32. Zhdanov , S.P. and Stanevich, A.E., 1974. Spectroscopic i n v e s t i Brodskii, I.A., g a t i o n s o f i n t e r i o n i c v i b r a t i o n s i n s y n t h e t i c f a u j a s i t e - t y p e z e o l i t e s . Sov. Phys. S o l i d S t a t e , 15 : 1771-1772. B r o d s k i i , I . A . , Z h d a y v , S.P., Krosavtseva and Samulevich, N.N., 1977. Longwavelength i n f r a r e d s p e c t r a o f c a t i o n - s u b s t i t u t e d c h a b a z i t e type z e o l i t e s . Sov. Phys. S o l i d S t a t e , 19: 549-550. B r o d s k i i , I . A . and Zhdanov, S.P., 1980. A p p l i c a t i o n o f f a r i n f r a r e d spectroscopy f o r a s t u d y of c a t i o n p o s i t i o n s i n z e o l i t e s . Proc. I n t . Conf. Z e o l i t e s , 5th: 234-241. B u t l e r , Wayne, M., A n g e l l , Charles, L., M c A l l i s t e r , Warren and Risen, W i l l i a m M. Jr., 1977. F a r i n f r a r e d study o f c a t i o n motion i n d r y and s o l v a t e d mono and d i v a l e n t c a t i o n c o n t a i n i n g z e o l i t e s X and Y . J . Phys. Chem., 81: 2061-2068. Calvet, R . , 1972. H y d r a t a t i o n de l a m o n t m o r i l l o n i t e e t d i f f u s i o n des i o n s compensateurs. These de d o c t o r a t , F a c u l t e des Sciences de P a r i s . Chourabi, B., 1980. C o n t r i b u t i o n a l ' e t u d e de l a s t r u c t u r e f i n e des p h y l l o s i l i cates 2 : l p a r spectroscopie I.R. 3 r d Cycle Thesis, C.R.S.O.C.I., CNRS, 45045 Or1 eans Cedex, France. Farmer, V.C., 1974. The i n f r a r e d s p e c t r a o f m i n e r a l s . M i n e r a l o g i c a l S o c i e t y Monograph 4, The M i n e r a l o g i c a l S o c i e t y , London; F r i p i a t , J . J . , Cruz, M . I . , Bohor, B.F. and Thomas, J . , 1974. I n t e r l a m e l l a r adsorpt i o n o f carbon d i o x i d e by smectites. Clays and Clay M i n e r a l s , 22: 22-30. Gervais, F., 1981. High temperature i n f r a r e d r e f l e c t i v i t y spectroscopy by scann i n g i n t e r f e r o m e t r y . I n f r a r e d and M i l l i m e t e r Waves Series, V I I , Academic Press,

210 i n press. I s h i i , M., Nakahira, M. and Takeda, H., 1969. F a r i n f r a r e d a b s o r p t i o n s p e c t r a o f micas. Proc. I n t . Clay Conf., 1969, Tokyo ( L . H e l l e r , Ed.), 1: 247-259. I s r a e l U n i v . Press, Jerusalem. Landau, L. and L i f c h i t z , E., 1967 . Mecanique q u a n t i q u e . T h @ o r i e non r e l a t i v i s t e . DeuxiPme E d i t i o n , E d i t i o n M . I . R . Moscow. Larson, S y l v i a , J., Pardoe, G.W.F., Gebbie, H,A. and Larson, E.E., 1972. Use o f f a r i n f r a r e d i n t e r f e r o m e t r i c s p e c t r o s c o p y f o r m i n e r a l i d e n t i f i c a t i o n . Amer. M i n e r a l , 57: 998-1002. Lazarev, A.N., 1972. V i b r a t i o n a l s p e c t r a and s t r u c t u r e o f s i l i c a t e s . Plenum Press, New York. M o e l l e r , K., Kunath, D. and Spangenberg, H.J., 1971. F a r i n f r a r e d a b s o r p t i o n by e x t e r n a l v i b r a t i o n o f sorbed w a t e r on t y p e A z e o l i t e s . Spectrochim. Acta, P a r t A , 27: 353-355. Roth, C . , 1978. E f f e c t s o f exchangeable c a t i o n s on t h e f a r i n f r a r e d s p e c t r a o f c l a y m i n e r a l s . P o s t e r session, Book o f summaries o f t h e 6 t h I n t e r n a t i o n a l Clay Conference, Oxford, p. 255.

211 Chapter 9

E.S.C.A.

STUDIES OF CLAY MINERALS

Paul CANESSON U n i v e r s i t i . de P o i t i e r s , L a b o r a t o i r e de Chimie X I , 40 Avenue du Recteur Pineau, 86022 - P o i t i e r s - France. INTRODUCTION

9.1

Since t h e f i r s t r e v i e w p u b l i s h e d b y K. Siegbahn e t a l . (1967) summing up t h e main r e s u l t s o b t a i n e d i n Uppsala by X-ray P h o t o e l e c t r o n Spectroscopy (X.P.S.)

a

l o t o f l a b o r a t o r i e s have been concerned w i t h t h i s t e c h n i q u e . The main f e a t u r e s of t h i s new t o o l r e s t i n i t s v e r y h i g h s e l e c t i v i t y f o r t h e s u r f a c e o f t h e sample t o be a n a l y s e d and i n i t s a b i l i t y t o d e t e c t a l l t h e elements except hydrogen. T h i s l a s t c a p a c i t y has a l l o w e d K. Siegbahn t o c a l l t h i s t e c h n i q u e E l e c t r o n Spect r o s c o p y f o r Chemical A n a l y s i s (ESCA)

.

I n a d d i t i o n t o t h e v a l e n c e e l e c t r o n s , each atom i n a compound has c o r e e l e c t r o n s t h a t a r e n o t i n v o l v e d i n t h e b o n d i n g ( e x c e p t hydrogen). The e n e r g i e s o f t h e s e e l e c t r o n s a r e c h a r a c t e r i s t i c o f t h e i n d i v i d u a l atoms; y e t , t h e y a r e a l s o r e s p o n s i v e t o changes i n t h e e l e c t r o n i c environment and t h e y may be used t o p r o v i d e i n f o r m a t i o n on bonding. S i n c e so much r e l e v a n t i n f o r m a t i o n seems t o be d i r e c t l y a v a i l a b l e f r o m t h e e l e c t r o n energy d i s t r i b u t i o n , i t i s n o t s u r p r i s i n g t h a t d i r e c t o b s e r v a t i o n o f e l e c t r o n energy l e v e l s by e l e c t r o n s p e c t r o s c o p y s h o u l d s t a n d o u t a s t h e most p o w e r f u l t o o l t o answer t h e q u e s t i o n s "What elements ? " "How many o f them ? " "What t y p e o f b o n d i n g ?". N e v e r t h e l e s s , anyone w o r k i n g w i t h an ESCA system must keep i n mind t h a t , i n most cases, t h e r e s u l t s o b t a i n e d f r o m t h i s t e c h n i q u e can be o f i n t e r e s t o n l y by comparison w i t h t h o s e o b t a i n e d b y any o t h e r means t h a t we know o f . Moreover, i n o r d e r t o o b t a i n good r e s u l t s and because o f i t s h i g h s e l e c t i v i t y f o r t h e s u r f a c e , i t i s necessary t h a t X.P.S.

(e.g.

induced problems s h o u l d be s a t i s f a c t o r i l y s o l v e d

c h o i c e o f a s u p p o r t , c h a r g i n g e f f e c t and c h o i c e o f a r e f e r e n c e , carbon

c o n t a m i n a t i o n o v e r l a y e r and i t s e f f e c t s . . . ) . B e a r i n g i n mind t h e p a r t i c u l a r case o f c l a y s and c l a y m i n e r a l s , t h e b a s i c p r i n c i p l e s o f X.P.S.

(ESCA) and t h e p r e c a u t i o n s t o be t a k e n f o r spectrum i n t e r -

p r e t a t i o n s w i l l be summarized i n t h e f i r s t p a r t o f t h i s s t u d y , b e f o r e r e v i e w i n g t h e main r e s u l t s t h a t have a l r e a d y been o b t a i n e d i n o u r p a r t i c u l a r t o p i c o f interest.

212

B A S I C PRINCIPLES OF PHOTOELECTRON SPECTROSCOPY.

9.2 9.2.1

Fundamental r e l a t i o n and consequences.

The fundamental proce'ss o c c u r i n g i n X . P . S .

( o r ESCA) i s a p h o t o e l e c t r i c e f f e c t .

An X-ray o f known energy, hv, impinges on a sample, e j e c t i n g e l e c t r o n s f r o m i t , w i t h k i n e t i c e n e r g i e s (EK) g i v e n by t h e fundamental r e l a t i o n : EK = hv - E l B

where E l B i s t h e energy t o be s u p p l i e d t o a p a r t i c u l a r atom i n o r d e r t o b r i n g o u t a p a r t i c u l a r e l e c t r o n f r o m i t s fundamental l e v e l i n s i d e t h e sample t o t h e s t a t e o f a s i n g l e e l e c t r o n w i t h a k i n e t i c energy equal t o z e r o under vacuum. I f t h e Fermi l e v e l i s t a k e n as r e f e r e n c e f o r t h e b i n d i n g energy s c a l e , E l B i s t h e sum o f two terms, namely EB, t r u e b i n d i n g energy o f t h e e l e c t r o n o f i n t e r e s t , and @ which r e p r e s e n t s t h e work f u n c t i o n o f t h e sample. The k i n e t i c energy o f t h e e j e c t e d e l e c t r o n can be w r i t t e n :

An a c c u r a t e measurement o f EK i s o b t a i n e d w i t h an e l e c t r o n s p e c t r o m e t e r and d e t a i l e d r e v i e w s o f i n s t r u m e n t a t i o n may be f o u n d elsewhere (Siegbahn, 1967; J e n k i n , 1977). An X.P.S.

spectrum r e p r e s e n t s a number o f p h o t o e j e c t e d e l e c t r o n s f r o m t h e

sample versus t h e i r k i n e t i c energy o r t h e i r b i n d i n g energy. A s h a r p l i n e appears f o r e v e r y e l e c t r o n i c l e v e l (with, o f course, a b i n d i n g energy l o w e r t h a n h v ) o f a l l t h e elements p r e s e n t i n t h e a n a l y z e d p r o d u c t . X.P.S.

i s sensitive t o a l l the

elements w i t h an a t o m i c number g r e a t e r t h a n 2 and t h e t a b l e o f b i n d i n g e n e r g i e s g i v e n by Siegbahn e t a l . (1967) i s always c o n v e n i e n t f o r t h e a t t r i b u t i o n o f t h e observed peaks i n a spectrum, t h u s X.P.S. The e x a c t p r o f i l e o f an X.P.S.

appears as an a n a l y t i c a l t o o l .

l i n e i s a complicated convolution product bet-

ween t h e n a t u r a l p r o f i l e o f t h e l e v e l o f i n t e r e s t , t h e r e s o l u t i o n o f t h e a n a l y z i n g energy system and t h e p r o f i l e o f t h e e x c i t i n g r a d i a t i o n . Working w i t h v e r y narrow X-ray l i n e s as a s o u r c e w i l l enhance r e s o l u t i o n . F o r t h i s reason, A1 Ka ( h v = 1486.6 eV)

and Mg Ka (1353.6 eV) l i n e s a r e c u r r e n t l y used as X-ray

sources. This basic p r i n c i p l e reveals t h e f i r s t l i m i t a t i o n o f e l e c t r o n spectroscopy. A g o o d d e t e r m i n a t i o n o f t h e k i n e t i c e n e r g y o f t h e e j e c t e d e l e c t r o n s c a n o n l y b e achieved

i f t h e i r mean f r e e p a t h i n s i d e t h e s p e c t r o m e t e r i s g r e a t e r t h a n t h e l e n g t h t h e y have t o t r a v e l between t h e sample and t h e d e t e c t o r ; p r e s e n t l y s p e c t r o m e t e r s a r e o p e r a t e d under vacuum b e t t e r t h a n 10-8

-

10-9 t o r r .

