Structural transformation of 2:1 dioctahedral layer silicates during dehydroxylation-rehydroxylation reactions.

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Structural transformation of 2:1 dioctahedral layer

silicates during dehydroxylation-rehydroxylation

reactions.

Fabrice Muller, Victor A. Drits, Alain Plançon, Jean-Louis Robert

To cite this version:

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Clays and Clay Minerals, Vol. 48. No. 5, 572-585. 2000.

S T R U C T U R A L T R A N S F O R M A T I O N OF 2:1 D I O C T A H E D R A L L A Y E R

S I L I C A T E S D U R I N G D E H Y D R O X Y L A T I O N - R E H Y D R O X Y L A T I O N

R E A C T I O N S

FABRICE MULLER, VICTOR DRITS, t ALAIN PLAN~ON, AND JEAN-LOuIS ROBERT 1STO, CNRS-University of Orlrans, 1A rue de la Frrollerie. 45071 Orldans, Cedex 2, France

Geological Institute of the Russian Academy of Sciences, Pyzhevsky per.7, Moscow, Russia A b s t r a c t - - T h e structural transformation of dioctahedral 2:1 layer silicates (illite, montmorillonite, glauconite, and celadonite) during a dehydoxylation-rehydroxylation process has been studied by X-ray diffraction, thermal analysis, and infrared spectroscopy. The layers of the samples differ in the distribution of the octahedral cations over the cis- and trans-sites as determined by the analysis of the positions and intensities of the 11l, 02l reflections, and that of the relative displacements of adjacent layers along the a axis (c cos ~3/a), as wei1 as by dehydroxylation-temperature values. One illite, glauconite, and celadonite

consist of trans-vacant (tv) layers; Wyoming montmorillonite is composed of cis-vacant (cv) layers, where-

as in the other illite sample ta, and cv layers are interstratified. The results obtained show that the rehydroxylated Al-rich minerals (montmorillonite, illites) consist of tv layers whatever the distribution of octahedral cations over cis- and trans-sites in the original structure. The reason for this is that in the dehydroxylated state, both tv and cv layers are transformed into the same layer structure where the former trans-sites are vacant.

The dehydroxylation of glauconite and celadonite is accompanied by a migration of the octahedral

cations from former cis-octahedra to empty trans-sites. The structural transformation of these minerals

during rehydroxylation depends probably on their cation composition. The rehydroxylation of celadonite preserves the octahedral-cation distribution formed after dehydroxylation. Therefore, most 2:1 layers of celadonite that rehydroxylate (--75%) have cis-vacant octahedra and, only in a minor part of the layers, a reverse cation migration from former trans-sites to empty octahedra occurred. In contrast, for a glauconite sample with a high content in WA1 and VJA1 the rebydroxylation is accompanied by the reverse cation migration and most of the 2:1 layers are transformed into tv layers.

Key W o r d s - - C a t i o n Migration, Celadonite, Cis-Vacant Octahedra, Dehydroxylation, Glauconite, Illite,

Rehydroxylation, Smectites, Structure, Trans-Vacant Octahedra.

I N T R O D U C T I O N T h e s t u d y o f clay m i n e r a l s b y t h e r m a l a n a l y s i s h a s a l o n g h i s t o r y d u r i n g w h i c h a r i c h e m p i r i c a l m a t e r i a l h a s b e e n c o l l e c t e d . D i o c t a b e d r a l s m e c t i t e s a n d finely d i s p e r s e d m i c a s are c h a r a c t e r i z e d b y a w i d e r a n g e o f d e h y d r o x y l a t i o n t e m p e r a t u r e s ( B r i n d l e y , 1976; G r i m e t al., 1951; G r i m , 1968; G u g g e n h e i m , 1990; G u g g e n - h e l m a n d K o s t e r v a n G r o s s , 1992; H e l l e r - K a l l a i e t al., 1962; K o s t e r v a n G r o s s a n d G u g g e n h e i m , 1987, 1990; M a c k e n z i e , 1957, 1982; T s i p u r s k y et al., 1985). It is also r e m a r k a b l e t h a t t h e s e d e h y d r o x y l a t e d m i n e r a l s c a n r e g a i n O H g r o u p s after h e a t i n g in w a t e r v a p o r ( M a c k e n z i e , 1957; G r i m , 1968).

T h e m o s t reliable s t r u c t u r a l results h a v e b e e n ob- t a i n e d for a l u m i n o u s m i n e r a l s , W a r d l e a n d B r i n d l e y ( 1 9 7 2 ) a n d U d a g a w a et al. ( 1 9 7 4 ) d e t e r m i n e d the de- h y d r o x y l a t e d s t r u c t u r e for p o w d e r e d p y r o p h y l l i t e a n d m u s c o v i t e . P y r o p h y l l i t e a n d m u s c o v i t e c o n s i s t o f t r a n s - v a c a n t (tv) 2:1 layers. T h e i r d e h y d r o x y l a t e d s t r u c t u r e s are s i m i l a r to t h e i r o r i g i n a l f o r m , b u t c o n t a i n f i v e - c o o r d i n a t e d A1 c a t i o n s i n s t e a d o f s i x - c o o r d i n a t e d ones. G u g g e n h e i m e t al. (1987), K o s t e r v a n G r o s s a n d G u g g e n h e i m (1987, 1990), a n d G u g g e n h e i m ( 1 9 9 0 ) u s e d a d e t a i l e d c r y s t a l l o - c h e m i c a l a p p r o a c h to c o n s i d -

