mechanism of illite formation during smectite-to-illite conversion in a

American Mineralogist, Volume 73, pages 1325-1334' 1988

Mechanism of illite formation during smectite-to-illiteconversion in a hydrothermal system

Arsuvurr Iuoup*Laboratoire de P6trologie des Alt6rations Hydrothermales, Universit6 de Poitiers, UA721 CNRS,

40, Avenue du Recteur Pineau, 86022 Poitiers Cedex, France

Bnucn Vnr,onLaboratoire de G6ologie, Ecole Normale Sup6rieure, ER224 CNRS,46, Rue d'[J[m,752230 Paris Cedex 05, France

Ar-lrN MnuNImnLaboratoire de P6trologie des Alt6rations Hydrothermales, Universit6 de Poitiers, UA721 CNRS'


Gpru,no ToucnlnoLaboratoire de Physiques et Mecanique Fluids, Universit6 de Poitiers, UAlgl CNRS,


Ansrn-lcr

Grain size was measured on interstratified illite/smectite (I/S) with 55-00/o expandable

layers and of hydrothermal origin. The grainJength and grain-width histograms vary sys-

tematically as iunctions of percent expandable layers, and the length and width distribu-

tions normalized to the modes give steady-state profiles. These observations convincinglyprove that coarsening in the lateral faces ofI/S particles in the hydrothermal setting ex-

amined is controlled by an Ostwald ripening process. The shape of steady-state profiles

also shows that a spiral-growth mechanism dominates the growth of the I/S minerals.

The growth of the I/S minerals occurs in two stages. One is the growth of lath-shaped

particlei present in I/S with 55-20o/o expandable layers. This stage can continue metastably

up to 090 expandable layers. The second stage is the growth ofhexagonal particles in I/S

with 20-00/o expandable layers. The first stage corresponds to the evolution from lMoto

tM illire and the second to the evolution of 2M, illite. The Srowh of both particle types

can be described by the same Ostwald ripening mechanism. The polytypic transformation

ftom lM to 2M, that occurs between 20o/o and l2o/o expandable layers probably requires

dissolution oflath particles having a lM polylype-

INrtnonucrroN

Smectite-to-illite conversion is an important mineral-ogical reaction that occurs during the diagenesis ofargil-laceous sediments (Burst, 1959; Perry and Hower, 1970;Dunoyer de Segonzac, 1970; Weaver and Beck, l97l;Foscolos and Kodama, 1974; Hower et al., 1976; Bolesand Franks, 1979; Hofman and Hower, 1979; Hower,l98l). This conversion is also important to an under-standing of hydrothermal alteration, especially the ther-mal history of rocks in active and fossil geothermal fields(Steiner, 1968; Mufler and White, 1969; Browne and El-lis, 1970; Eslinger and Savin, 1973; Inoue et al., 19781'McDowell and Elders, 1980; Inoue and Utada, 1983;Horton, 1985; Jennings and Thompson, 1986). Althoughthe conversion of smectite to illite has been well docu-mented, the mechanism of the conversion is still contro-versial. Hower et al. (1976) proposed a continuous trans-

* Present address: Geological Institute, College of Arts andSciences, Chiba University, Chiba 260, Japan.

formation of smectite to illite by means of cationsubstitution in precursor smectite layers; this process hasbeen called a solid-state transformation mechanism. Incontrast. Nadeau et al. (1985) advocated a mechanism ofstepwise dissolution and recrystallization. Inoue (1986)

and Inoue et al. (1987) used scanning and transmissionelectron microscopic data for a series of I/S minerals ofhydrothermal origin to support the dissolution and re-crystallization mechanism. According to Inoue et al.(1987), randomly interstratified I/S grains with 100-500/oexpandable layers are smectites that are undergoing Kfixation and grain dissolution, and short- and long-range-ordered I/S grains with 50-00/0 expandable layers are im-mature illites that are still growing. An understanding ofthe smectite-to-illite conversion requires an understand-ing of the mechanisms and kinetics of smectite dissolu-tion and illite growth in a given natural environment. Thepurpose ofthe present paper is to describe the mechanismof illite growth during the smectite-to-illite conversion forI/S with 50-00/o expandable layers.