Under t h e s e c o n d i t i o n s t h e u s e r must make s u r e t h a t t h e s u r f a c e o f t h e sample does n o t undergo any t r a n s f o r m a t i o n under u l t r a h i g h vacuum. S i n c e t h e r e i s no

213 t e m p e r a t u r e l i m i t a t i o n , i t i s always p o s s i b l e t o a v o i d a m o d i f i c a t i o n o f t h e s u r f a c e by c o o l i n g t h e sample a t temperatures as l o w as t h a t o f l i q u i d n i t r o g e n . Moreover, t h e vapour p r e s s u r e o f t h e sample must be l o w e r t h a n t h e r e s i d u a l p r e s sure i n s i d e t h e spectrometer, p r a c t i c a l l y l i m i t i n g e l e c t r o n spectroscopy t o t h e study o f s o l i d s . Working w i t h c l a y s , t h i s l i m i t a t i o n may be a s o u r c e o f d i f f i c u l t i e s s i n c e t h e s e m i n e r a l s most o f t e n e x i s t w i t h a h i g h w a t e r c o n t e n t and keeping c l a y m i n e r a l s under vacuum can i n d u c e d e h y d r a t i o n r e a c t i o n s t h a t may change t h e s u r f a c e p r o p e r t i e s . F o r example, when m e s o - t e t r a p h e n y l p o r p h y r i n (TPP) i s adsorbed i n t h e i n t e r l a y e r s p a c e , o f m o n t m o r i l l o n i t e , t h e X.P.S.

spectrum o f t h e N 1 s l e v e l i s s u e d

from t h e adsorbed m o l e c u l e suggests t h a t t h e r e e x i s t s some e x t r a p r o t o n a t i o n of t h e TPP m o l e c u l e (Canesson e t a l . ,

1978). Vacuum b e t t e r t h a n lo-* t o r r and X-ray

bombardment would i m p l y a d e s o r p t i o n o f adsorbed w a t e r , and i t i s w e l l known t h a t c l a y s w i t h a v e r y l o w w a t e r c o n t e n t have t h e i r a c i d i t y enhanced ( F r i p i a t e t al.,

1965; B a i l e y e t a l . ,

1976). T h e r e f o r e i n s i d e t h e s p e c t r o m e t e r t h e c l a y

s u r f a c e a c t s as a super a c i d medium f o r t h e adsorbed TPP. 9.2.2

R e l a x a t i o n process.

I f a vacancy i s c r e a t e d i n an i n n e r e l e c t r o n s h e l l b y X-ray i r r a d i a t i o n , t h e

e x c i t e d atom w i l l r e v e r t t o t h e ground s t a t e by e i t h e r e m i t t i n g c h a r a c t e r i s t i c X-ray r a d i a t i o n o r by r a d i a t i o n l e s s t r a n s i t i o n s , t h e s o - c a l l e d Auger t r a n s i t i o n s . I n t h e X-ray case, t h e i n n e r vacancy i s f i l l e d w i t h an e l e c t r o n f r o m an o u t e r s h e l l and t h e r e l e a s e d energy i s e m i t t e d as e l e c t r o m a g n e t i c r a d i a t i o n i n an X-ray quantum. I n t h i s case, t h e r e l a x a t i o n process i s t h e s o - c a l l e d X-ray f l u o r e s c e n c e . I n t h e Auger case, t h e r e l e a s e d energy i s i n s t e a d t r a n s f e r r e d t o a n o t h e r e l e c t r o n i n one o f t h e o u t e r s h e l l s . T h i s e l e c t r o n i s t h e n r e l e a s e d and l e a v e s t h e atom. The Auger process can be c o n s i d e r e d as t h e sum o f X-ray f l u o r e s c e n c e and t h e p r i m b y process o f X.P.S..

A c o m p l i c a t i o n i n X.P.S.

spectroscopy i s t h a t

Auger e l e c t r o n s e m i t t e d f r o m t h e sample a r e a l s o a n a l y z e d i n t h e s p e c t r o m e t e r and appear i n t h e X.P.S.

s p e c t r a . S i n c e t h e Auger process i n v o l v e s t h r e e d i f f e -

r e n t l e v e l s i n s i d e one atom, t h e c o r r e s p o n d i n g Auger peaks a r e r a t h e r broad w i t h r e s p e c t t o t h e p r i m a r y X.P.S. s c a l e o f a s o - c a l l e d X.P.S.

e l e c t r o n s . Ift h e p o s i t i o n on t h e k i n e t i c energy e l e c t r o n depends on t h e n a t u r e o f t h e anode o f t h e

X-rays source, Auger e l e c t r o n s do n o t , because t h e i r k i n e t i c energy i s o n l y dependent on t h e n a t u r e o f t h e c o n s i d e r e d atom. 9.2.3

X.P.S.

as a s u r f a c e t e c h n i q u e

The e x c i t i n g r a d i a t i o n used i s a b f e t o e j e c t e l e c t r o n s f r o m atoms a t r a t h e r i m p o r t a n t depths, b u t i t i s w e l l known t h a t e l e c t r o n s w i t h e n e r g i e s o f a few keV o r l e s s p e n e t r a t e o n l y v e r y t h i n l a y e r s o f s o l i d m a t t e r . P h o t o e l e c t r o n s

2 14 produced by a Mg K a l i n e f o r example emerge f r o m a s u r f a c e l a y e r c o n t a i n i n g some hundred a t o m i c l a y e r s . The f r a c t i o n t h a t emerges w i t h o u t energy l o s s becomes e x c e e d i n g l y s m a l l as t h i s l i m i t i s approached. The average d e p t h a t w h i c h t h o s e e l e c t r o n s observed i n t h e e l e c t r o n l i n e s a r e produced may be o n l y a few t e n s o f an

i.The

s o - c a l l e d escape d e p t h o f p h o t o e l e c t r o n s depends on s e v e r a l f a c t o r s ,

e.g. t h e n a t u r e , d e n s i t y and c r y s t a l s t r u c t u r e o f t h e sample m a t e r i a l , t h e energy o f the radiation involved f o r e x c i t a t i o n

... .

I t i s w e l l known t h a t t h e escape

depth i s s u b s t a n t i a l l y g r e a t e r i n s i d e an o x i d e t h a n i n t h e c o r r e s p o n d i n g m e t a l , and t h e more e n e r g e t i c t h e X-ray s o u r c e i s , t h e g r e a t e r t h e escape depth. The v a r i a t i o n s o f t h e mean f r e e p a t h o f e l e c t r o n s versus t h e i r k i n e t i c energy must be t a k e n i n t o account f o r q u a n t i t a t i v e i n t e r p r e t a t i o n s o f X.P.S. 9.2.4

spectra.

Choice o f a r e f e r e n c e l i n e

We have seen t h a t X.P.S.

can be used as a q u a l i t a t i v e a n a l y t i c a l t e c h n i q u e .

L e t us now emphasize t h a t t h e e x a c t p o s i t i o n on t h e b i n d i n g energy s c a l e o f a p a r t i c u l a r l e v e l does n o t depend o n l y upon t h e n a t u r e o f t h i s l e v e l o f t h e c o n s i dered element b u t a l s o on i t s chemical environment. O x i d a t i o n s t a t e , t y p e o f bonding, n a t u r e o f t h e n e a r e s t neighbour have an i n f l u e n c e on t h e k i n e t i c energy o f t h e p h o t o e j e c t e d e l e c t r o n s , and i n my o p i n i o n one o f t h e p r i n c i p a l advantages o f X.P.S.

r e s t s here. One can e a s i l y a d m i t t h a t t h e more p o s i t i v e an element i n

a sample i s , t h e more d i f f i c u l t a f u r t h e r i o n i s a t i o n w i l l be, even i n c o r e l e v e l s . T h i s f a c t induces chemical s h i f t s i n t h e p o s i t i o n o f t h e observed l e v e l . T h i s s h i f t i s always l o w (e.g.

5 eV f r o m m e t a l l i c molybdenum t o molybdenum t r i o x i d e

(Cimino and De A n g e l i s , 1975) b u t i t c a r r i e s o u t some fundamental i n f o r m a t i o n . I t f o l l o w s t h a t t h e e x a c t p o s i t i o n o f a g i v e n l e v e l can be s a i d t o be o n l y depen-

d e n t on t h e n a t u r e o f t h i s l e v e l and t h e r e a l charge o f t h e element o f i n t e r e s t i n s i d e t h e sample. I n o r d e r t o a v o i d any u n c e r t a i n t y i n t h e i n t e r p r e t a t i o n o f s p e c t r a , t h e b i n d i n g e n e r g i e s d e t e r m i n a t i o n s must be as a c c u r a t e as p o s s i b l e . T h i s r a i s e s one o f t h e m a j o r problems w i t h X.P.S.:

any l a c k o f p r e c i s i o n i n

t h e d e t e r m i n a t i o n o f k i n e t i c energy o f p h o t o e j e c t e d e l e c t r o n s w i l l i n d u c e t h e same i n a c c u r a c y f o r b i n d i n g e n e r g i e s . A c c o r d i n g t o t h e b a s i c p r i n c i p l e o f X.P.S., some e l e c t r o n s a r e e j e c t e d f r o m t h e sample, t h e r e s u l t o f w h i c h i s a n e t p o s i t i v e charge on t h e a n a l y z e d s u r f a c e , even i f t h e sample i s i n good e l e c t r i c c o n t a c t w i t h t h e s p e c t r o m e t e r ; w o r k i n g w i t h i n s u l a t o r s o r semi-conductors does n o t cancel t h i s charging e f f e c t . I n a s t a t i o n a r y s t a t e , a c o n s t a n t p o s i t i v e charge remains on t h e s u r f a c e , which i n t u r n c r e a t e s a r e t a r d i n g e l e c t r i c f i e l d f o r t h e e l e c t r o n s l e a v i n g t h e sample. I t can be e a s i l y understood t h a t i t i s o f p r i m e i m p o r t a n c e t o a c c o u n t f o r t h i s effect,

and t h e b i n d i n g energy s c a l e must be c a l i b r a t e d f o r e v e r y sample.

Various methods have been proposed i n t h e l i t t e r a t u r e f o r t h e c a l i b r a t i o n o f X.P.S. s p e c t r a (Ebel, 1974; Hnatowich e t a l . ,

1971; O g i l v i e and Wolberg, 1972).