er the d y n a m i c s o f the d e h y d r o x y l a t i o n p r o c e s s o f alu- m i n o u s d i o c t a h e d r a l 2:1 l a y e r silicates a s s u m i n g t h a t m o n t m o r i l l o n i t e s , b e i d e l l i t e s , a n d illites h a v e the s a m e s t r u c t u r e o f 2:1 layers as p y r o p h y l l i t e a n d m u s c o v i t e . H o w e v e r , it h a s b e e n f o u n d b y o b l i q u e - t e x t u r e e l e c t r o n d i f f r a c t i o n a n d the s i m u l a t i o n o f X - r a y d i f f r a c t i o n ( X R D ) p a t t e r n s t h a t m o s t m o n t m o r i l l o n i t e s as w e l l as s o m e illites a n d interstratified i l l i t e - s m e c t i t e s (I-S) are c i s - v a c a n t ( c v ) or h a v e c o e x i s t e n c e o f tv a n d e v l a y e r s ( T s i p u r s k y a n d Drits, 1984; Z v y a g i n e t al., 1985; Drits e t al., 1996, 1998; R e y n o l d s a n d T h o m p s o n , 1993; M c C a r t y a n d R e y n o l d s , 1995; C u a d r o s a n d Altaner, 1998). T h e d e r i v a t i v e t h e r m a l g r a v i m e t r i c ( D T G ) c u r v e s o f m o s t m o n t m o r i l l o n i t e s h a v e o n e e n d o t h e r - m i c p e a k n e a r 700~ w h e r e a s b e i d e l l i t e s a n d illites d e h y d r o x y l a t e n e a r 550~ I-S are o f t e n c h a r a c t e r i z e d b y t w o e n d o t h e r m i c r e a c t i o n s n e a r 5 0 0 a n d 700~ ( M a c k e n z i e , 1957; G r i m , 1968). In c v l a y e r s t h e O H - O H e d g e is not s h a r e d a n d thus O H - O H f o r m s a c o m - m o n e d g e to o c c u p i e d a n d v a c a n t c i s - o c t a b e d r a . T h e O H - O H e d g e l e n g t h is c o n s i d e r a b l y l o n g e r t h a n the l e n g t h o f the s h a r e d O H - O H e d g e o f tv layers. Drits et al. ( 1 9 9 5 ) a s s u m e d t h a t t h e r m a l e n e r g y n e e d e d for a p r o t o n to j u m p to the n e a r e s t O H g r o u p to f o r m a w a t e r m o l e c u l e s t r o n g l y d e p e n d s o n the d i s t a n c e be-

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Vol. 48, No. 5, 2000 Dehydroxylation-rehydroxylation of 2:1 dioctahedral layer silicates 573

tween the nearest OH groups. Therefore c v montmo-

rillonite requires a higher dehydroxylation energy than tv illite because of the longer O H - O H distance. These authors also showed that, contrary to tv layers, the

dehydroxylation of c v aluminous 2:1 layers occurs in

two stages. During the first stage the adjacent OH groups are replaced by a single residual oxygen atom, Or, and the A1 cations, which originally occupied c i s - and t r a n s - s i t e s become, respectively, five- and six-coordinated. During the second stage, the A1 cations migrate from the former t r a n s - s i t e s to vacant pentagonal prisms. As a result, the dehydroxylated structures of

the former c v and tv aluminous layers are similar.

Concerning the dehydroxylation of Fe3+-rich dioctahedral 2:1 phyllosilicates (glauconites, celadonites, and nontronites), there are two contradictory interpre- tations. Heller-Kallai and Rozenson (1980) and Roz- enson and Heller-Kallai (1980) concluded that there is no cation migration during dehydroxylation whatever the cation composition. They assumed that in Fe3*-rich 2:1 phyllosilicates, Fe 3~ occupies both c i s - a n d t r a n s - octahedra. However, the structural study of nontronites, glauconites, and celadonites by diffraction meth- ods has shown that these minerals always consist of

tv 2:1 layers (Tsipursky and Drits, 1984, Sakharov et

al., 1990, Drits e t al., 1997, Mancean et al., 2000). In

addition, Tsipursky e t aI. (1985) showed that octahe-

dral cations migrate from cis- to t r a n s - s i t e s in the dehydroxylated Fe3--rich dioctahedral 2:1 layer silicates, assuming that, as in the case of the aluminous varieties, Or locates midway between two closest Fe 3+ with the same z coordinate. As a result, Fe 3+ cations, which formerly occupied the c i s - s i t e s , become six-coordinat-

ed after migration. Muller et al. (2000) studied the

dehydroxylation process of several Fe-rich minerals using XRD. The diffraction patterns were simulated using the C A L C I P O W computer program designed for the simulation of the intensities diffracted by layer structures containing different kinds of layers and stacking faults (Planqon, 1981). Several models were calculated, differing by the site occupancies and location of residual oxygen anions. An occupancy of the former vacant t r a n s - s i t e s and the location of the residual oxygens in one of the two possible positions of the former OH groups yields the best fit between experimental and calculated patterns, confirming the migration of Fe 3" cations for the dehydroxylated celadonites and glauconites. The Fe 3- cations in the former t r a n s - and c i s - s i t e s preserve their five-coordination and the location of the residual anions provides structural stability for glauconite and celadonite dehydrox- ylates.

In contrast to the study of dehydroxylation reactions, there are only a few studies devoted to the re- hydration of dehydroxylated dioctahedral 2:1 phyllosilicates having different original structure and com-

position (Mackenzie, 1957; Grim, 1968; Emmerich e t

al., 1999). Montmorillonites with an endotherrnic peak near 700~ can regain their initial hydroxyls by cool- ing in the presence of water vapor. However, the D T G curves of these rehydroxylated montmorillonites have either one peak near 600~ or two peaks near 500 and

650~ respectively. Dehydroxylated beidellites and il-

lites can also be rehydroxylated. In this case, the loss of hydroxyls of the rehydroxylated samples in D T G analysis occurs at temperatures near the dehydroxylation temperature of the original samples. An expla- nation to the difference in thermal behavior of rehydroxylated montmorillonites and rehydroxylated beidellites and illites was proposed by Drits e t al. (1995) where they suggested that original c v montmorillonites can be transformed into tv rehydroxylates. This hy- pothesis was confirmed experimentally for smectites (Emmerich e t al., 1999), but not for illites. The structure of rehydroxylated Fe-rich dioctahedral 2:1 layer silicates has not been examined.

In this paper, XRD, D T G analysis, and infrared (IR) spectroscopy was used to study natural (N), dehydroxylated (D), rehydoxylated (R), and de-rehydroxylated (DR) dioctahedral 2:1 clay minerals consisting of tv a n d c v or interstratified t v / c v layers of different cation compositions. The fundamental method for the study

of the polymorphous transformation of c v layers into

tv layers (or v i c e v e r s a ) is XRD. Thermal gravimetric (TG) analysis and IR were used to complement the interpretation of the XRD data.

E X P E R I M E N T A L

Two illites (identified below as tv and c v l t v ) , a Wy- oming montmorillonite, a glauconite, and a celadonite were studied. Structural formulae and references where these samples are described are presented in Table 1.

Powder X R D patterns were recorded using MoKet radiation with an I N E L CPS 120 diffractometer equipped with a curved position-sensitive detector. This detector permits a simultaneous recording of the X R D pattern in a 20 interval from 6 to 30 ~ The sample holder was a Lindemann glass tube with a 1-mm di- ameter. The diffusion background arising from the empty tube was subtracted in the experimental X R D patterns.