0003404x/88 / | | 12-1325502.00 r325

INOUE ET AL.: ILLITE FORMATION DURING SMECTITE-TO-ILLITE CONVERSION

Baronnet (1974,1982,1984) showed that micas in high-temperature and high-pressure autoclave experiments (600'C, I kbar) grow by both ripening and coalescence andthat the growth can be described by Ostwald ripening.According to Baronnet (1982), Ostwald ripening is char-acteized by the simultaneous growth and dissolution ofgrains in a closed system. After nucleation, a system maycontain alarge number of crystallites with different sizes.The tendency then is to minimize surface free energy bydissolving small particles and growing large particles viamaterial transfer from the former to the latter, based uponthe well-known Gibbs-Thomson relation. The coales-cence phenomenon involves a discontinuous increase ofsize of individual crystals accompanied by a decrease oftheir total number. In systems in which growth is con-trolled by Ostwald ripening, grain-size histograms broad-en, flatten, and shift toward greater size with time (Niel-sen, 1964). Grain-size distributions normalized to themodes in the histograms typically have a steady-stateshape (Lifshitz and Slyozov, 196l; Wagner, 1961; Exnerand Lukas, 1971; Chai, 1973). Baronnet (1982) pointedout that natural clays could grow by the Ostwald ripeningmechanism, but little work has been done to investigatethis possibility. To examine whether Ostwald ripeningdescribes I/S reactions, we have carried out grain-sizeanalysis for a series of I/S minerals from the Shinzan,Japan, hydrothermal area previously studied by Inoue(1986) and Inoue et al. (1987).

Slvrpr,ns AND EXpERIMENTAL METHODS

Samples

In the present study, nine samples of I/S from the Shinzanalteration area (Akita Prefecture, Japan) were used (Table 1).These samples have previously been described by Inoue et aI.(1978), Inoue and Utada (1983), Inoue (1986), and Inoue et al.(1987). The percent expandable layers in these samples are be-tween 550/o and 00/0, and the ordering type (Reichweite: g) rangesfrom random interstratification (8 : 0) to pure illite throughshort- and long-range--ordered interstratifications (g : 1,2, and=3). The definition of Reichweite follows that of Jagodzinski(1949). Transmission-electron micrographs of these samples areshown in Figure 1. The randomly interstratified samples containclay particles with a thin lathlike habit coexisting with moreabundant flakes. The flaky and lathlike particles correspond torandomly interstratified I/S and short-range-ordered clays, re-spectively (Inoue et al., 1987). In samples with 4G-200lo expand-able layers, the particles are chiefly thin laths. In samples withl5-l2o/o expandable layers, there are tiny hexagonal or lozenge-shaped particles (<0.2 pm in length), together with lath-shapedones (Fig. l). The pure illite contains plates with hexagonal hab-its together with laths wider than 0. I pm. The K contents of the

t327

Taale 1 . Percent expandable layers, ordering type, and K con-tent in interstratified l/S

Expandable OrderingSample layers ('/.) (Reichweite) l(O'o(OH),


ws-2-383 55+5 0ws-4-392 50 + 10 0WS-2-423 40+5 1WS-4-440 30+5 1ws-5-240 20+5 2w s - 7 - 1 1 5 1 5 + 5 - 3ws-5-185 12+3 >3w s - 7 - 1 9 7 5 + 5 > 3ws-8-452 0

0.29 + 0 070.31 + 0.090 34 + 0.070.43 + 0.100.71 + 0.02

0.73 + 0.060.76 + 0.060 80 + 0.01

lath- and hexagonal-shaped samples range from 0.29 to 0.80 ionsper half formula unit.