215 One o f t h e f i r s t methods c o n s i s t s i n u s i n g t h e C 1 s l e v e l o r i g i n a t i n g f r o m a carbon c o n t a m i n a t i o n o v e r l a y e r . E s p e c i a l l y w i t h t h e f i r s t commercial X.P.S. systems, t h e vacuum was n o t o f v e r y good q u a l i t y and t h e samples were c o a t e d w i t h a l a y e r o f carbon c o n t a i n i n g p r o d u c t s ( p r o b a b l y e s s e n t i a l l y hydrocarbons f r o m pump o i l s ) . T h i s carbon c o n t a m i n a t i o n has been used w i d e l y f o r c a l i b r a t i o n purpose, t h e b i n d i n g energy o f t h e C 1 s l e v e l b e i n g a r b i t r a r i l y f i x e d between 283.5 eV and 285.5 eV, depending on t h e a u t h o r s . P r e c i s i o n i n b i n d i n g energy d e t e r m i n a t i o n s has been e s t i m a t e d a t 0.2 eV i n t h i s case (Ebel, 1974; Contour and Mouvier, 1975). T h i s method

i s o b v i o u s l y t h e e a s i e s t t o use, b u t i t f a i l s

now w i t h new ESCA systems because t h e carbon c o n t a m i n a t i o n o v e r l a y e r i s t o o small, i f n o t absent. I t does n o t work e i t h e r when t h e r e a r e more t h a n one carbon spe-

c i e s i n t h e spectrum o f t h e C 1 s l i n e ; t h i s i s t h e case f o r s t u d i e s o f adsorbed carbon c o n t a i n i n g molecules (Defosse and Canesson, 1976). The second method c o n s i s t s i n e v a p o r a t i n g a g o l d l a y e r as t h i n as p o s s i b l e o n t o t h e s u r f a c e o f t h e sample, t h e 4 f d o u b l e t o f g o l d b e i n g used as a standard. T h i s t e c h n i q u e has been much d e s c r i b e d (Urch and Weber, 1974); n e v e r t h e l e s s , i t can be used p r o v i d e d some p r e c a u t i o n s a r e t a k e n . The g o l d l a y e r i s n o t evenly spread on t h e s u r f a c e b u t i t shows i s l a n d s (Brunner and Zogg, 1974) and t h e e l e c t r i c c o n t a c t between t h e sample and t h e s e i s l a n d s may n o t be as good as expected. Gold i s a b l e t o r e a c t w i t h some compounds, e s p e c i a l l y m e t a l l i c species, l e a d i n g t o t h e f o r m a t i o n o f a l l o y s which, i n t u r n , i n d u c e a s h i f t o f t h e Au 4 f l e v e l s (Friedman e t a l . ,

1973 a; Friedman e t a l . ,

1973 b ) . Gold e v a p o r a t i o n can

a c t as argon bombardment and a m o d i f i c a t i o n o f t h e s u r f a c e may occur, o r some s u p e r f i c i a l r e a c t i o n s such as r e d u c t i o n can show up. B e f o r e u s i n g t h i s g o l d d e c o r a t i o n t e c h n i q u e f o r t h e b i n d i n g energy s c a l e c a l i b r a t i o n , i t i s o f t h e g r e a t e s t i m p o r t a n c e t o r e c o r d a l l X.P.S.

s p e c t r a f o r q u a n t i t a t i v e purposes b e f o r e

and a f t e r g o l d e v a p o r a t i o n and t h i s p r a c t i c a l l y m u l t i p l i e s a p p r e c i a b l y t h e t i m e f o r r e c o r d i n g t h e s p e c t r a f o r one sample. Whenever p o s s i b l e , t h e l a s t method i s t h e b e s t i n my o p i n i o n .

A w e l l defined

l e v e l o f one element p r e s e n t i n t h e sample i s t a k e n as r e f e r e n c e and i t s b i n d i n g energy v a l u e i s a r b i t r a r i l y f i x e d . T h i s method i m p l i c i t e l y assumes t h a t , when s t u d y i n g v a r i o u s samples, one element i s p r e s e n t i n a l l o f them and t h e b i n d i n g energy v a l u e o f i t s c o r e l e v e l s does n o t b r i n g a b o u t any m o d i f i c a t i o n , which means t h a t t h e chemical environment o f t h i s element i s t h e same f r o m one sample t o an o t h e r . Working w i t h c l a y s i s an enormous advantage, s i n c e p r a c t i c a l l y a l l c l a y m i n e r a l s c o n t a i n s i l i c o n i n an oxygen t e t r a h e d r a l environment, and one can assume t h a t t h e b i n d i n g energy o f t h e S i 2p l e v e l i s p r a c t i c a l l y t h e same f o r a l l samples. I n t h i s case, p r e c i s i o n ’ i n b i n d i n g energy d e t e r m i n a t i o n s can be e s t i m a t e d a t 0.1 eV ( O g i l v i e and W o l b e r t , 1972). Moreover, when adsorbed compounds a r e i n v e s t i g a t e d , i t has been demonstrated t h a t a d s o r p t i o n does n o t b r i n g about any s h i f t i n t h e b i n d i n g e n e r g i e s o f t h e adsorbant l e v e l s (Brundle, 1974).

216 I n c o n c l u s i o n , i f t h e b i n d i n g energy s c a l e c a l i b r a t i o n remains a problem i n

X.P.S.,

w o r k i n g w i t h c l a y s p e r m i t s t h e use o f an i n t e r n a l s t a n d a r d and t h i s i s

t h e b e s t method f o r t h i s purpose. 9.2.5

P o s s i b i l i t i e s o f quantitative analysis.

B e f o r e c o n s i d e r i n g t h e v a r i o u s f a c t o r s t h a t a r e i n v o l v e d i n t h e i n t e n s i t y of peak, l e t us f i r s t examine t h e s e n s i t i v i t y of t h i s t e c h n i q u e . To b e g i n

an X.P.S.

w i t h , i t must be p o i n t e d o u t t h a t X.P.S.

i s n o t a b l e t o d e t e c t t r a c e s . The extreme

d e t e c t i o n l i m i t i s about 0 . 1 % o f t h e a n a l y z e d l a y e r , b u t , i n o r d e r t o deduce some i n f o r m a t i o n from an X.P.S.

spectrum, t,he s i g n a l - t o - n o i s e r a t i o must n o t

be t o o bad and i t i s r a t h e r d i f f i c u l t t o p u t f o r w a r d any c o n c l u s i o n s on t h e chemical environment o f s p e c i e s w i t h a c o n t e n t l o w e r t h a n 0.3 analyzed l a y e r . N e v e r t h e l e s s when w o r k i n g w i t h X.P.S.,

-

0.5

I

i n the

a d i s t i n c t i o n must be

made between t h e b u l k c o m p o s i t i o n and what i s s t u d i e d , namely t h e s u r f a c e compos i t i o n . I f t h e r e i s some s u p e r f i c i a l e n r i c h m e n t i n t h e elements o f i n t e r e s t , t h e d e t e c t i o n l i m i t can be as l o w as p.p.b. e t al.,

expressed as b u l k c o n t e n t r a t i o s ( H e r c u l e s

1973).

L e t us now examine what i n f l u e n c e s i n t e n s i t y , expressed as t h e i n t e g r a t e d s u r face a r e a of an X.P.S.

-

peak. T h i s i n t e n s i t y i s p r o p o r t i o n a l t o :

t h e p h o t o e l e c t r o n i c c r o s s s e c t i o n ux o f t h e c o n s i d e r e d l e v e l t h e mean f r e e p a t h Ax o f t h e e j e c t e d p h o t o e l e c t r o n s i n s i d e t h e sample t h e s o - c a l l e d l u m i n o s i t y Lx o f t h e system f o r t h e l i n e o f i n t e r e s t t h e c o n c e n t r a t i o n Cx o f t h e c o n s i d e r e d element i n s i d e t h e a n a l y z e d l a y e r of

t h e sample. The i n t e n s i t y Ix o f a l i n e x can b e w r i t t e n :

Ix = k ox X, Lx C x

k b e i n g a p r o p o r t i o n a l i t y c o n s t a n t , depending on t h e s p e c t r o m e t e r dhd i t s w o r k i n g conditions.

I f t h e various

CT

v a l u e s can be c a l c u l a t e d f o r t h e X-ray s o u r c e o f i n t e r e s t

( S c o t f i e l d , 1976), t h e v a l u e s o f k and Lx cannot be e a s i l y determined. I n t e n s i t i e s a r e used as i n t e n s i t y r a t i o s between t h e peak o f i n t e r e s t and a peak o f reference,

n o t e d r. The r a t i o R,

between t h e s e two v a l u e s i s equal t o :

I n t h i s e q u a t i o n , X i s a f u n c t i o n o f t h e k i n e t i c energy o f t h e c o n s i d e r e d electrons. I n t h e f i r s t approximation, Ax i s p r o p o r t i o n a l t o

(Carter e t a l . KX

217 1975), w h i c h means t h a t R x can be w r i t t e n as

L x a l s o depends upon t h e E

KX

value; from t h i s c o r r e l a t i o n , i t i s n o t possible

t o o b t a i n any i n f o r m a t i o n because t h e c o r r e c t r e l a t i o n s h i p i s f u n c t i o n o f t h e w o r k i n g mode o f t h e c o n s i d e r e d ESCA system. Over t h e r e c e n t y e a r s , some compilat i o n s have been made, which a r e v e r y u s e f u l . Wagner (1972) compared t h e i n t e n s i t y r a t i o s o f t h e l e v e l s issued from various s a l t s , t h e i n t e n s i t y o f the

F 1s

l i n e b e i n g t a k e n a r b i t r a r i l y as u n i t y and he has d e t e r m i n e d t h e r e l a t i v e u n i t a r y i n t e n s i t i e s o f v a r i o u s l e v e l s o f most c u r r e n t l y s t u d i e d elements. The same k i n d of work was a l s o done by Nefedov e t a l . ,

(1973, 1975, 1977). These a u t h o r s d i d

use t h e Na 1 s l e v e l s as r e f e r e n c e f o r t h e r e l a t i v e u n i t a r y i n t e n s i t i e s s c a l e . I n a l l t h e s e r e f e r e n c e s t h e e f f e c t o f t h e carbon c o n t a m i n a t i o n o v e r l a y e r was n e g l e c t e d . I f t h e a n a l y z e d s u r f a c e i s covered by such a l a y e r o f t h i c k n e s s d, t h e t r u e Rx r a t i o i s t r a n s f o r m e d i n t o a measured R I x r a t i o , the- r e l a t i o n s h i p between them b e i n g : Rlx=Rx

XIr

exp[

1

and A ' x b e i n g t h e mean f r e e p a t h s o f e l e c t r o n s i n s i d e t h e c o n t a m i n a t i n g

l a y e r . R e s p e c t i v e l y f r o m ( 6 ) , i t i s p o s s i b l e t o d e f i n e c l e a r l y when t h e contam i n a t i o n o v e r l a y e r can be n e g l e c t e d , namely :

-

when d i s equal t o zero, namely when t h e r e i s no c o n t a m i n a t i o n and, when 1 / X l r

-

1 / A I x i s n e a r l y equal t o z e r o , which means t h a t A\

cs

A;(.