The unit-cell parameters were determined by using the following procedure. The values of b were determined by measuring d(060) values. For mica samples, the positions of 003 reflections permitted the determination of the d(001) = c sin [3 values, whereas the 113 and 112 reflections were used to estimate a and

[3 using well-known relationships between h k l indices,

d ( h k l ) , and the monoclinic unit-cell parameters (Sak-

harov et al., 1990; Muller et al., 2000). This procedure

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574 Muller, Drits, Plangon, and Robert Clays and Clay Minerals

Table 1. Structural formula of samples under study calculated per

Ol0(OH)2.

[llite c W t v W y o m i n g Glauconite 4 Celadonite 5

(sample 60) illite'- montmorillonite 3 (sample 6869) (sample 69)

Tetrahedral cations Si 3.63 3.47 3.96 3.78 3.94 AI 0.37 0.53 0.04 0.22 0.06 Octahedral cations A1 1.41 1.71 1,53 0.55 0.05 Fe 3- 0.10 0.04 0.18 0.89 1.15 Fe 2- 0.07 - - 0.18 0.36 Mg 0.42 0.26 0,26 0.39 0.41 Ti - - 0.01 0,01 - - Interlayer cations K 0.77 0.63 0.80 0.83 Na 0.07 0.07 0.35 - - 0.01 Ca 0.01 0.07 - - 0.03 Ivanovskaya et al. (1989). 2 Horton (1983).

c Private communication of R. Glaeser. 4,~ Drits et al. (1997).

layers induces a d i s p l a c e m e n t of the 11l reflections f r o m the positions corresponding to periodic tv and cv dioctahedral micas, in which case the a and 13 values d e t e r m i n e d by this p r o c e d u r e do not correspond to the actual parameters. T h e criteria described by Drits and M c C a r t y (1996) w h i c h use e x p e r i m e n t a l d ( l l l ) ,

d(OOl), d(060), and calculated c cos 13/a values h a v e b e e n applied to distinguish cv, tv, and interstratified

cv/tv m i c a structures. Errors in determination of the

d(hkl) values do not e x c e e d 0.002 .~.

T h e natural samples w e r e heated to 650~ (glau-

conite and celadonite) or 750~ (illites and m o n t m o -

rillonite) for dehydroxylation. T h e heating rate was

100~ each sample r e m a i n i n g heated at the upper

temperature for 1 h, and then c o o l e d in air. To rehy- d r o x y l a t e samples, g o l d capsules w e r e filled with the d e h y d r o x y l a t e plus 20 wt % water. The capsules w e r e p l a c e d in a Tuttle-type pressure vessel at 400~ and 1 kbar for 10 d. A f t e r r e m o v a l f r o m the pressure vessel, the r e h y d r o x y l a t e d samples w e r e heated at 120~ for several hours to d r y the sample.

T h e r m a l g r a v i m e t r i c analyses w e r e p e r f o r m e d with a S e t a r a m T G A 92 microanalyser, with a heating rate

of 10~ A 30-rag sample was used for each m e a -

surement. T h e derivatives of the T G c u r v e s ( D T G ) c o m p a r e the w e i g h t loss, w h i c h was assigned to water e v o l v e d f r o m h y d r o x y l groups.

I R spectra in the OH-stretching region w e r e obtained w i t h a Perkin E l m e r Paragon 1000PC spectrom- eter. A 3 - m g sample (except for glauconite: 0.5 nag) was m i x e d with 150 m g of K B r in a steel capsule (0.5 m g o n l y for cv/tv illite). The disk obtained after press- ing was heated at 140~ for 15 h to r e m o v e m o s t of the absorbed water. T h e transmitted intensities w e r e r e c o r d e d f r o m 3000 to 4 0 0 0 c m 1 at 1-cm-~ intervals.

R E S U L T S

F i g u r e 1 shows the X R D patterns obtained for the four states (N, D, R, D R ) o f the five studied samples. Table 2 p r o v i d e s the unit-cell parameters of these sam-

ples and the c cos 13/a value w h i c h characterizes the relative d i s p l a c e m e n t o f the adjacent layers along the a axis. W y o m i n g m o n t m o r i l l o n i t e has a turbostratic structure. Therefore, the determination of its unit-cell parameters is limited to d(001) and b; a is taken as b~ "x/-3. Figure 2 c o m p a r e s the T G and D T G c u r v e s of the original and r e h y d r o x y l a t e d samples. F i g u r e 3 shows the IR spectra of the original and r e h y d r o x y - lated samples in the stretching-vibration region. T h e w a v e n u m b e r s corresponding to the centers of the c o m - plex OH-stretching bands are s h o w n at each transmit- tance m i n i m u m .

tv illite ( s a m p l e 60)

T h e X R D patterns and unit-cell parameters (Figure la; Table 2) o f the N and R s p e c i m e n s reflect the diffraction and structural characteristics o f dioctahedral tv 1M mica, i.e., strong 111, 112, and 112 reflections, almost zero intensity o f the 111 reflection, and relative displacements of adjacent layers along the a axis be- tween - 0 . 3 9 and - 0 . 4 0 (Bailey, 1984; Drits et aL, 1984a, 1993). The X R D patterns for the D and D R forms are quite similar and p r e s e r v e the m a i n diffraction features o f the N and R samples (Table 2). Thus, the X R D data show that there is no octahedral-cation migration. In all states the trans-octahedral sites re- main vacant.

This c o n c l u s i o n is consistent with the D T G c u r v e (Figure 2a). The shift in peaks of the m a x i m u m water

lost in N (600~ and R (575~ is probably related to

a partial decrease in the particle size after d e h y d r o x - ylation. The IR spectra of the N and R s p e c i m e n s are v e r y similar (Figure 3a). T h e center o f the OH-stretching band o f N (3606 c m -~) shifts to h i g h e r frequencies for R (3621 c m 1).

cv/tv iltite

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Vol. 48, No. 5, 2000 Dehydroxylation-rehydroxylation of 2:1 dioctahedral layer silicates 575 t a 6 9 11 t4 16 19 21 24 26 29 20 (~ m

b

)O2 6 060 o o , ~ 1 1 1 1 ~ I12 _ D

~

R 11 14 16 19 21 24 26 29 20 (~ C 0 I I, ~" 11~003112 J l A =

1

6 9 11 14 16 19 21 24 26 29 6 9 12 15 18 21 24 27 20 (~ 20 (~ 6 9 11 14 16 19 21 24 26 29 20 (~

Figure 1. XRD patterns (kKaMo = 0.70926 A) of a) illite 60, b) c v / t v illite, c) Wyoming montmorillonite, d) glauconite 6869, and e) celadonite 69 in natural (N), dehydroxylated (D), and rehydroxylated (R) states. XRD patterns of illite 60, c v / tv illite, Wyoming montmorillonite, and celadonite 69 are also presented in de-rehydroxylated (DR) state.