Methods

Transmission electron microscopic (reIra) examination was

made with a rsot- loosx transmission electron microscope on clay

fractions (<1 pm) of each sample, which were newly isolated

from the rock materials by centrifugation and dispersal in deion-ized water by means of ultrasonic vibration. This preparation

was deposited on C-coated Cu grids. The proper concentration

ofclay suspension to obtain good dispersion on a grid was de-

termined by trial and error.For samples with 55-500/0 expandable layers, only lath-shaped

particles were measured for grain size. The flakes were neglected,

because only the laths correspond to illitic mineral, as mentioned

above. In the other samples, all the particle types were measured.

The particle sizes and shapes were measured using a digitizing

table. The corners of a polygonal particle were marked with the

stylus. The length (I), wrdth (W), and aspect ratio (R) were then

calculated. The particle length is defined as the longest distanceparallel to the longest edge of a particle. The width is defined as

the longest distance perpendicular to the direction ofthe length.

The aspect ratio, R, is defined as the value of L/W'The percent expandable layers of each sample was determined

by applying ttre L20, - A20, diagram of Watanabe (1981) to

X-ray powder diffraction (xno) patterns of glycolated samples(see Inoue and Utada, 1983; Inoue et a1., 1987). The accuracy

of the determination of percent expandable layers is +50/o for

most I/S samples.Chemical analysis of I/S particles was made \Mith a Hitachi

H-500 transmission electron microscope equipped with a Kevex

5000 solid-state detector for energy-dispersive X-ray analysis

and a microcomputer for quantitative data processing (Inoue et

al., 1987). The accelerating voltage and beam current were 100

kV and about 240 pA, respectively. The measured X-ray inten-

sity of each element was corrected by using the methods of Cliff

and Lorimer (1975) and using k values obtained from standard

clay specimens ftaolinite, muscovite, celadonite, and chlorite).

For a quantitative analysis, particles with a thickness ofat least

a few hundred angstrom units were used to obtain useful X-ray

signal/noise ratios.

Fig. 1. Transmission-electron micrographs of illite/smectitewith (a) 500/o expandable layers, O) 400/o expandable layers, (c)300/o expandable layers, (d) 20o/o expandable layers, (e) 15o/o ex-pandable layers, (f) l2olo expandable layers, (g) 50/o expandable

layers, and (h) 00/o expandable layers. The scale bars are 0.5 pm.

Tiny hexagonal and lozenge-shaped particles are indicated by

arrows.

a

o

o

d

xo

oIq

o

@

o

r

Foh

o9 o

: > .

9 ga i . o

E !

9 F

v t r4 0t r 9. x 9

a d

o ! a

o 9F a

> o

. i o

. Z oo q

U H

. 2 o

o0 bt)h ;

6 t a

c.l ?- i 9; F RA A ;

=J

9F

FoUJo.o

o€ o€| o ( \ |

soo|

6e .€o ot ( '

( o z o ) A C N f O O l l 8 l

o 9 o o o o o o o o o g 9 9 i 9 ; c 9 ; ; o ; ; ; ; ; ; ; ; - - -t ' i d -

( o / o ) A C N 3 n O f U l

E

o@

-Fo

=

i i";":o{ o€ o€o o ot e ) A l

o{

oroo€1l)r4t

Eo E

o OT

F ( O

Io F

o- zo u J

J

(e " ) Ac Nf nosu I

o o.5 1.oL E N G T H ( P m )

Fig. 3. Length as a function ofpercent expandable layers.

Rnsur-rsHistograms of length, width, and aspect ratio for sam-

ples with different percent expandable layers are shownin Figures 2a,2b,and2c, respectively. The means, modes,and observed maximum values of the length, width, andaspect ratio are summarized in Table 2.