L f one

assumes t h a t A ' v a l u e s behave i n t h e same way as r e a l A , t h e l a t t e r c o n d i t i o n b e i n g o b t a i n e d i f t h e p h o t o e j e c t e d e l e c t r o n s f r o m t h e r e f e r e n c e l i n e and t h e l i n e o f i n t e r e s t have t h e same k i n e t i c energy. T h i s case i s t h e most f a v o u r a b l e f o r q u a n t i t a t i v e a n a l y s i s purposes because i n t h i s case Ax # Ar and t h e sampling d e p t h i s t h e same f o r t h e two l i n e s o f i n t e r e s t . On t h e c o n t r a r y , e.g. EK,

if

>z E K ~ , t h e sampling d e p t h i s n o t e x a c t l y t h e same f o r t h e r and x l i n e s .

T h i s e f f e c t o f a s u p e r f i c i a l l a y e r was c l e a r l y shown by Defoss& e t a l . (1975). Working w i t h n i c k e l exchanged z e o l i t e s , t h e c r a c k i n g o f benzene on t h e s u r f a c e induces a carbon l a y e r w h i c h d i m i n i s h e s b y a f a c t o r o f 5 t h e r a t i o

R N i = ' N i 2p 3 / 2 / I S i

2p

A l l t h e f a c t o r s t h a t may i n f l u e n c e q u a n t t a t i v e d e t e r m i n a t i o n s by X.P.S.

were

s t u d i e d i n d e t a i l by Wagner ( 1 9 7 7 ) . A f t e r t h e s e g e n e r a l c o n s i d e r a t i o n s a b o u t t h e X.P.S.

t e c h n i q u e , l e t us now

2 18 examine what i n f o r m a t i o n has a l r e a d y been o b t a i n e d w i t h t h i s p o w e r f u l and v e r s a t i l e technique. 9.3

APPLICATIONS OF X.P.S. Since X.P.S.

TO CLAYS AND CLAY MINERALS.

can be used as an a n a l y t i c a l t o o l f o r q u a n t i t a t i v e s u r f a c e ana-

l y s i s as w e l l as f o r d e t e r m i n a t i o n o f t y p e o f bonding, i t i s d i f f i c u l t t o a r b i t r a r i l y separate t h e r e s u l t s obtained r e s p e c t i v e l y . A l l i n f o r m a t i o n i s contained i n t h e same spectrum and I have chosen t o examine s e p a r a t e l y s t a n d a r d a n a l y s i s o f b u l k m i n e r a l s and what i s t y p i c a l l y s u r f a c e a n a l y s i s , namely a d s o r p t i o n s t u d i e s . 9.3.1

Minerals analvsis

A t t e n t i o n must be drawn on t h e f a c t t h a t X.P.S.

does n o t e n a b l e us t o d i s t i n -

g u i s h between two c o o r d i n a t i o n numbers f o r t h e same c a t i o n w i t h t h e same n a t u r e o f l i g a n d s i n t h e c o o r d i n a t i o n sphere. I n s p i t e o f r e s u l t s p u b l i s h e d i n t h e e a r l y t i m e s of X.P.S.

(Nicholls e t al.,

1974; L i n d s a y e t a l . ,

1972), s e v e r a l a u t h o r s (Anderson and Swartz,

1973) have shown t h a t t h e r e i s no d i f f e r e n c e i n t h e b i n d i n g

energy v a l u e o f t h e A1 2p l e v e l i n an oxygen environment whatever i t s c o o r d i n a t i o n number : e i t h e r f o u r o r s i x . Keeping i n mind t h a t t h e e x a c t p o s i t i o n on t h e b i n d i n g energy s c a l e o f a p a r t i c u l a r l e v e l i s o n l y dependent on t h e r e a l charge o f t h e element o f i n t e r e s t , such r e s u l t i s n o t s u r p r i z i n g . I n c r e a s i n g t h e c o o r d i n a t i o n number o f A13+ f r o m f o u r t o s i x w i l l i n d u c e an i n c r e a s e i n t h e mean A1-0 bond l e n g t h and so t h e s t a t i s t i c a l e f f e c t o f t h e oxygen environment on t h e r e a l charge o f t h e c e n t r a l c a t i o n w i l l be t h e same. The r e s u l t s o b t a i n e d by Nicholls

e t a l . (1972) a r e r a t h e r n i c e l y e x p l a i n e d by L i n d s a y e t a l . (1973).

The d i f f e r e n c e i n b i n d i n g energy o f t h e A1 2p l e v e l ( 1 . 4 eV) between m i c r o l i n e (aluminum i n f o u r f o l d c o o r d i n a t i o n ) and A1203 (aluminum i n s i x f o l d c o o r d i n a t i o n ) i s a consequence o f t h e presence o f potassium i n m i c r o l i n e ; i t reduces t h e e l e c tron

-

a t t r a c t i n g a b i l i t y o f t h e oxygen atoms, so t h e r e a l charge o f A13+ i o n s

i s lowered w i t h r e s p e c t t o t h a t i n A1203 and t h e b i n d i n g energy o f t h e A1 2p l e v e l i s a l s o lowered i n m i c r o l i n e .

I f t h e c o o r d i n a t i o n number has no i n f l u e n c e on t h e b i n d i n g energy, on t h e c o n t r a r y , Urch and Murphy (1974) have i n t e r p r e t e d t h e d i f f e r e n c e s i n b i n d i n g e n e r g i e s o f A1 2p and A1 2s c o r e l e v e l s o f a s e r i e s o f a l u m i n o s i l i c a t e s m i n e r a l s as i n f l u e n c e d by t h e A1-0 bond l e n g t h s . These a u t h o r s concluded t h a t b i n d i n g energy i n c r e a s e s w i t h t h e A1-0 d i s t a n c e . When w o r k i n g w i t h ESCA on a l u m i n o s i l i c a t e s m i n e r a l s , one can g e n e r a l l y a d m i t t h a t t h e b i n d i n g e n e r g i e s o f c o r e l e v e l s o f t h e main c o n s t i t u e n t s , namely S i , A1 and 0 do n o t v a r y t o o much f r o m one sample t o an o t h e r . T h i s assumption was v e r i f i e d many t i m e s i n t h e l i t e r a t u r e b y s e v e r a l a u t h o r s on a l o t o f m i n e r a l s l i k e k y a n i t e , s i l l i m a n i t e and m u l l i t e (Anderson e t a l . ,

1974), k a o l i n i t e , c h l o r i t e

219 and i l l i t e (Koppelman and D i l l a r d , 1975) and 12 n a t u r a l s i l i c a t e s and a l u m i n o s i l i c a t e s (Carriere e t al.,

1977). T a k i n g t h e S i 2p l i n e as a s t a n d a r d , t h e A1 2s

and 0 1s l i n e s o f m o n t m o r i l l o n i t e do n o t show any m o d i f i c a t i o n i n b i n d i n g energy and i n t e n s i t y , whatever t h e charge b a l a n c i n g c a t i o n : Na+, Fez+, N i 2 + , Mn2+, Cu2+, Co2+, Sn4+. These r e s u l t s were o b t a i n e d f r o m a s t u d y o f c a t i o n exchanged montm o r i l l o n i t e s (Canesson e t a l . ,

1978).

About t h e b u l k chemical c o m p o s i t i o n o f s e v e r a l m i n e r a l s , Adams e t a l . , have demonstrated t h a t X.P.S.

(1977)

can p r o v i d e b u l k q u a n t i t a t i v e c o m p o s i t i o n o f a i r -

s t a b l e homogeneous s o l i d s w i t h an accuracy o f c.a.

5 %. From t h e i r c o m p i l a t i o n

o f a l o t o f p o l y c r i s t a l l i n e and m o n o c r i s t a l l i n e samples, t h e y concluded t h a t t h e r e i s no s u p e r f i c i a l s e g r e g a t i o n o f any o f t h e c o n s t i t u e n t s o f k a o l i n i t e , t a l c , m o n t m o r i l l o n i t e , l a p o n i t e , l e p i d o l i t e , c r y o l i t e and z e o l i t e . The o n l y dev i a t i o n f r o m b u l k c o m p o s i t i o n i s observed f o r m o n o c r i s t a l l i n e p h l o g o p i t e , which e x h i b i t s a h i g h e r c o n t e n t i n aluminum i o n s i n t h e analyzed r e g i o n t h a n i n t h e b u l k , even a f t e r r e p e a t e d c l e a v a g e s . The reason would be t h a t t h i s m i n e r a l cleaves i n r e g i o n s w i t h a h i g h aluminum c o n t e n t . More s u r p r i s i n g i s t h e l a c k o f d i s c r e pancy f o r Y z e o l i t e s , s i n c e , as i t was shown by s e v e r a l a u t h o r s , a d e p l e t i o n i n aluminum i o n s i s observed i n t h e f i r s t a t o m i c l a y e r s o f s y n t h e t i c Y z e o l i t e s (Tempere e t a1 . , 1977; Defosse e t a1

. , 1977).

S i m i l a r r e s u l t s were p u b l i s h e d by Koppelman and D i l l a r d ( 1 9 7 8 ) . These a u t h o r s found t h a t t h e S i / A l s u r f a c e r a t i o i s t h e same as i n t h e b u l k f o r k a o l i n i t e and c h l o r i t e . F o r i l l i t e , t h e y n o t e d a d e p l e t i o n i n potassium i n t h e analyzed l a y e r s i n c e t h e S i / K r a t i o deduced f r o m X.P.S.

i n t e n s i t i e s i s l o w e r t h a n t h a t determined

by more c l a s s i c a l a n a l y t i c a l methods. F o r t h e o t h e r elements (Mg, A1 and Fe), which a r e c o n s t i t u t i v e o f t h e t e t r a h e d r a l and o c t a h e d r a l l a y e r s , t h e S i / M e t a l r a t i o s were w i t h i n 6 % o f t h e b u l k chemical c o m p o s i t i o n . I t was suggested t h a t t h e observed d i s c r e p a n c i e s a r o s e because no c o r r e c t i o n f o r e l e m e n t a l depth i n t h e sample was made..Potassium i n t h e i n t e r l a y e r space i n i l l i t e i s a t a depth g r e a t e r t h a n o t h e r exposed c a t i o n s , r e d u c i n g t h e S i / K X.P.S. The c a t i o n t h a t has focused most X.P.S.

intensity ratio.

workers e f f o r t s i n t h e f i e l d o f n a t u r a l

m i n e r a l s , i s u n d o u b t e d l y i r o n . I t s chemical s t a t e (Fez+ o r Fe3+) was t e n t a t i v e l y s t u d i e d by Adams e t a l . (1972); t h e y were u n a b l e t o d i s t i n g u i s h between f e r r i c and f e r r o u s i r o n s p e c i e s i n t h e m i n e r a l s t h e y examined. T h i s i s n o t s u r p r i s i n g s i n c e t h e Fe 2p 3/2 l e v e l i s r a t h e r b r o a d and t h e i r o n c o n t e n t , e x c e p t i n some m i n e r a l s o f t h e mica f a m i l y i s r a t h e r l o w i n a l u m i n o s i l i c a t e s . The c o r r e s p o n d i n g X.P.S.

peaks a r e ill d e f i n e d , w i t h a l o w s i g n a l t o n o i s e r a t i o . Moreover, t h e

observed s h i f t s between Fez+ and Fe3+ i n p u r e i r o n o x i d e s a r e r a t h e r l o w (Mac I n t y r e and Z e t a r u k , 1977; Asami and Hashimoto, 1977). N e v e r t h e l e s s , Koppelman and D i l l a r d (1975) were a b l e t o d i s t i n g u i s h between Fez+ and Fe3+ i n i l l i t e . The comparison of t h e Fe 2p 3/2 l i n e i s s u e d f r o m n o n t r o n i t e (Fez+ s p e c i e s o n l y ) ,

220 c h l o r i t e (Fez+ s p e c i e s o n l y ) and i11 it e , a1 lowed t h e a u t h o r s t o decompose t h e Fe 2p 3/2 X.P.S.

p r o f i l e o f i l l i t e i n t o two i r o n s p e c i e s , namely Fe2+ and Fe3+.