( 1 9 9 3 ) a n d D r i t s e t al. ( 1 9 9 3 ) s h o w e d t h a t the X R D p a t t e r n s o f d i o c t a h e d r a l tv 3 T a n d c v 1 M m i c a p o l y - t y p e s are v e r y similar. B e c a u s e , for the s t u d i e d s a m p l e , d ( 0 6 0 ) -- 1.501 ,~, a n d the m i n i m u m p e r i o d i c i t y a l o n g

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576 Muller, Drits, Plan~on, and Robert Clays and Clay Minerals Table 2. Unit-cell parameters and ccos ~/a values of samples under study.

Sample State a (]~) b (A) c (A) [3 (~ ccos [3/a d(001) (A)

Illite (sample 60) cv/tv illite Wyoming montmorillonite Glauconite (sample 6869) Celadonite (sample 69) Average error N 5.238 9.018 10.14 101.48 0.385 9.933 D 5.265 9.024 10.24 100.98 - 0 . 3 7 4 10.050 R 5.252 9.042 10.16 101.46 - 0 . 3 8 7 9.956 DR 5.291 9.048 10.26 101.22 - 0 . 3 8 2 10.065 N 5.214 9.006 10.12 99.97 0.337 9.966 D 5.299 9.108 10.34 102.11 - 0 . 4 1 3 10.110 R 5.265 9.018 10.17 101.52 - 0 . 3 9 0 9.966 DR 5.352 9.120 10.33 102.31 - 0 . 4 1 8 10.090 N 5.193 8.994 - - - - - - 12.560 D 5.241 9.078 - - - - - - 9.708 R 5.196 9.000 - - - - - - 9.558 DR 5.252 9.096 - - - - 9.681 N 5.270 9.090 10.19 101.28 - 0 . 3 7 8 9.993 D 5.242 9.054 10.23 99.25 - 0 . 3 1 4 10.098 R 5.271 9.096 10.17 100.69 - 0 . 3 5 8 9.996 N 5.220 9.060 10.22 100.92 -0.371 10.062 D 5.233 9.030 10.21 98.78 - 0 . 2 9 9 10.089 R 5.235 9.060 10.26 99.24 0.315 10.128 DR 5.240 9.036 10.17 98.55 - 0 . 2 9 0 10.062 _+ 0.005 ,~ +- 0.006 A + 0.02 ]~ r~ 0.05

v a l u e s o f the 3 T u n i t cell, the d i f f e r e n c e s b e t w e e n the c a l c u l a t e d s p a c i n g s , dcal~, a n d e x p e r i m e n t a l values, dexp, o f the 101, 102, 104, a n d 105 r e f l e c t i o n s v a r y f r o m 0 . 0 0 4 to 0 . 0 0 9 A. T h e s e d i f f e r e n c e s are h i g h e r t h a n the e r r o r o f the m e a s u r e m e n t o f the d values. H e n c e , the 3 T - p o l y t y p e d e s i g n a t i o n m u s t b e d i s c a r d e d . I n d e x - ing o f the e x p e r i m e n t a l X R D p a t t e r n b a s e d o n a o n e - l a y e r m o n o c l i n i c u n i t cell, w i t h a = 5 . 2 1 4 A, b = 9 . 0 0 6 A, c = 10.12 A, a n d [3 = 9 9 . 9 7 ~ p r o v i d e s the b e s t a g r e e m e n t b e t w e e n t h e e x p e r i m e n t a l a n d c a l c u - l a t e d d ( 0 2 l , 111); the d i f f e r e n c e b e t w e e n dca~c a n d dexp d o e s n o t e x c e e d 0 . 0 0 2 A. N o t e t h a t the b p a r a m e t e r o f the u n i t cell is n o t e q u a l to aN/3, as it m u s t for the 3 T u n i t cell. O n e o f the m a i n d i f f e r e n c e s b e t w e e n u, 3 T a n d c v I M p o l y t y p e s is the c cos ~3/a v a l u e w h i c h is - 0 . 3 3 3 for tv 3 T ( t r a n s f o r m e d i n t o a o n e l a y e r m o n o c l i n i c u n i t - c e l l ) a n d - 0 . 3 0 0 for a l u m i n o u s cv 1 M p o l y t y p e s (Drits et al., 1993). T h e c cos [3/a v a l u e c a l c u l a t e d for the m o n o c l i n i c u n i t cell ( - 0 . 3 3 7 ) o f the s t u d i e d s a m p l e is c l o s e to that o f the 3 T p o l y t y p e w h i c h h a s b e e n d i s c o u n t e d . T h e m o s t r e l i a b l e inter- p r e t a t i o n o f the d i f f r a c t i o n f e a t u r e s is t h a t the s a m p l e is c o m p o s e d o f r a n d o m l y i n t e r s t r a t i f i e d tv a n d cv 2:1 l a y e r s w i t h r e l a t i v e p r o p o r t i o n s n e a r 0.4 a n d 0.6 as d e t e r m i n e d b y t h e m e t h o d p r o p o s e d b y Drits a n d M c C a r t y ( 1 9 9 6 ) u s i n g the e x p e r i m e n t a l d ( l l / ) values. S u c h a n i n t e r p r e t a t i o n is c o n f i r m e d b e l o w b y D T G . T h e p o s i t i o n s a n d i n t e n s i t i e s o f the 1 l l a n d 0 2 / r e - flections in the X R D p a t t e r n o f the D s p e c i m e n are typical o f d e h y d r o x y l a t e d tv 1M illites (Figure l b )

w i t h a c cos [3/a v a l u e ( - 0 . 4 1 3 , T a b l e 2) w h i c h sig- n i f i c a n t l y differs f r o m c cos [3/a = - 0 . 3 3 7 as deter- m i n e d f o r the N s p e c i m e n . T h e m o d i f i c a t i o n o f the X R D p a t t e r n o f the D s p e c i m e n in c o m p a r i s o n to t h a t o f the N p a t t e r n c o n f i r m s a p r e d o m i n a n c e o f cv 2:1 l a y e r s in the N structure. T h e X R D p a t t e r n s o f N a n d D tv-illites a n d m u s c o v i t e s are quite s i m i l a r b e c a u s e the d e h y d r o x y l a t i o n o f tv 2:1 a l u m i n o u s l a y e r s is n o t a c c o m p a n i e d b y a n o c t a h e d r a l - c a t i o n m i g r a t i o n ( G u g - g e n h e i m et al., 1987, 1990; Drits et al., 1995). I n c o n - trast, the X R D p a t t e r n s o f N a n d D cv illite s h o u l d b e s u b s t a n t i a l l y d i f f e r e n t o w i n g to t h e m i g r a t i o n o f the A1 c a t i o n s f r o m f o r m e r t r a n s - o c t a h e d r a into e m p t y cis- p r i s m s (Drits et al., 1995). A n illite s a m p l e c o n s i s t i n g o f cv a n d tv layers is t h e n e x p e c t e d to s h o w the tv c h a r a c t e r i s t i c s in t h e D state, as is the c a s e for this sample.