Grain-length distribution

As shown in Figure 2a,the grain-length histograms forI/S with 55-500/o expandable layers have an asymmetricaldistribution and become more symmetrical with decreas-ing percent expandable layers. In addition, these histo-grams broaden and flatten with decreasing percent ex-pandable layers. The mean length and the mode increasewith decreasing percent expandable layers in the range of55-300/0, then decrease in the range of 20-12o/o expand-able layers, and increase again in the range of l24olo (Fig.3).

Grain-width distribution

The grain-width distributions of the samples with moreexpandable layers are narrow, but become broader with

r329

g O.1 O.2 O'3w t D T H ( I m )

Fig. 4. Width as a function of percent expandable layers.

decreasing percent expandable layers (Fig. 2b). The meanvalue increases with decreasing percent expandable layersin the range of 55-150/0, slightly decreases at l2o/o, andthen significantly increases in the range of l24oh (Fig.4). These features are similar to those of the grain-lengthdistribution.

Aspect-ratio distribution

The aspect-ratio distributions broaden and flatten withdecreasing percent expandable layers in the range of 55-300/0, but become asymmetrical, sharpen, and shift to-ward smaller values with decreasing percent expandablelayers (Figs. 2c and 5). The aspect-ratio mode is approx-imately constant at 6.5 in the range of 55-300/o expand-able layers and decreases with decreasing percent ex-pandable layers (Fig. 5). The increase in length and widthvalues in the range of 55-300/o (Figs. 3 and 4) means thatmorphologically anisotropic lath particles were growingin this range such that they keep a constant aspect ratio.The decrease in aspect-ratio value in the range of20-l2o/oexpandable layers may be due to two factors: growth oflath-shaped particles toward a more equant form or thenucleation and growth of new, more equant particles. In


6060

50

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30lrJ

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TABLE 2. Length, width, and aspect ratio obtained from grain-size analysis of interstratified illite/smectite

Length (lm) Width (pm) Aspect ratioMea-sureo

Sample particle Mean (STD) Mode Max Mean (STD) Mode Max Mean (STD) Mode Max

ws-2-383ws-4-392ws-2-423ws-4-440ws-5-240ws-7-115ws-5-185ws-7-197ws-8-452

0.23 (0.1 0)0.28 (0.14)0.32 (0.14)0.43 (0 21)o.48 (O.2710.41 (O.22)0.32 (0.21)0.49 (0.23)0.86 (0 47)

0.027 (0.010)0.035 (0.014)0.043 (0.019)0.048 (0.020)0.069 (0.046)0.088 (0.043)0.066 (0.027)0 .111 (0 .041)0.260 (0.156)

8.73 (4.02)8.77 (4.6518.13 (3.56)

10.15 (6.34)8.02 (4.4515.26 (2.98)5.36 (3.65)4.81 (2.s4)3.81 (2.10)

16827914319126712423910235

021o.21o.210.300.300.300.210.300.38

0.0260.0260.0260.0430.0430.0600.0600.0770.145

0.0530.098o.1210.1270.4160.2530.1860.222o.732

25.929.021.944.127 119.519.516.69.8

0.610.990.961.201.541.370.981.302.12

o.c6.56.5b.c5 . 5

3.51 . 53.53

Nofe: STD means the standard deviation.

r330

o 5 1 0ASPECT RATIO (r - rw)

..Jtt. t. Aspect ratio as a function ofpercent expandable lay-

fact, many tiny hexagonal or lozenge-shaped particles co-exist with lath-shaped particles in the samples having lessthan 150/o expandable layers (Fig. l). The theoretical Rvalue ofan ideal hexagon is 1.15. Therefore, the decreasein R value at about l2o/o expandable layers is due to theappearance of tiny hexagonal or lozenge-shaped particles.This observation is consistent with the decrease in I andW valtrcs at about 120lo expandable layers, as mentionedpreviously.