The r e l a t i v e abundance o f Fe3+ as determined by t h e r e l a t i v e s u r f a c e a r e a o f t h e c o r r e s p o n d i n g X.P.S. mineral .

peak i s i n good agreement w i t h Mossbauer d a t a on t h e same

S t u c k i e t a l . ( 1 9 7 6 ) , s t u d y i n g n o n t r o n i t e and b i o t i t e , a l s o observed a s h i f t between t h e Fe 2p 3 / 2 s p e c t r a o f t h e s e m i n e r a l s . Upon r e d u c t i o n by e i t h e r h y d r a z i n e o r d i t h i o n i t e t h e Fe 2p l e v e l s o f n o n t r o n i t e e x h i b i t a broadening t o g e t h e r w i t h a s h i f t towards

l o w e r b i n d i n g e n e r g i e s . On t h e c o n t r a r y , o x i d a t i o n o f b i o -

t i t e by bromine induces a s h i f t towards h i g h e r b i n d i n g e n e r g i e s . W i t h such compound, t h e observed s p e c t r a a r e o f r a t h e r p o o r q u a l i t y and any c o n c l u s i o n on t h e r e l a t i v e q u a n t i t i e s o f t h e v a r i o u s i r o n s p e c i e s would be r a t h e r s p e c u l a t i v e .

Si2

P

C r u d e kaalinitc

___.x 'I i n c r e a s i n g

1

:time I I

I

0.8

I I

deferrating treatment

/

0.6

0.4 0.2 0.1

0:2

013

014

015'

si2p

F i g . 9.1. Behaviour o f t h e Fe 2p/Si 2p r a t i o v e r s u s t h e A 1 2p/Si 2p r a t i o f o r t h e YBi k a o l i n i t e a f t e r v a r i o u s c o n t a c t t i m e s w i t h ammonium o x a l a t e s o l u t i o n .

221 I r o n i n k a o l i n i t e i s a n o t h e r problem many c l a y r e s e a r c h e r s have t a c k l e d (e.g. see Angel and V i n c e n t (1978); Angel e t a l . (1977); H e r b i l l o n e t a l . (1976) and r e f e r e n c e s t h e r e i n ) . Working w i t h X.P.S.

on a p a r t i c u l a r t r o p i c a l s o i l k a o l i n i t e ,

namely t h e Y B i sample d e s c r i b e d by H e r b i l l o n e t a l . (1976), i t i s p o s s i b l e t o s t u d y t h e Fe 2p l e v e l s i n c e t h e i r o n c o n t e n t o f t h i s p a r t i c u l a r c l a y i s h i g h . Moreover, t h i s k a o l i n i t e can be c l e a n e d t o some e x t e n t by e x t r a c t i n g t h e c o a t i n g i r o n o x i d e by ammonium o x a l a t e , as a l r e a d y mentioned by H e r b i l l o n e t a l . (1976). On F i g u r e 9.1 t h e e v o l u t i o n o f t h e Fe 2p/Si 2p r a t i o i s p l o t t e d a g a i n s t t h e A1 2p/Si 2p r a t i o f o r v a r i o u s c o n t a c t t i m e s between t h e c l a y and t h e s o l u t i o n . The c u r v e c l e a r l y shows two d i s t i n c t p a r t s . I n t h e i n i t i a l s t a g e s o f t h e a t t a c k , i t can be n o t e d t h a t t h e Fe 2p/Si 2p r a t i o decreases, t h e A1 2p/Si 2p r a t i o r e -

m a i n i n g p r a c t i c a l l y c o n s t a n t . F o r l o n g e r c o n t a c t t i m e s , t h e two r a t i o s behave i n t h e same way. The c r u d e k a o l i n i t e p a r t i c l e s seem c o a t e d w i t h a h i g h i r o n and a l o w aluminum c o n t e n t g e l . A f t e r t h i s c o a t i n g has been d i s s o l v e d , t h e k a o l i n i t e c r i s t a l i s a t t a c k e d by t h e complexing s o l u t i o n and i r o n and aluminum i o n s a r e s i m u l t a n e o u s l y removed. These r e s u l t s c o n f i r m t h e f a c t a l r e a d y p o i n t e d o u t by E.P.R.,

t h a t a f r a c t i o n o f the t o t a l i r o n content o f kaolinites i s inside the

s t r u c t u r e o f t h e m i n e r a l , p r o b a b l y s u b s t i t u t i n g A13+ i o n s . From t h i s study, i t was n o t p o s s i b l e t o draw any c o n c l u s i o n a b o u t t h e o x i d a t i o n s t a t e o f i r o n s i n c e t h e X.P.5.

Fig.9.2.

peaks were t o o broad.

Geometrical c o n s i d e r a t i o n s f o r X-rays p h o t o e l e c t r o n s d i f f r a c t i o n .

222 W i t h p o l y c r i s t a l l i n e samples, as i t has been a l r e a d y p o i n t e d o u t , X.P.S.

i s not

a b l e t o d i s t i n g u i s h between t h e v a r i o u s p o s s i b l e environments o f a g i v e n i o n . Working w i t h s i n g l e c r y s t a l s , Evans e t a l .

(1979) have developed a method which a l l o w s some

comparisons between t h e v a r i o u s l o c a t i o n s i n s i d e t h e s t r u c t u r e . The s o - c a l l e d X-rays p h o t o e l e c t r o n s d i f f r a c t i o n i s based on t h e a n g u l a r d e p e n d e n c e o f t h e v a r i o u s i n t e n s i t y r a t i o s o f a w e l l o r i e n t e d sample. By r o t a t i n g t h e s i i l g l e c r y s t a l a l o n g t h e c a x i s (see f i g u r e 9 . 2 ) ,

the angle

Q

between t h e e x c i t i n g r a d i a t i o n a n d t h e a n a l y z e d

electrons being f i x e d b y t h e geometryof t h e spectrometer, i t i s p o s s i b l e t o vary the

e a n g l e c o n t i n u o u s l y f r o m 0 t o 90". The o u t g o i n g p h o t o e l e c t r o n s c a n b e d i f f r a c -

t e d by t h e atoms s u r r o u n d i n g t h e e m i t t i n g one and t h e n u m b e r o f c o l l e c t e d p h o t o e l e c t r o n s a t t h e e n t r a n c e s l i t o f t h e s p e c t r o m e t e r depends on t h a t a n g l e . I f two elements A and B a r e l o c a t e d i n s i d e t h e c r y s t a l l a t t i c e o n t h e same k i n d o f s i t e s , i . e . w i t h

t h e same number o f t h e same s u r r o u n d i n g i o n s , t h e A / B X.P.S. w i l l n o t v a r y when

e

peaks i n t e n s i t y r a t i o

changes. I f t h e two s i t e s a r e n o t e q u i v a l e n t , a dependence

oftheintensity ratio with

e w i l l be observed. I f , f o r example, t h e A e l e m e n t i s

d i s t r i b u t e d between two k i n d s o f s i t e s , one b e i n g e q u i v a l e n t t o t h e B environment, w h i l e t h e o t h e r i s n o t , an i n t e r m e d i a t e s i t u a t i o n w i l l be observed and a c c o r d i n g l y the i n t e n s i t y r a t i o w i l l bemodified. T a k i n g t h e i n t e n s i t y o f t h e S i 2p l i n e as r e f e r e n c e , Evans e t a l . (1979) s t u d i e d t h e l o c a t i o n o f t h e v a r i o u s c a t i o n s i n muscovite, l e p i d o l i t e , p h l o g o p i t e and b o t h n a t u r a l and Pb exchanged v e r m i c u l i t e . R e s u l t s o b t a i n e d by t h e X-rays p h o t o e l e c t r o n d i f f r a c t i o n patterns confirm those p r e d i c t e d by t h e s t r u c t u r a l determination

o f t h e isomorphous s u b s t i t u t i o n s i n m u s c o v i t e and l e p i d o l i t e . They a l s o

c o n f i r m t h e s u r f a c e e n r i c h m e n t i n A13+ i o n s i n t h e r e g i o n s o f f a c i l e cleavage f o r p h l o g o p i t e and v e r m i c u l i t e , aluminum b e i n g a l m o s t e x c l u s i v e l y i n f o u r f o l d coordination i n these regions. A s i m i l a r s t u d y on t i t a n i u m c o n t a i n i n g b i o t i t e and p h l o g o p i t e (Evans and

R a f t e r y , 1980) has shown t h a t t i t a n i u m i s more l i k e l y T i 3 + t h a n T i 4 + i n t h e s e micas. Moreover, a l l t h e t i t a n i u m i o n s a r e l o c a t e d i n o c t a h e d r a l s i t e s , n o t e x a c t l y e q u i v a l e n t t o t h o s e o f Mg and Fe i o n s s i n c e t h e a u t h o r s observed a var i a t i o n with the

e a n g l e o f t h e T i 2p/Mg

2s i n t e n s i t y r a t i o .

T h i s a p p l i c a t i o n i s , i n my o p i n i o n , one o f t h e most p o w e r f u l i n X.P.S.. U n f o r t u n a t e l y , i t i s l i m i t e d t o t h e s t u d y o f s i n g l e c r y s t a l s , o r , perhaps, i n t h e most f a v o u r a b l e cases, t o w e l l o r i e n t e d p o l y c r i s t a l l i n e samples. 9.3.2

AdsorDtion studies

Because i t s h i g h s e l e c t i v i t y f o r t h e s u r f a c e a n a l y s i s , X.P.S.

i s undoubtedly

an i d e a l t o o l f o r t h i s k i n d o f s t u d i e s . Working w i t h t h r e e c l a y m i n e r a l s , namely k a o l i n i t e , i l l i t e and c h l o r i t e , Koppelman and D i l l a r d have s t u d i e s t h e adsorpt i o n o f Fe3+ and C r 3 + (1975), N i 2 + and Cu2'

(1977),

Cr3+

(1980 a ) , two complexes

o f C r 3 + i o n s (1980 b ) and v a r i o u s c o b a l t i o n s (1978 b ) . The comparison o f b i n d i n g

223 e n e r g i e s between an adsorbed i o n c o r e l e v e l and t h e same i o n c o r e l e v e l i n s i d e t h e s t r u c t u r e , r e v e a l e d a g e n e r a l l o w e r i n g i n b i n d i n g energy o f t h e photopeak i s s u e d f r o m adsorbed s p e c i e s , i f a d s o r p t i o n i s performed a t l o w pH v a l u e s .