T h e X R D p a t t e r n o f the R s p e c i m e n is t y p i c a l o f a tv 1 M illite ( F i g u r e l b ; T a b l e 2). A s e x p e c t e d , the de- h y d r o x y l a t i o n o f the r e h y d r o x y l a t e r e p r o d u c e s the m a i n d i f f r a c t i o n f e a t u r e s o f the D s p e c i m e n ( F i g u r e l b ) . T h i s c o n c l u s i o n a g r e e s w i t h the t h e r m a l e f f e c t s (Figure 2b). T h e D T G c u r v e o f the N s a m p l e c o n t a i n s t w o m a x i m a : the w e a k e r at 550~ a n d t h e s t r o n g e r at 700~ T h e s e m a x i m a r e s p e c t i v e l y c o r r e s p o n d to t h e d e h y d r o x y l a t i o n o f tv a n d cv 2:1 l a y e r s (Drits et al., 1995, 1998). T h e D T G c u r v e o f the r e h y d r o x y l a t e c o n t a i n s o n l y o n e m a x i m u m at 550~ w h i c h c o n f i r m s the e x i s t e n c e o f o n l y o n e t y p e o f l a y e r in t h e r e h y - d r o x y l a t e d s a m p l e . T h i s r e s u l t i n d i c a t e s also t h e t r a n s - ____)

Figure 2. TG and DTG curves of a) illite 60, b) cv/tv illite, c) Wyoming montmorillonite, d) glauconite 6869, and e)

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f o r m a t i o n o f cv into tv layers during dehydroxylation. T h e c o m p a r i s o n of the IR spectra o f the original and r e h y d r o x y l a t e d s p e c i m e n s shows that r e h y d r o x y l a t i o n is a c c o m p a n i e d by a significant decrease of the O H band and a displacement of its center f r o m 3620 to 3636 c m -I (Figure 3b).

W y o m i n g m o n t m o r i l l o n i t e

T h e turbostratic stacking of the sample m a k e s it im- possible to d e t e r m i n e the octahedral cation distribution o v e r the trans- and cis-octahedra using X R D . H o w - ever, it is noted that the X R D patterns of the N and R s p e c i m e n s are quite similar (Figure lc), e x c e p t for 00l reflections. T h e N s p e c i m e n of F i g u r e l c is in its natural state [d(001) = 12.56 A] and the R sample is in a dried state [d(001) = 9.55 A, Table 2]. T h e structure o f the 2:1 layers o f the N and R s p e c i m e n s can be d e d u c e d f r o m the D T G c u r v e s (Figure 2c). T h e D T G c u r v e o f the N sample contains one strong m a x i m u m

at 700~ This m a x i m u m corresponds to the dehy-

droxylation o f cv layers (Drits et al., 1995). In con- trast, the d e h y d r o x y l a t i o n of the R s p e c i m e n occurs at

a l o w e r temperature (540~ which corresponds to the

d e h y d r o x y l a t i o n o f tv layers. Thus, the thermal effects s h o w that the d e h y d r o x y l a t i o n - r e h y d r o x y l a t i o n process o f the W y o m i n g m o n t m o r i l l o n i t e induces the transformation o f cv to tv layers. As for the tv illite and the cv/tv illite described previously, the main in- tensity band in the IR spectrum of the rehydroxylate shifts to a h i g h e r value c o m p a r e d to that o f the original s p e c i m e n f r o m 3632 to 3664 c m -~.

G l a u c o n i t e ( s a m p l e 6 8 6 9 )

T h e m a i n diffraction features o f the sample in its N, D, and R states (Figure ld) s h o w that d e h y d r o x y l a t i o n is a c c o m p a n i e d by a m i g r a t i o n o f octahedral cations f r o m cis- into trans-sites. F o r the D specimen, the rath- er w e a k 111, 112, 112, and 113 m a x i m a typical for cv 1M dioctahedral micas replace the strong l i T , 112, and 112 reflections o b s e r v e d for the N specimen, and typical o f tv 1M dioctahedral micas (Figure ld). A significant decrease in the relative displacement of ad- j a c e n t layers f r o m - 0 . 3 7 8 for the N s p e c i m e n to - 0 . 3 1 4 for the D sample confirms that cation migra- tion occurs. Drits et al. (1997) s h o w e d that the c cos

B/a value o f dioctahedral 1M micas depends on their

c h e m i c a l composition. The h i g h e r the content o f oc- tahedral Fe 2+ and M g , the l o w e r the [c cos B/a[ value. F o r this reason, the Ic cos

B/a]

value for the N speci- m e n is l o w e r than the values o f tv 1M illites. A c c o r d - ing to the p r o c e d u r e described by Drits et al. (1984a), a transformation o f tv 1M to cv 1 M dioctahedral m i c a

with the c h e m i c a l c o m p o s i t i o n o f sample 6869 should change the c cos B/a values f r o m - 0 . 3 7 8 to - 0 . 3 1 1 . T h e latter is close to the - 0 . 3 1 4 value d e t e r m i n e d for the D s p e c i m e n (Table 2). The X R D patterns o f the N and R s p e c i m e n s are quite similar with respect to the reflection-intensity distribution. This indicates that the r e h y d r o x y l a t i o n process is a c c o m p a n i e d by a reverse cation migration. H o w e v e r , the ]c cos B/a] v a l u e for the R s p e c i m e n ( - 0 . 3 5 8 ) is significantly l o w e r than that ( - 0 . 3 7 8 ) for the N specimen. A c c o r d i n g to Drits and M c C a r t y (1996), the R s p e c i m e n should contain 2 5 - 3 0 % of cv 2:1 layers based on this value. Thus, after rehydroxylation, m o s t layers h a v e t r a n s - v a c a n t octahedra. As a side effect, the d e h y d r o x y l a t i o n - r e - h y d r o x y l a t i o n process slightly disturbs the structure, with the characteristic appearance o f a slight increase in b a c k g r o u n d at the 02l and 1 II reflections.