DrscussroN

Growth mechanism of illite

As discussed above, Ostwald ripening predicts thatgrain-size histograms broaden, flatten, and shift towardgreater size with time. In the present study, the lengthand width histograms change as a function of percentexpandable layers instead of time. This observation sup-

INOUE ET AL.: ILLITE FORMATION DURING SMECTITE-TO-ILLITE COIWERSION

60 ports but does not prove a ripening mechanism for thegrowth of the illitic minerals examined here.

Ostwald ripening theory predicts that when a grain-sizehistogram is plotted in reduced coordinates, such as/(r)//(r)-"- and r/r, in which the grain-size distribution is nor-malized to the mode, the distribution forms a steady-state profile (Lifshitz and Slyozov, 1961; Wagner, 19611,Exner and Lukas, l97l; Chai, 1973; Baronnet, 1982,1984). Here flr) and flr)^.. are the frequency of a givengrain size and the maximum frequency encountered, andr andr are the grain size and the mean grain size, respec-tively. Such steady-state profiles are independent ofrip-ening time and initial grain-size distribution. The shapeof a steady-state profile is indicative of the type of thegrowth-controlling mechanism (Baronnet, 1982, 1984).Grain-size distributions normalized to the modes of thelengths and widths of our I/S samples as functions ofpercent expandable layers have steady-state profiles (Figs.6 and 7).In these diagrams, the data for the 0olo expand-able layers sample were omitted, because the number ofparticles measured is too small (35 grains) compared tothe others (>100 grains), and therefore the definition ofthe grain-size distribution is statistically poor (Fig. 2).Both steady-state profiles are characterized by a maxi-mum value of about 0.8 with respect to L/L and W/Wand by the tailing of profile toward greater L/Land W/Wvalues.

According to Baronnet (1982), the ripening process in-volves three kinetic steps, i.e., dissolution, solute transfer,and growth. The growth step is generally rate limiting inOstwald ripening kinetics. The growth rate of crystal par-ticles at a constant temperature can be expressed by

dr/dt: Ko:.

where K: growth-rate constant, o, : relative supersatu-ration ofthe bulk solution with respect to a particle sizer. and I < n < 2. where n : the order of the kinetics.

^ 5 0df f 4 0

t 3 0

UJJc0

8 2 0z

* r olrl

o

to

I

trT

O 55 o/o

O 50 oro

frcov"A 30 o/o

L 2ootoV 15o/oA 1 2 o t otr 5or"

g

X#.T I

t a

o t rvaft A

.V

IA I

A V

L ILFig. 6. Length ofillite/smectite grains normalized to the mode as a function ofpercent expandable layers. The curve has been

smoothed.

1 . O

e

r o .5J

7tJF

IJ

A

v

o 55 o/o

O 50 o/o

*co" r .A 30 o/o

L 2o"t"V 15 o/o

a 12"toO 5o lo

frIt r -

INOUE ET AL.: ILLITE FORMATION DURING SMECTITE-TO-ILLITE CONVERSION 133 1

oE

=

\ o.5=

Boundary cases ,? : I and 2 are accounted for by the BCFtheory of spiral growth (Burton et al., l95l) of crystalfaces controlled by a screw-dislocation mechanism. Thefirst- and second-order kinetics operate under high- andlow-supersaturation conditions, respectively. Volume dif-fusion-controlled ripening predicts the second-order ki-netics as well (Greenwood, 1956). The present steady-state profiles for length and width fit exceedingly well thetheoretical profile for a spiral-growth mechanism underlow-supersaturation conditions (second-order kinetics;Fig. 8). No spiral patterns have yet been observed on thelateral faces of micas (Baronnet, 1984). However, Sunand Baronnet (pers. comm.) recently demonstrated frommeasurements of growth rates of synthetic mica singlecrystals that the lateral faces grow according to the samespiral-growth mechanism as that on the basal faces.

t t r r l

3

Inoue et al. (1987) measured the thickness distributionof some of the present specimens by a Pt-Pd shadowingmethod. These thickness data do not give a steady-stateprofile because of the small number of data. No spiralgrowth pattems were observed on the basal faces of thesesamples. Nevertheless, it is reasonable to assume that thegowth on the basal faces followed the same Ostwaldripening process as the other directions, probably con-trolled by a spiral-growth mechanism. K-Ar dating of thesamples (our unpub. data) shows that the ripening oc-curred about I Ma in the present hydrothermal setting.