The r e l a t i v e

l o w e r i n g o f t h e b i n d i n g energy f o r adsorbed i o n s w i t h r e s p e c t t o t h e same l a t t i c e i o n s i s i n t e r p r e t e d as an i n c r e a s e i n e l e c t r o n d e n s i t y on t h e adsorbed metal due t o the negative surface p o t e n t i a l o f t h e various clays. For adsorption a t higher pH values, t h e X.P.S.

photopeaks o f adsorbed s p e c i e s a r e s i m i l a r i n b i n d i n g ener-

g i e s t o t h o s e o f t h e c o r r e s p o n d i n g h y d r o x i d e s . The q u a n t i t y o f adsorbed i o n s was found t o v a r y always i n t h e f o l l o w i n g o r d e r : c h l o r i t e > i l l i t e > k a o l i n i t e . The b e h a v i o u r o f Cu2+ i s q u i t e d i f f e r e n t . I n t h e adsorbed s t a t e , Koppelman and D i l l a r d (1977) n o t e d an i n c r e a s e i n t h e b i n d i n g energy o f t h e Cu 2p 3/2 l e v e l w i t h r e f e r e n c e t o t h e same l e v e l i n d i o p t a s e . T h i s i n c r e a s e i s a t t r i b u t e d t o t h e f o r m a t i o n o f Cu(OH)+ s u p e r f i c i a l s p e c i e s . R e s u l t s c o n c e r n i n g a d s o r p t i o n o f some m i n e r a l complexes such as [ C O ( H ~ O ) ~ I ~ + and [ C O ( N H ~ ) ~ ]on ~ +c h l o r i t e (Koppelman and D i l l a r d , 1978 b ) and [Cr(NH3)513+ and [ C r ( e t h y l e n e diamine) 313'

on c h l o r i t e , i l l i t e and k a o l i n i t e (Koppelman and

D i l l a r d , 1980 b ) l e a d t o t h e same c o n c l u s i o n t h a t t h e adsorbed s p e c i e s behave as h y d r a t e d c a t i o n s . C o n t a c t i n g c h l o r i t e w i t h s o l u t i o n o f [ C O ( N H ~ ) ~3' ] induces an i n c r e a s e i n t h e pH o f t h e s o l u t i o n t o g e t h e r w i t h a r e d u c t i o n o f C03'

species i n t o

h y d r a t e d Co2+ s p e c i e s . The l o s s o f n i t r o g e n can be p u t f o r w a r d by a decrease o f t h e

N ls/Co 2p 3 / 2 r a t i o . A d s o r p t i o n o f chromium complexes i s a l s o c h a r a c t e r i z e d by an i n c r e a s e i n t h e pH o f t h e s o l u t i o n and a c l a y c a t a l y z e d h y d r o l y s i s o f t h e amine complexes. S t u d y i n g t h e exchange p r o p e r t i e s o f b e i d e l l i t e , Adams and Evans (1979) deduced t h e c a t i o n exchange c a p a c i t i e s o f t h a t c l a y f r o m X.P.S.

i n t e n s i t i e s measurements.

The r e s u l t s t h u s o b t a i n e d a r e i n c l o s e r e l a t i o n w i t h t h e C . E . C . by chemical methods f o r Na+ and Ca2+ c a t i o n s . F o r K', an a p p a r e n t excess o f t h e C.E.C.

values determined

Pb2+ and Ba2+ t h e y n o t e d

w i t h r e s p e c t t o t h e chemical v a l u e . T h i s excess

i s i n t e r p r e t e d as a consequence o f a s t r o n g a d s o r p t i o n o f t h e s e c a t i o n s on t h e edges o r on t h e e x t e r n a l s u r f a c e o f t h e c l a y p a r t i c l e s . Working w i t h l a r g e o r g a n i c molecules such as p o r p h y r i n s , Canesson e t a l . (1978) were a b l e t o s t u d y t h e r e a c t i v i t y o f t h e s e molecules i n t h e i n t e r l a m e l l a r space o f m o n t m o r i l l o n i t e . Meso-tetraphenyl p o r p h y r i n (TPP) and m e s o - t e t r a p y r i d y l p h y r i n (TPyP) can undergo w i t h i n

por-

t h e c l a y s t r u c t u r e e i t h e r p r o t o n a t i o n o r com-

p l e x a t i o n . S i n c e t h e p o s i t i o n on t h e b i n d i n g energy s c a l e o f t h e N Is l e v e l i s s u e d from adsorbed molecules i s s e n s i t i v e t o t h e s u r f a c e environment (Defosse and Canesson,

1976), i t i s p o s s i b l e t o d i f f e r e n c i a t e p y r o l i t i c from t h e aza n i t r o g e n

atoms. Moreover, t h e i n t e n s i t y r a t i o between t h e two N 1s l e v e l s p e r m i t s a quant i t a t i v e d e t e r m i n a t i o n o f t h e v a r i o u s s p e c i e s . I t was concluded t h a t TPP undergoes p r o t o n a t i o n and t h a t t h e e x t e n t o f m e t a l l a t i o n i n t h e i n t e r l a m e l l a r space depends upon t h e n a t u r e o f t h e exchangeable c a t i o n , t h e s t a b i l i t y o r d e r b e i n g

224 Cu2+ > Co2+, Sn4+ > Fez+, Mn2'.

The s i t u a t i o n i s q u i t e d i f f e r e n t w i t h TPyP s i n c e

t h i s molecule can undergo p r o t o n a t i o n on t h e p y r i d y l s u b s t i t u e n t s c o m p a t i b l e w i t h m e t a l l a t i o n o f t h e p o r p h i n r i n g . T h i s i s observed w i t h Co2' and Cu2+ i n t e r l a y e r as c a t i o n s . W i t h o t h e r c a t i o n s , c o m p l e x a t i o n i s n o t complete and r e s u l t s o b t a i n e d by X.P.S. al.,

c o n f i r m t h o s e o b t a i n e d by U . V .

and v i s i b l e s p e c t r o s c o p y (Van Damme e t

1978).

From t h i s survey o f X.P.S. concluded t h a t X.P.S.

t e c h n i q u e s a p p l i e d on c l a y

m i n e r a l s , i t can be

emerges as a p r o m i s i n g t o o l .

Since s u r f a c e p r o p e r t i e s o f c l a y s and c l a y m i n e r a l s a r e o f m a j o r i m p o r t a n c e

X.P.S.

cannot f a i l t o occupy a more p r o m i n e n t p l a c e i n t h e f u t u r e f o r a b e t t e r un-

derstanding o f t h e various i n t e r a c t i o n s o f these minerals w i t h t h e external environment. The use o f X.P.S.

i s t h u s e x p e c t e d t o spread more b r o a d l y , n o t o n l y i n

t h e p a r t i c u l a r t o p i c o f c l a y m i n e r a l s , b u t a l s o i n a l l f i e l d s o f s c i e n c e f o r which surface p r o p e r t i e s a r e o f major i n t e r e s t .

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K a r i c k h o f f , S.W. and B a i l e y , G.W., 1976. P r o t o n a t i o n o f o r g a n i c bases i n c l a y w a t e r systems. Clays and C l a y Miner., 24: 170. Koppelman, M.H. and D i l l a r d , J.G., 1975. An ESCA s t u d y o f sorbed metal i o n s on c l a y m i n e r a l s . Marine Chemistry i n t h e c o a s t a l environment, ACS symposium s e r i e s , 18: 186-201. Koppelman, M.H. and D i l l a r d , J.G., 1977. A s t u d y o f t h e a d s o r p t i o n o f N i ( 1 1 ) and Cu (11) by c l a y m i n e r a l s . Clays and C l a y Miner., 25: 457-462. Koppelman, M.H. and D i l l a r d , J.G., 1978 a. The a p p l i c a t i o n o f X-ray p h o t o e l e c t r o n s p e c t r o s c o p y (X.P.S. o r E.S.C.A.) t o t h e s t u d y o f m i n e r a l s u r f a c e c h e m i s t r y . Proc. VIth I n t e r n . C l a y Conf., M o r t l a n d , M.M. and Farmer, V.C., e d i t o r s , E l s e v i e r , Amsterdam, pp. 153-166. Koppelman, M.H. and D i l l a r d , J.G., 1978 b . An X-ray p h o t o e l e c t r o n s p e c t r o s c o p i c (X.P.S.) s t u d y o f c o b a l t adsorbed on t h e c l a y m i n e r a l c h l o r i t e . J. C o l l o i d and I n t e r f . S c i . , 66: 345-351. Koppelman, M.H., Emerson, A.B. and D i l l a r d , J.G., 1980 a. Adsorbed C r (111) on c h l o r i t e , i l l i t e , and k a o l i n i t e : an X-ray p h o t o e l e c t r o n s p e c t r o s c o p i c s t u d y . Clays and Clay Miner., 28: 119-124. Koppelman, M.H. and D i l l a r d , J.G., 1 9 8 4 b. A d s o r p t i o n o f C r (NH3),j3+ and Cr(en)33t on c l a y m i n e r a l s and t h e c h a r a c t e r i z a t i o n o f chromium by X-ray p h o t o e l e c t r o n spectroscopy. Clays and C l a y Miner., 28: 211-216. Mc I n t y r e , N.S. and Z e t a r u k , D.G., 1977. X-ray p h o t o e l e c t r o n s p e c t r o s c o p i c s t u d i e s o f i r o n o x i d e s . A n a l . Chem., 11: 1521-1529.

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227 SUBJECT I N D E X A

164-166, 168

Absorption c o e f f i c i e n t

163

A b s o r p t i o n edge Accessory m i n e r a l s

A c i d base r e a c t i o n s

B

12

A c t i v a t i o n energy b y T.G.

B a c k s c a t t e r geometry i n Mossbauer

207

A c t i v a t i o n energy f o r i o n i c hop Adsorbed w a t e r i n . z e o l i t e s , external vibrations

206

frequencies i n Beidellite

222, 223

-

176 124, 125 174

106

13

Amphiboles

130

A n t i ferromagnetic o r d e r i n g

124

A r c h a e l o g i c a l samples, Wjssbauer spectra

199, 200

Benzidine

-

176, 177

Fe3+ i n t e r a c t i o n

181 180, 181

214 Biotite 123, 130, 132, 133, 180 B i o t i t e - Ht and F- NMR 102, 103 2 , 2 ' - b i p y r i d i n e complexes 176 Boehmite 20 Boehmite - NMR second moment 106 B r i 11ia n t green (dye a d s o r p t i o n ) 182 Brucite 22 i n ESCA measurements

176, 179, 180

A l u m i n o s i l i c a t e s - Na and A1 NFlR Alunite

...

alkylamnonium

B i n d i n g energy s c a l e c a l i b r a t i o n

106

NFlR second moment

A1 on c l a y s

-

Benzo-acetophenone ( i n d i c a t o r )

5, 16, 73

Allophane Allophane

n

Benzene - copper complexes

164

Aggregation o f c l a y p a r t i c l e s

Alkylammonium exchange

-

107

NMR

A d s o r p t i o n processes s t u d i e d

Akaganei t e , B-Fe203

21

Beidellite, interlayer cation

169, 171, 175

polyamines

125, 126

spectroscopy Bauxite

A d s o r p t i o n o f w a t e r and a l i p h a t i c

Ag on c l a y s

176

A u ( I I 1 ) on c l a y s

180

A c i d i t y , Lewis

213

Auger r e l a x a t i o n process

180

A c i d i t y , Bransted

by ESCA

31

microscopy)