N o t e that the IR spectra o f the N and R s p e c i m e n s in the stretching vibration r e g i o n are quite similar (Figure 3d), with two bands located near 3538 and 3558 c m 1 for the N specimen, and near 3539 and 3567 c m 1 for the R specimen. T h e D T G patterns o f the N and R samples h a v e a broad m a x i m u m at 480 and 560~ respectively. T h e small d e h y d r o x y l a t i o n

peak at 700~ probably corresponds to a carbonate

impurity.

C e l a d o n i t e ( s a m p l e 69)

T h e X R D patterns for the N and D s p e c i m e n s o f the celadonite and glauconite samples are quite similar (Figure l d and le). This shows that the d e h y d r o x y l a - tion o f both minerals is a c c o m p a n i e d by an octahedral- cation migration f r o m cis into f o r m e r trans-sites. A significant decrease of ]c cos B/a] f r o m 0.371 for the N s p e c i m e n o f celadonite to 0.299 for the D s p e c i m e n confirms this conclusion. T h e v a l u e o f [c cos B/a[ = 0.371 for the N s p e c i m e n is related to a high content o f M g and Fe 3+, as for the glauconite sample. T h e similarity of the X R D patterns for the D and R specimens (Figure l e ) shows that their 2:1 layers h a v e a near identical cation distribution. Also, their c cos B/a values are similar ( - 0 . 2 9 9 vs. - 0 . 3 1 5 ) . I f the relative d i s p l a c e m e n t o f adjacent cv layers in dioctahedral mi- cas h a v i n g a celadonite c o m p o s i t i o n is - 0 . 2 9 9 or - 0 . 2 9 0 as found in the D and D R specimens, respec- t i v e l y (Table 2), then, according to Drits and M c C a r t y (1996), the R s p e c i m e n should contain 2 5 - 3 0 % o f tv 2:1 layers. Then, after rehydroxylation, there is no re- verse cation migration f r o m f o r m e r trans- to e m p t y

c i s - o c t a h e d r a for m o s t of the layers. In this respect, the structural transformation o f the d e h y d r o x y l a t e d celadonite and glauconite samples during the rehy-

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580 Muller, Drits, Planqon, and Robert Clays a n d Clay Minerals

droxylation process differs substantially. A small reverse cation migration occurs in celadonite whereas considerable reverse cation migration occurs in glauconite. The dehydroxylation o f the R specimen of celadonite is not accompanied by significant cation migration (c cos [~/a = - 0 . 2 9 0 , Table 2), and the former t r a n s - s i t e s remain occupied.

The D T G curves of the celadonite sample (Figure 2e) agree with the transformations described above. The D T G curve of the original sample (Figure 2e) can

be described by two peaks near 550 and 650~ which

cannot be assigned, respectively, to tv and c v layers

because X D R shows unambiguously that the sample contains only tv layers. A water loss showing two peaks has been observed in pyrophyllite and muscovite by Guggenheim (1990), and in illite-smectites by

Drits e t al. (1998). Guggenheim (1990) explained this

effect by the change of coordination of the A1 next- nearest neighbors of the OH pairs during dehydroxylation. In celadonite, the existence, within the layers, of domains differing in the octahedral cation distri-

bution (Drits e t al., 1997), can also induce a heterog-

enous loss o f water. The maximum water lost at 650~ is significantly higher than that observed for tv Al-rich

dioctahedral 2:1 layer silicates. Drits e t al. (1998)

showed that for the Al-rich c v and tv layers, the de-

hydroxylation temperatures (To) are, respectively, be-

low and above 600~ Two factors can be responsible

for a higher value of Td for celadonites. First, according to crystal structure refinements (Zvyagin, 1967; D r i t s et al., 1984b), octahedral sheets of celadonite 2:1 layers are significantly thicker than sheets of illites. The thicker the octahedral sheet, the longer the OH- OH shared edges. More thermal energy is then required for dehydroxylation. Second, a local compen- sation of the negative charge of the residual oxygen atoms in a mica dehydroxylate is more difficult to oc- cur if these oxygen atoms are coordinated by divalent cations. Therefore, it is likely that the greater the amount of divalent octahedral cations in 2:1 layers, the greater the resistance of these layers to be transformed and the higher the thermal energy required for the layer dehydroxylation. The D T G curve of the R specimen has a m a x i m u m water loss at 700~ (Figure 2e). This increase of Ta in comparison with that of the original

sample confirms a presence of c v 2:1 layers in the

rehydroxylated structure. The maximum in water loss observed at 450~ in the D T G curve of the rehydroxylate corresponds to the dehydroxylation of tv 2:1 layers.

Figure 3e shows that the well resolved IR spectrum of the original sample is replaced by a single wide OH band at 3530 cm 1. The "center of gravity" of the OH band in the R sample shifts to a lower frequency in comparison with that in the N specimen (Figure 3). This is the opposite behavior of the studied Al-rich samples.

D I S C U S S I O N

The data obtained by T G A and IR spectroscopy confirm that the experimental rehydroxylation process produces hydroxytation of the samples. The results obtained here show that the structural features of dehydroxylated and rehydroxylated dioctahedral 2:1 layer silicates depend on their chemical composition as well as on the original distribution of octahedral cations over c i s - and t r a n s - s i t e s . Dioctahedral 2:1 phyllosilicates in their initial state can be subdivided into three

main groups: Al-rich tv, Al-rich cv, and Fe 3+, Mg-rich

(V~Fe3-, VtMg > V~A1) tv 2:1 minerals. Rehydroxylated dioctahedral 2:1 layer silicates are subdivided into two groups. The first group consists of rehydroxylated A1- rich minerals, which are composed of tv layers regard- less of the distribution of the octahedral cations over c i s - and t r a n s - s i t e s in the original structures. The second group is represented by the Fe 3+, Mg-rich minerals

for which tv 2:1 layers are transformed into c v ones

during dehydroxylation, and preserve this structure partially after rehydroxylation. Table 3 summarizes the main structural transformations of the studied samples during dehydroxylation and rehydroxylation reactions. To trace the structural transformations of dioctahedral 2:1 layer silicates during the dehydroxylation-rehydroxylation reactions, it is useful to consider the caus- es of the structural transitions within each subgroup. T r a n s - v a c a n t illites