Crystallization generally involves both nucleation andcrystal-growth pro@sses. All the VS samples studied here,except the 00/o expandable layer sample, have attained asteady state in their growth. rnr'r observations show thatin the studied I/S samples, nucleation of lath-shaped par-

t l

4l l l l l l

o 1l l l l l l l l

2

w/wFig. 7. Width of illite/smectite grains normalized to the mode as a function of percent expandable layers. The curve has been

smoothed.

oE

: O.5

f / r

Fig. 8. Comparison ofthe experimental curves (Figs. 6 and 7) with theoretical steady-state profiles calculated for various growth-

controlling mechanisms (see text).

- - - - vo lume d i f fus ion- - - f i r s t o rder- second Ordel- leng th in th is s tudy. . . . . . . - w i d t h i n t h i s s t u d y

t332

1 0 2 0

o.2 0.3 0.4 0.s 0.6 0.7 0.8K /Oro(OH)z

Fig. 9. (a) Relationship between the percent expandable lay-ers and the percent of all grains with the most frequent length(mode) in illite/smectite. (b) Relationship between the percentexpandable layers and the K content per half formula unit inillite/smectite.

ticles began at the stage when the I/S contained more than550/o expandable layers (Inoue et al., 1987). Nucleation ofthe hexagonal particles apparently started at about 200/oexpandable layers, because many tiny hexagonal particlesoccur in samples with l5-l2olo expandable layers.

I M-to-2 M, illite polytypic transformation

The growth of illitic minerals during smectite-to-illiteconversion in this hydrothermal setting consists of twodistinct growth sequences (Fig. 9). One corresponds togrowth of laths between 550/o and 200/o expandable layers,and the other to that of the hexagonal-shaped crystalsbetween 20o/o and 00/o expandable layers. Prevrous xnpand rru examinations indicate that the lath-shaped illitehas I M o or /M symmetry and the hexag onal illite has 2 M,symmetry flnoue et al., 1987). Therefore, the size discon-tinuity in the growth evolution of illite particles is relatedto the lM-to-2M, polytypic transformation.

Several hypotheses have been proposed for the mech-anism of the lM-to-2Mr polytypic transformation inmuscovite, e.9., lattice reconstruction in the solid state(Hunziker et al., 1986) and dissolution-recrystallization(Baronnet, 1980; Mukhamet-Galeyer et al., 1985). Bar-onnet (1980) assumed the IM-to-2Mt transformation inhigh-temperature and high-pressure hydrothermal exper-iments to take place by a ripening process at a constanttemperature as follows: During an initial nucleation stage,

alarge number of lM, mica particles form; the grain-sizedistribution is characterized by a very dispersed aspect-ratio distribution. When the bulk supersaturation in thesolution decreases, the particles smaller than the criticalsize dissolve to form IM overgrowths on the larger lMoparticles. With further decrease in supersaturation, thedissolving, smaller I M particles feed 2M, overgrowths onthe larger lM particles. According to this hypothesis, thetransformation cannot go fully to completion, because thecores of the continually growing crystals record and retainall the successive growth forms from the nucleation stage.In other words, we can observe the three types of micamodification (IMd, lM, and 2M,) in one large, thick crys-tal. The variations in the length and width of the naturalI/S minerals of the present study (Figs. 3 and 4), whichpresumably occurred under nonisothermal conditions,however, suggest that dissolution of large laths and theformation of the small hexagons occur during rhe lM-to-2M, transformation. Such dissolution-recrystallizationduring the lM-to-2Mt polytypic transformation has beenemphasized by Mukhamet-Galeyer et al. (1985) on thebasis of rnvr observations of the morphology of syntheticmuscovite.