165, 166 11, 14 164, 181

Absorption o f l i g h t

Atomic r e s o l u t i o n ( e l e c t r o n

132

B u l k chemical c o m p o s i t i o n of

219

minerals

Area o f peaks i n M s s b a u e r

120, 132 Arene r a d i c a l c a t i o n s 154, 155 Asbestos 17

Calcite

Assignments o f exchangeable c a t i o n s

Ca on c l a y s

spectroscopy

v i b r a t i o n a l modes

205

AtmosFhere c o n t r o l i n t h e r m a l analysis

16

C

Carbonates

13

176

11, 13, 16, 17

Carbon c o n t a m i n a t i o n o v e r l a y e r C a t i o n exchange c a p a c i t i e s

223

217

228 139, 141, 143, 144

Cations v i b r a t i o n i n

C o p p e r ( I 1 ) ESR

199, 200, 201, 202 Ceramics 18, 19, 21 Chamosi t e 123 Charge d e n s i t y d i s t r i b u t i o n 172

Copper e t h y l e n e d i a m i n e

smectites

Charge t r a n s f e r , LMCT band,

163, 169, 174, 176, 177,

MLCT band

178, 179 Chemical t r a n s p o r t

15

214

Chemical s h i f t s i n X.P.S. Chemical s t a t e o f i r o n

219, 220

i n minerals Chernykhite

- H+

Chloramben Chlorite

NMR

105

Crystal f i e l d s t a b i l i z a t i o n

182 132, 180

energy

163

45

microscopy)

C r y s t a l imaging ( e l e c t r o n

182

C h r y s o i d i n e (dyes on c l a y s )

microscopy)

17

Chrysoti l e

172

C r y s t a l growth ( e l e c t r o n

46, 47

C h l o r i t e ( e l e c t r o n microscopy) Chromophore

149, 172, 173, 174 Copper p y r i d i n e complexes 149 C u ( I 1 ) on c l a y s 169, 171, 175, 176, 177 Cu2+-hectorite 140, 155 C u z + - v e r m i c u l i t e 140 Cu(H20)E+ on c l a y s 169 Cr(bip):+ on c l a y s 178 C r on c l a y s 176 C r i t e r i a f o r thermal a n a l y s i s 6 comp 1exes

Cummingtonite

C l a s s i f i c a t i o n o f thermal analysis

Curie p o i n t

37 131 23

6, 8

techniques

C l a y - o r g a n i c complexes

17

0

164 Clay s u r f a c e 169, 173, 174 Cold neutrons 52 Colloidal particles 53, 54, 72 Combination band o f w a t e r 164, 168 C o ( I 1 ) on c l a y s 169, 170, 176

163, 169, 172, 177, 178, 179 20 Defect s t r u c t u r e 152 D e f i n i t i o n s i n thermal analysis 6, 8 D e h y d r a t i o n curves 5, 9

Complexation w i t h t r a n s i t i o n

D e r i v a t i v e thermodilatometry

Clay p a r t i c l e

metals Complexes

181

D e r i v a t i v e thermogravimetry

169, 171, 174, 183

Coordination

C o o r d i n a t i o n number by Plossbauer spectroscopy Coordination s h e l l

127-129 163

Copper d i m e t h y l s u l f o x i d e complexes

Diamagnetism

150

174

Dichroic e f f e c t (general )

125 164, 169

C o o r d i n a t i o n numbers and X.P.S.

18

7, 9, 11,

13

Conversion e l e c t r o n s i n M6ssbauer spectroscopy

d-d band

Decrepitation

195

Dichroism o f i n t e r l a y e r c a t i o n

218

200, 201, 202 Dicinnamalacetone ( i n d i c a t o r ) 180, 181 Dickite 17, 18 D i e l e c t r i c constant 22 Diethylenetriamine (dien) 175, 176 D i f f e r e n t i a l s c a n n i n g c a l o r i m e t r y 7, 17, 18 v i b r a t i o n a l bands

2 2 9

5, 7,

D i f f e r e n t i a l thermal a n a l y s i s 15-17, 20, 21, 22

D i f f r a c t i o n , k i n e t i c measurements

70

Ethylenediamine (en)

171, 172, 174

Evolved gas a n a l y s i s

7, 8, 12, 13,

14, 15

Diffraction o f light

164

Evolved gas d e t e c t i o n

Diffraction,

51, 53, 71, 72

Exchangeable c a t i o n s l o c a t i o n s

neutron

D i f f r a c t i o n o f X-rays

53, 56

D i f f u s e r e f l e c t a n c e spectrum

i n zeolites 164,

206

60, 63, 64, 68

F

63, 64, 68

Diffusion, rotational Diffusion,

water

60, 6 1

Diffusion coefficients D i f f u s i o n , jump

60, 64

t r a c e r measurements

Factors i n f l u e n c i n g i n t e n s i t y o f 68

D i f f u s i o n , two-dimensional

an X.P.S.

68

line

Fe on c l a y s

180, 181

Fe3+ on c l a y s

d(0Ol) Distance

171

6-Fe ODH

181, 182

Dynamics o f c r y s t a l l a t t i c e

196

216

176, 178, 179, 183

Diphenylcarbinol Dyes

203

E x t e r n a l v i b r a t i o n o f adsorbed

165, 166 Diffusion, f i c k i a n

7, 12, 13

151 124'

F e r r i anni t e

133

F e r r ih y d r i t e

23

Ferromagnetic o r d e r i n g F-factor

E

174

interferometry

59, 60, 63, 67-69

E l e c t r i c f i e l d gradient

58, 59, 61, 62

F o u r i e r t r a n s f o r m scanning

E l a s t i c incoherent structure factor

120, 122, 123, 126

Fourier transform

E f f e c t i v e n u c l e a r charge

124

192

Framework paramagnetic c e n t e r s

116, 117, 118

Free energy of exchange

151

172, 173

129, 130, 133 E l e c t r i c quadrupole i n t e r a c t i o n

114

G

E l e c t r i c quadrupole i n t e r a c t i o n i n Fe2+

117

Gibbsite

E l e c t r i c quadrupole i n t e r a c t i o n i n Fe3+

Gibbsite

117

NMR second moment

Glasses

Electron-beam s e n s i t i v i t y

44

E l e c t r o n b i n d i n g energy measurement E l e c t r o n escape depth

214

E l e c t r o n mean f r e e p a t h

216

E l e c t r o n microscope imaging 73

E l e c t r o n s p i n resonance

139

22

Glauconite

21

-

123, 130 H+

NMR

Goethite, a-Fe DOH

105 16, 18, 22, 124,

125, 126, 127, 129

181

Emanation thermal a n a l y s i s

Glauconite

106

31

E 1e c t r o n m i c r o s copy

Electron transfer

22

-

H

Hal 1o y s i t e 7, 14

5, 18

H a l l o y s i t e ( e l e c t r o n microscopy)

45

2 3 0 Halloysite

-

water NMR

H-bonding H-clay

94

I n f r a r e d spectroscopy

182, 183

microscopy)

180

Heating curves Hectorite

8

reflectance)

70, 178

-

Hectorite - fluoro

-

Hectorite

Ag

-

Na NMR

peaks

Benzene NNX

107

89, 93

Hematite, a-Fe203

23, 124, 125,

126, 127, 129 169

High-flux reactor

51

microscopy

205, 206 149

164, 169, 171

178, 183 Interlayer cation vibration

38

frequencies

107

202

I n t e r s t r a t i f i c a t i o n (electron

Hydrated metal i o n s Hydrobiotite

139

microscopy)

10

55

169

I r o n ESR

150, 151, 152

I r o n oxides

14, 15, 17, 23, 124, 125

126, 127, 129

I d e n t i f i c a t i o n o f Fe-containing

I r o n oxyhydroxides

129- 13 1 123-126

I d e n t i f i c a t i o n o f oxidation states of Fe by Mossbauer spectroscopy

I s o b a r i c mass-change d e t e r m i n a t i o n Isomer s h i f t 114 Isomer s h i f t and c o o r d i n a t i o n number

’

121-123 123

Isomer s h i f t i n Fez+

121-123

Isomer s h i f t i n Fe3+

121-123

Isomorphic s u b s t i t u t i o n

water NMR

8

I n v e r s e h e a t i n g - r a t e curves

I d e n t i f i c a t i o n o f m i n e r a l species by Mijssbauer spectroscopy

124, 125, 126,

127, 129

s i t e s i n s i l i c a t e s by Mossbauer

89

Mossbauer s p e c t r a

Image processing ( e l e c t r o n

128

164 125, 129

Isothermal mass-change d e t e r m i n a t i o n

40

Incoherent s c a t t e r i n g f u n c t i o n

58-60

J

180, 181

I n f r a r e d a c t i v e modes

6, 7, 9

Isomorphous s u b s t i t u t i o n , e f f e c t on

23

microscopy)

17

I n t r a l i g a n d t r a n s i t i o n band

I

spectroscopy

45

I n t e r s t r a t i f i e d minerals

Hydrogen neutron c r o s s - s e c t i o n

Indicators

K, Y and E z e o l i t e s

I n t e r l a m e l l a r space

31

microscopy

-

193 192

in

High v o l t a g e e l e c t r o n

Illite

Interferogram Int e r f e rome t e r

51, 71, 72

I n t e r l a m e l l a r metal complexes

High-resolution-electron

Ilmenite

117, 118

I n t e r c a l a t i o n complexes

I n t e r i o n i c vibrations o f cations

Hexaquo complexes

Illite

165

I n t e n s i t i e s o f Mossbauer a b s o r p t i o n

H e c t o r i t e - water NMR

Humin

40

I n t e g r a t i o n sphere ( d i f f u s e

5, 7, 15

H e a t i n g - r a t e curves

54

Instrumental resolution (electron

195

Jahn-Teller e f f e c t

169, 170, 180

9

2 3 1 Janus green (dye)

182

Plet eor it e s

21

Method o f q u a s i - i s o t o p i c s u b s t i t u t i o n 196

K

Kaolinite

Mg on c l a y s

5, 17, 18, 123, 152,

Mica

Mica ( e l e c t r o n microscopy)

178, 180

-

Kaolinite

NMR

106

M i ca-montmori 11o n i t e

K a o l i n i t e - water NMR Kerogen

179

10, 17, 20

89

Mn2+-hectorite

22

Kimberl it e

145, 148

Mn2+-montmori 1 l o n i t e

Keying i n phenomenon

Mn2+ m o b i l i t y

178

23

145

Mn on c l a y s

5, 10, 18, 123, 130

132, 180

178

M o n t m o r i l l o n i t e , Ca2+

L a t t i c e t r a n s i t i o n metal i o n s

178

L a t t i c e v i b r a t i o n s i n micas L e p i d o c r o c i t e , y-Fe203

198 124, 125

170, 183

Ligand f i e l d s t r e n g t h

66, 69, 71

Montmori 1 l o n i t e , i n t e r 1 ayer c a t i o n frequency i n

...