As mentioned above, the cation distribution in the octahedral sheet in the dehydroxylated state of these minerals is the same as that in the natural state. No cation migration occurs. These cations become five- coordinated, the residual oxygen atoms Or being located at the same z coordinate as octahedral cations and midway between them. We assume that a structural rearrangement takes place in reverse order during the rehydroxylation process: water molecules migrate into the sample and react with the residual oxygen atom according to H20 + Or --* 2(OH). This reaction reconstructs the octahedral coordination of the A1 cations bonded to the newly formed OH groups. Thus, the original structural arrangement of the 2:1 layers is recovered. However, as shown in Table 2, the transi- tion from the N to R state of sample 60 leads to a noticeable increase of the a and b unit-cell parameters. This effect is unexpected. It is obvious that dehydroxylation alone cannot change the valency of the octahedral cations present in sample 60 (mainly A1 and Mg, Table 1). C i s - o c t a h e d r a l cation occupancy and composition should also be the same in the N and R specimens.

C i s - v a c a n t a n d c v / t v illite a n d s m e c t i t e s

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Table 3. The main structural transformations of the studied samples after dehydroxylation and rehydroxylation reactions. Status

S a m p l e N D R

Illite (sample 60) tv layers

c v / t v illite c v / t v

Wyoming montmorillonite c v

Glauconite (sample 6869) tv

Celadonite (sample 69) tv

no cation migration, five-fold coordination of A1 and Mg cations, location of a residual oxygen atom, Or, midway between two closest A1, Mg cations.

migration of the cations into vacant

cis-sites in c v layers and formation

of a structure similar to that of dehydroxylated tv illites.

migration of octahedral cations into vacant cis-sites and formation of a structure similar to that of dehydroxylated tv illites.

migration of octahedral cations into vacant former trans-sites, location of a residual oxygen atom, OF, in one of two possible positions of OH groups.

migration of octahedral cations into vacant former trans-sites, location of a residual oxygen atom, Or, in one of two possible positions of OH groups.

no cation migration, reconstruction of tv layers due to the reaction Or + H20 ~ 2(OH).

no cation migration, all layers are tv, as in the previous case.

no cation migration, all layers are tv.

partial reverse cation migration from former trans-sites into empty cis- sites and formation of 7 0 - 7 5 % tv layers. No cation migration in the remaining 25-30% of layers which are cv.

there is no cation migration in most layers, but - 2 0 - 3 0 % of them have been transformed into tv layers ow- ing to the cation migration.

are s h o w n s c h e m a t i c a l l y in F i g u r e 4. A s s e e n in F i g u r e 4a, e a c h o f the t w o a d j a c e n t O H g r o u p s f o r m n o n - s h a r e d e d g e s w h i c h are c o n s i d e r a b l y l o n g e r t h a n s h a r e d O H - O H e d g e s in tv layers. F o r the latter, the O H g r o u p s s c r e e n the e l e c t r o s t a t i c r e p u l s i o n b e t w e e n o c t a h e d r a l cations. F o r tv layers, the d e h y d r o x y l a t i o n o f c v l a y e r s is a c c o m p a n i e d b y the r e p l a c e m e n t o f t w o a d j a c e n t O H g r o u p s b y a r e s i d u a l o x y g e n a t o m . T h e A1 c a t i o n s t h a t o r i g i n a l l y o c c u p i e d c i s - a n d t r a n s - s i t e s b e c o m e five- a n d s i x - c o o r d i n a t e d , r e s p e c t i v e l y ( F i g u r e 4b). T h e s t r u c t u r e o f the d e h y d r o x y l a t e d 2:1 l a y e r s is u n s t a b l e b e c a u s e the b o n d l e n g t h s b e t w e e n the six- c o o r d i n a t e d A1 c a t i o n s a n d the o x y g e n a t o m s are un- r e a l i s t i c a l l y l o n g r e l a t i v e to u n h e a t e d structures. Drits e t al. ( 1 9 9 5 ) s h o w e d t h a t n o s h i f t o f o c t a h e d r a l c a t i o n s or a n i o n s c a n p r o v i d e an a p p r o p r i a t e c h a r g e s a t u r a t i o n o f t h e o x y g e n a t o m s o f the o c t a h e d r a l p o l y h e d r o n . S t r u c t u r a l s t a b i l i z a t i o n is o b t a i n e d b y the m i g r a t i o n o f the A1 c a t i o n s f r o m the f o r m e r t r a n s - s i t e s to the e m p t y f i v e - c o o r d i n a t e d prisms. T h e r e s u l t i n g s t r u c t u r e (Fig- u r e 4 c ) is i d e n t i c a l to t h a t o f the tv d e h y d r o x y l a t e s w h e r e the A1 c a t i o n s p r o v i d e h o m o g e n e o u s local- c h a r g e c o m p e n s a t i o n . D u r i n g r e h y d r o x y l a t i o n t h e s e h e a t - t r e a t e d l a y e r s r e m a i n as tv layers ( F i g u r e 4d). T h e X R D data, as w e l l as the D T G c u r v e s , o b t a i n e d f o r c v / t v illite a n d W y o m i n g s m e c t i t e are c o n s i s t e n t w i t h this m o d e l . T h e d i f f e r e n c e i n the O H - O H l e n g t h s in the o r i g i n a l c v l a y e r (Figure 4a) a n d r e h y d r o x y l a t e d tv l a y e r ( F i g u r e 4d) e x p l a i n s the s i g n i f i c a n t d e c r e a s e i n d e h y d r o x y l a t i o n t e m p e r a t u r e s for the R s p e c i m e n s o f c v / t v illite a n d W y o m i n g m o n t m o r i l l o n i t e ( F i g u r e

2 b a n d 2c) as p r o p o s e d b y Drits e t al. (1995). A n e x p l a n a t i o n b a s e d o n a n e n e r g y rise r e q u i r e d for oc- t a h e d r a l - c a t i o n m i g r a t i o n is d i s c a r d e d . D e h y d r o x y l a - t i o n i n d u c e s c a t i o n m i g r a t i o n in l a y e r s o f g l a u c o n i t e a n d c e l a d o n i t e . N e v e r t h e l e s s , F i g u r e 2 d a n d 2 e s h o w s t h a t d e h y d r o x y l a t i o n t e m p e r a t u r e s are l o w e r for the N s a m p l e s .

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582 Muller, Drits, Plan~on, and Robert Clays and Clay Minerals

may also be the case for all dioctahedral Al-rich smec-

rites; cv and tv 2:1 layers of identical octahedral cation

composition should have identical or nearly identical unit-cell lateral dimensions.