The present LM-to-2M, polytypic transformation pro-cess at the Shinzan area may be summarized as follows.The initially nucleated laths with the l Mo or 1M polytypegrew dominantly in two dimensions, keeping a nearlyconstant aspect ratio in the range between 55o/o and 20o/oexpandable layers. Laths larger than the critical size forthe actual fluid supersaturation continued to grow meta-stably up to the 0olo expandable layer stage, although theaspect ratio may have decreased slightly. The 2M, hex-agonal crystals began to form at about 20o/o expandablelayers, with dissolution oflaths occurring at between 200/oand l2o/o expandable layers. Growth of hexagonal crystalstended to a more equant shape, increasing the aspect ratioand thickness slightly.

Figure 9b shows that a large change in the K contentof the samples correlates with the change from the lMto2M, polytype at about 20o/o expandable layers. Thus, thetwo polytypes have different chemical compositions, inaddition to different morphologies and growth dynamics,especially in their early crystallization stages. The K con-tent of the laths gradually increases with growth, whereasthe K content ofthe hexagons is more nearly constant at0.7-0.8 ions per half formula unit. The overall transfor-mation observed in the present study is lMo - lM -2M,, consislent with the previous observations made onsynthetic experiments of muscovite (Yoder and Eugster,1955; Smith and Yoder, 1956; Velde, 1965).

SulrvHny AND coNcLUSroNS

Grain-size analysis of interstratified I/S confirms thefollowing conclusions regarding the mechanism of illiteformation during smectite-to-illite conversion in hydro-thermal systems.

l. The coarsening of illitic minerals in the lateral facesand possibly basal faces follows an Ostwald ripening pro-

INOUE ET AL.: ILLITE FORMATION DURING SMECTITE-TO-ILLITE COTWERSION

f lOST FREAUENT LENGTH(%)

cess in which the growth rate is probably controlled by aspiral-growth mechanism. The present data indicate thatI/S minerals with less than 550/o expandable layers haveattained to a steady state in their gowth. The nucleationofparticles having a lath-shaped habit begins rather earlyin the smectite-to-illite conversion, possibly at 80-70o/oexpandable layers.

2. The growth of illitic minerals during the smectite-to-illite conversion from 550/o to 0o/o expandable layersconsists of two processes: growth of lath-shaped particleshaving lMo or 1M symmetry and the nucleation andgowth of hexagonal-shaped particles having 2M, sym-metry. The growth of lM crystals takes place in the rangeof 55-20o/o and continues metastably up to 00/o expand-able layers, whereas 2M, crystals begin to form at about200/o expandable layers. The development of 2M' llliteprobably occurs simultaneously with dissolution of lMlaths between 20o/o and l2o/o expandable layers. Thecoarsening of both crystal populations is controlled byOstwald ripening. These two polytypes have different in-terlayer K contents in addition to different morphology,especially in their early crystallization stages.

AcrNrowLnocMENTS

We greatly thank A. Baronnet, Universit6 de Aix-Marseille III, and D.Beaufort, Universit6 de Poitiers, for their valuable suggestions in the pro-cess of completing this study and critical reading of the manuscript.

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- (1980) Polytypism in micas: A survey with emphasis on the cryslalgrowlh aspect In E. Kaldis, Ed., Current topics in material science,vol. 5, p. 447-548 North Holland, Amsterdam.

-(1982) Ostwald ripening in solution The case of calcite and mica.Estudios Geologicos, 38, 185-198.

- (1984) Growth kinetics ofthe silicates. A review ofbasic concepts.Fortschritte der Mineralogie, 62, 187-232.

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Burton, W.K., Cabrera, N., and Frank, F C (1951) The growth of crystalsand the equilibrium structure of their faces Philosophical Transactionsofthe Royal Society,243, 299-358

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