200

Montmorillonite, Li+

169, 173, 175, 176, 177

Limestone

145

Montmorillonite 169

Lattice sites

Ligand

176

Mn2+-vermicul it e

L a t t i c e oxygen

Ligand f i e l d

145

147

Mn*+-nontronite

L

41, 43, 46

17

172

70, 72

Montmori 1l o n i t e , Mg2+

66, 67

P l o n t m o r i l l o n i t e , Na+

70-72

Montmori 1 l o n i t e , Na+-deuteropyridine

21

M o n t m o r i l l o n i t e , p y r i d i n e on,

L i n e a r image approximation ( e l e c t r o n microscopy)

Montmori 1 l o n i t e - w a t e r NMR 36

L i n e shapes o f Mossbauer s p e c t r a L i n e w i d t h i n Nossbauer spectroscopy

113 - 137

Mdssbauer spectroscopy and mineral a l t e r a t i o n reactions

114, 119, 120, 121

L o r e n t z i a n l i n e shape

119

72 89, 94

Mossbauer spectroscopy 119

131-132

Mossbauer spectroscopy i n q u a n t i t a t i v e analysis

126, 127

M o b i l i t y o f i n t e r l a y e r metal i o n s

Loss tangent

19, 22

Luminescence

165, 178, 180

Mobility o f nitroxides

156

Mu 1t i s 1ice met hod ( e l e c t r o n microscopy)

ri

Muscovite

41 123, 179, 180

Magnetic h y p e r f i n e i n t e r a c t i o n 114, 118, 124 Magnetite, Fe304 Manganese ESR Manganese oxides Marble

N

23, 124 145, 148

Na on c l a y s

Maxwell i a n energy spectrum

176

Neutron d i f f r a c t o m e t e r

17

21

Neutron d i f f r a c t i o n 52

70

56-58 164

145

2 3 2 ldeutron f l u x d i s t r i b u t i o n s Neutron

-

52

nucleus i n t e r a c t i o n

52

51 - 75

Neutron s c a t t e r i n g

: k u t r o n s c a t t e r i n g coherent

51, 53

Neutron s c a t t e r i n g e l a s t i c

53,

Organic m a t t e r

14

Order o f r e a c t i o n

12

Organic r a d i c a l ESR

154

Overtone ( v i b r a t i o n )

164, 168

56-58, 63 Neutron s c a t t e r i n g i n c o h e r e n t

53, 56

Neutron s c a t t e r i n g i n e l a s t i c

53, 57

58 Neutron s c a t t e r i n g q u a s i - e l a s t i c 54, 58-60,

51, 51, 53

169, 174, 176, 184

Ni(en)3+ on c l a y s

154, 155, 156

Nomenclature i n thermal a n a l y s i s

6-8

197

125

-

N.M.R.

- C13 spectra

N.M.R.

-

N.M.R.

- Electron-nucleus c o u p l i n g

C o r r e l a t i o n times

85, 88,

89, 94 107

Doublet s p l i t t i n g

82, 88,

Phlogopite

complexes

H+ and

Phonons

F-

183 183

88, 89, 93,

178, 180

P h o t o e l e c t r o n i c cross s e c t i o n

216

Polyamines

169, 171

182, 183, 223, 224

Portlandit e

22

77, 8 1

Proton a s s o c i a t i o n t i m e Proton t u n n e l i n g 85,

Pyrophyllite

174

51

chromatography-mass

14

spectrometry Pyroxenes

70

13

Pyrolysis-gas 0

213

22

Pulsed neutron sources Pyrite

89, 94, 107

46

53, 54, 72

Pressure l i m i t a t i o n o f X.P.S.

Spin-lattice relaxation

180

Photo-chemical s u b s t i t u t i o n

81, 106

18

168 164, 166, 167

Photo-chemical chemistry

Principles

Optical electronegativity

102, 103

Photo-chemical physics

Second moment

Opal

NMR

54

P o r p h y r i ns

94, 102

19, 21, 22

176

128, 132

-

Pores s i z e s

84, 102

O i l shale

180, 181

180, 182

Polytypes ( e l e c t r o n microscopy)

93, 102

Local o r d e r

Pesticides

Photo-chemical c a t a l y t i c p r o p e r t i e s

54, 68, 70, 164 N.M.R.

N.M.R.

e f f e c t on

Photoacoustic spectroscopy

Nuclear magnetic resonance (N.M.R.)

-

-

Mossbauer spectrum

Photoacoustic spectrometer

123, 128, 130, 174

Normal coordinates i n micas

N.M.R.

71

Phlogopite

157

N.M.R.

51

Particles orientation

1,lO-Phenanthroline

174, 175

N i t r o x i d e s p i n probes

-

5, 10, 17, 180

P-Dimethylamino-azobenzene

72, 73 N i ( I 1 ) on c l a y s

N.M.R.

Palygorskit e

P a r t i c l e s accelerators Particle size

66-71

Neutron scattering, small-angle

Nontronite

P

5 130, 131, 132, 133

233

Q Quadrupole s p l i t t i n g

114, 116-118

132, 133 Quadrupole s p l i t t i n g and coordination

number 128 Quadrupole s p l i t t i n g in Fez+

117,

1 2 2 , 132 Quadrupole s p l i t t i n g in Fe3+

117,

1 2 2 , 132 Quadrupole s p l i t t i n g v a r i a t i o n s with temperature 132 Quantum y i e l d 178 Quartz 14, 16 Quartzite 20 Quasi-elastic broadening curve 60 Quenching (luminescence) 178

R Racah's i n t e r e l e c t r o n i c repulsion parameter 174, 184 Raman spectroscopy Random walk method Reaction k i n e t i c s

54 62 11, 17

21 Refractories Reciprocal l a t t i c e vector 56, 57 Reciprocal space 56 Recoil-free f r a c t i o n 120, 122, 123 126 Redox reactions

164, 181 Reflection, specular r e f l e c t i o n 164, 165 Refraction of l i g h t 164 Relationship between a c t i v a t i o n energy f o r c a t i o n i c hop and vibration frequency 207 Relationship between cation s e l f d i f f u s i o n and vibration 208, 209 Relaxation e f f e c t s 119-120 Residence t i me 68, 69

Resolution function (neutron scattering) 58 Rhodamine B (dye) 182 Rotational c o r r e l a t i o n time 60 Ru(bip)i+ on clays 176, 178

S

Safranine (dye)

182

Sample preparation f o r f a r infrared spectroscopy 194 Sampling depth (ESCA) 214, 217 Scattered wave amplitude 52, 53 S c a t t e r i n g cross-section coherent 54,55 S c a t t e r i n g cross-section incoherent 54-56 S c a t t e r i n g length coherent 54, 55 S c a t t e r i n g length incoherent 54, 55 S c a t t e r i n g vector 56 Scherzer defocus ( e l e c t r o n microscopy 37 Schists 13 Schuster-Kubel ka-Munck equation 166 Selection r u l e s (neutron s c a t t e r i n g ) 54 S e l f - c o r r e l a t i o n function (incoherent neutron s c a t t e r i n g ) 58, 59, 62 S e n s i t i v i t y of X.P.S. 216 Sepiolite 10 S e p i o l i t e ( e l e c t r o n microscopy) 45 Serpentine ( e l e c t r o n microscopy) 45 Shales 13 Shear modulus 19 Silica 164 Simultaneous techniques i n thermal 8 , 9 , 10, 12, 13, 14, ana 1ys i s 15, 16, 18, 19, 21, 22 Small-angle s c a t t e r i n g (neutron) 51, 53, 72, 73 Small-angle s c a t t e r i n g (X-rays) 72 Smectite 18, 21, 68, 151, 163, 178

2 3 4 Smectites, redox r e a c t i o n s Soils

131, 132

Thermodilatometry Thermoelectric power

23

22

Thermoelectrometry

6, 7, 22, 23

10, 14

Solid-state transitions Solid-state reactions

Specimens i n e l e c t r o n microscopy Spin magnetic moment

6, 7, 8, 9-12,

Thermogravimetry

17

11, 17

Soluble s a l t s

7, 18, 19

42

Thermoluminescence

21, 23

Thermomagnetometry

23

Thermomechanical measurement

53

Spin-orbit coupling

170

Thermomicroscopy

Spin p a i r i n g energy

174

Thermoparticulate a n a l y s i s

microscopy) Stereospecifi c i ty

8

21 7, 20

T h i n s e c t i o n n i n g ( e l e c t r o n microscope

Structure resolution (electron microscopy)

7, 21

Thermorefractometry Thermosonimetry

178

7, 14, 15 21

T hermopt omet r y

45

S t a n d a r d i z a t i o n i n thermal a n a l y s i s

7, 19

21

Thermophotometry

Stacking sequences ( e l e c t r o n

13, 18

spec imen )

38

44

Study o f surfaces

125, 126

T i m e - o f - f l i g h t spectrometers

57, 58, 66

Surface a c i d i t y

180

Torsional b r a i d analysis

19

Surface charging e f f e c t Surface oxygen

Transfer function (electron

214

microscopy)

164, 170, 172, 181, 183

~i

Surface p r o p e r t i e s ( X . P . S . )

214

Symbols i n thermal a n a l y s i s

8

-

TI*

35

transition

164, 169, 176, 177, 183

n

-f

n* t r a n s i t i o n

165, 169, 183

T r a n s i t i o n metals

163, 164, 169, 171, 178, 183

T

Triphenylcarbinol

Temperature programmed d e s o r p t i o n

14

Tetraethylenepentamine ( t e t r e n )

T r i p l e - a x i s spectrometers ( n e u t r o n )

U

172, 174

T h e o r e t i c a l expression f o r i n t e r l a y e r c a t i o n frequency

203

Thermal gas t i t r i m e t r y

13

Thermal neutrons

52

58

175,

176 Tetragonal d i s t o r t i o n

180, 181

U l t r a v i o l e t and v i s i b l e l i g h t spectroscopy UV-vis-N.I.R.

Thermally s t i m u l a t e d c o n d u c t i v i t y

163- 189

spectroscopy

163, 164,

178

22, 23 Thermally s t i m u l a t e d c u r r e n t

22

v

Thermally s t i m u l a t e d d e p o l a r i z a t i o n 22

Vanadium (neutron c r o s s - s e c t i o n )

Thermal v o l a t i l i z a t i o n a n a l y s i s Thermoacousti metry

7, 20, 2 1

12

Vanadyl ESR

55

142, 146, 147, 148

Vanadyl i o n m o b i l i t y

147

2 3 5 V02+-hectori t e

142, 146, 148

V a r i a t i o n s w i t h temperature o f Mossbauer s p e c t r a

132

Vermiculite

132, 179, 180

V e r m i c u l i t e Ca2+

66, 67, 69

V e r m i c u l i t e Co2+

72

Vermiculite, i n t e r l a y e r cation frequencies

200, 201

Vermicul ite-water N M R

89, 93, 106

V i b r a t i o n a l modes (neutron

52

scattering)

V i b r a t i o n a l modes o f potassium i n micas

197, 198

w 51, 61,

Water (neutron s c a t t e r i n g ) 66-71 Water (UV and v i s i b l e l i g h t spectroscopy)

168, 169, 171,

172, 114, 175, 178 Weakly s c a t t e r i n g o b j e c t ( e l e c t r o n microscopy)

33

X X.P.S.

fundamental r e l a t i o n s h i p s

212

X-rays fluorescence process

213

X-rays p h o t o e l e c t r o n s d i f f r a c t i o n

222

Z

Zeol it e s

14, 170

Z e o l i t e , exchangeable c a t i o n s locations i n

...

203

Z e o l i t e , s i t e I1 c a t i o n frequency

207

Z e o l i t e , s i t e I11 c a t i o n frequency 207

Zn on c l a y s

176