Fe3+-rich d i o c t a h e d r a l tv 2 : ] l a y e r s i l i c a t e s

The structural transformations in celadonites and glauconites from the original to the dehydrated state and then to the rehydroxylated state are shown sche- matically in Figure 5. As previously, it might be assumed that during dehydroxylation, adjacent OH groups are replaced by a residual Or oxygen atom. The Or atom locates midway between and at the same z- coordinate level as the octahedral cations, and occupied octahedra are transformed into five-coordinated prisms (Figure 5b). In such cases, the octahedral cat-

ion-Or distance will become b/6, i.e., 1.51-1.52 A, and

this distance is too short for cations with pentagonal coordination. Therefore, the octahedral cations must m o v e away from each other, as observed in dehydroxylated muscovite where the A1-Or distance is 1.69

(Udagawa e t aL, 1974). According to Drits et al.

(1995), such a rearrangement of the A1 and Or atoms is the main reason for the larger a and b parameters compared to the original values. The cations Fe 2+, Mg, and Fe 3+ are relatively large in comparison with AP ~. Thus the replacement of AI by Fe and Mg should increase the cation-Or distance and increase the b parameter in a way that is not compatible with the limited lateral expansion o f the tetrahedral sheets. Tsipursky et al. (1985) argued that this factor is responsible for the cation migration from cis- to t r a n s - s i t e s . T h e main reason for this assumption was that cation migration is absent in glauconites where the content of V~(Fe + Mg) is lower or equal to the V~A1 content. However,

Muller e t al. (2000) showed by simulation of XRD

patterns, that for Fe-rich dioctahedral layer silicates [Vl(Fe + Mg) -> VIA1], O, atoms in the D structure are located in former OH positions, eliminating the prob- lem of the short Fe, Mg-Or distances. Figure 5c shows a model where the Or atoms occupy one of the two former hydroxyl sites distributed along the [100] di- rection with a period equal to the a parameter; Fe, Mg cations have five-coordination and are located in the

former cis-sites. Figure 5d shows an arrangement for

the dehydroxylated octahedral sheet, assuming an ad- ditional cation migration from cis- to t r a n s - s i t e s , in comparison with Figure 5c. These two structural models (Figure 5c and 5d), having different cation distri- butions, are characterized by different stability fields, which can be estimated by using the Pauling electro- static valency principle. In both models the residual anions are strongly undersaturated with respect to pos- itive charge because they receive this charge only from two five-coordinated cations. The degree o f undersa- turation may be decreased by a shortening of the cation-residual anion bonds because the shorter the bond

Figure 4. Schematical representation of octahedral sheet during the dehydroxylation-rehydroxylation reaction of A1- rich cv 2:1 layers silicates. Natural state (a), dehydroxylation (b), cations migration (c), and rehydroxylated tv state (d).

length, the greater the bond strength, i.e., the higher

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because only one Or anion provides the required screening of repulsions caused by each o f the two nearest cations along the b axis (Figure 5c). In addition, the Or anion is not at the s a m e z level as the cations. In contrast, in the d e h y d r o x y l a t e c i s - v a c a n t m o d e l (Figure 5d), the repulsion of cations b o n d e d to the Or anion is screened by shortened anion edges, which occurs also in non-heated dioctahedral 2:1 layers. Thus, the octahedral-cation m i g r a t i o n and location o f the residual o x y g e n atoms in the f o r m e r O H sites stabilize the d e h y d r o x y l a t e structure o f celadonites and glauconites. T h e migration of octahedral cations f r o m cis- to t r a n s - s i t e s was o b s e r v e d in these d e h y d r o x y - lated minerals by Tsipursky et aL (1985). T h e f o r m e r tv layers are transformed into layers with vacant five- fold prisms f o r m e d f r o m the f o r m e r c i s - o c t a h e d r a . T h e o b s e r v e d decrease of b for d e h y d r o x y l a t e d samples 69 and 6869 is in accordance with cation m i g r a t i o n (Table 2). T h e values of c cos f~/a for the original and de- h y d r o x y l a t e samples also agrees with the location o f Fe, M g cations in the sites as described above.

Figure 5e illustrates the r e h y d r o x y l a t i o n process with the r e p l a c e m e n t o f each Or anion by t w o O H groups in cv 2:1 layers. T h e positions and intensity o f h k l reflections in the X R D patterns o f r e h y d r o x y l a t e d celadonite (Figure l e ) as w e l l as the unit-cell parameters and the mutual disposition of adjacent layers (Ta- bles 2) are consistent with the m o d e l described. N o t e that after rehydroxylation a portion o f the d e h y d r o x - ylated layers ( 2 5 - 3 0 % ) in the celadonite sample re- turns to a tv sheet. A m o r e c o m p l e x transformation takes place for the glauconite sample. F o r d e h y d r o x - ylated glauconite, r e h y d r o x y l a t i o n of the 2:1 layers is a c c o m p a n i e d by reverse cation m i g r a t i o n f r o m the for- m e r t r a n s - s i t e s into e m p t y prisms and the r e p l a c e m e n t of Or anions by pairs o f O H groups. As a result, the glauconite rehydroxylate consists of the interstratifi- cation of 7 0 - 7 5 % tv and 2 5 - 3 0 % cv layers. This effect m a y be related to partial h e t e r o g e n e i t y in the cation c o m p o s i t i o n o f individual 2:1 layers in the original samples. During rehydroxylation, the layers with a h i g h e r A1 content are transformed into tv layers whereas layers with a l o w e r A1 content p r e s e r v e the cation distribution o f the dehydroxylate. Probably, the reverse migration o f octahedral A1 cations to vacant c i s - s i t e s o f R layers p r o v i d e s a m o r e stable configuration.

A C K N O W L E D G M E N T S

The authors are very grateful to J. Cuadros and L. Heller- Kallai for their valuable comments and English corrections. The authors thank also M. Pioffet for his participation during his D.E.A. training and D. Horton for providing the cv/tv

Figure 5. Schematical representation of octahedral sheet during the dehydroxylation-rehydroxylation reaction of Fe- tich tv 2:1 layers silicates. Natural state (a), dehydroxylation (c), cations migration (d), and rehydroxylated cv state (e). The

6---

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584 Muller, Drits, Planqon, and Robert Clays and Clay Minerals

sample. V. Drits thanks the Russian Fundamental Science Foundation and Orlrans University for financial support.

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E-mail of corresponding author: Fabrice.Muller @

univ-orleans.fr