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Clays and Clay Minerals, Vol. 35, No. 5,353-362, 1987.

PARAMETERS INFLUENCING LAYER STACKING TYPES IN SAPONITE AND VERMICULITE: A REVIEW

HELIENE SUQUET AND HENRI PEZERAT

Laboratoire de R6activit6 de Surface et Structure, C.N.R.S.U.A. 1106, Universit6 Pierre et Marie Curie 4 Place Jussieu, 75252 Paris Cedex 05 France

Abstract--Saponites and vermiculites may assume at least 11 ordered or semi-ordered layer stacking sequences. For a given relative humidity, the layer stacking type assumed is a function of the nature of the interlayer cation, the layer charge density, the mean size of the particles, and the di- or trioctahedral character of the sheets. For each interlayer cation, a succession of layer stacking types can be observed as relative humidity increases. For high relative humidity, some particular layer stacking types exist, but only for low-charge minerals. No other differences have been found for saponites and vermiculites in each successive layer stacking type. The degree of order that these layer stacking types imply is probably due to the existence of electrostatic bonds between hydrated interlayer cations and surface oxygens of the substituted tetrahedra. For octahedrally substituted 2:1 phyllosilicates, however, the disorder of the layer stacking sequences is related to a highly delocalized distribution of negative charges on the surface oxygens of the layers.

A study of the superstructures detected in saponites and vermiculites indicates that the interlayer cations tend to be located as far as possible from one another. The superstructures exist only with some cations and some layer stacking types and if the layer charge density is compatible with the charge produced by the cation distribution in this kind of superstructure.

Key Words--Layer charge, Layer stacking, Relative humidity, Saponite, Vermiculite.

INTRODUCTION

Swelling 2:1 phyllosilicates consist of layers whose surfaces are negatively charged and of interlayer spaces containing more or less hydrated cations. The layer stacking sequences depend upon the nature of the bond that exists between the layers and the interlayer space. This article reviews the different layer stacking sequences that may exist in saponites and vermiculites and analyzes the factors which determine the layer stacking type assumed by the sample. In addition, different types of abnormal diffusions between the Bragg spots are described and interpreted to explain the distribution of the exchangeable cations in the interlayer space.

MATERIALS AND METHODS

Materials

Their origin, structural formula, and layer charge of the natural saponites and vermiculites studied are listed in Table 1. Their layer charge ranges from 0.45 to 0.86. Eight clays having the general formula Nax (Si4_x qAlx+q)(Mg3 qAlq)O~0(OH)2, where 0.33 --< x --< 1, q = 0 and 0.2, were synthesized under hydrothermal conditions (Suquet et al., 1977). The synthetic clays are called saponites here because they are chemically similar to minerals in this group. They differ only by their layer charge, which ranged between 0.33 and 1.0.

Methods

The samples were examined by X-ray powder diffraction (XRD) and by Weissenberg-camera single

Copyright �9 1987, The Clay Minerals Society

crystal techniques. An exhaustive description of the layer stacking sequences was reported for two-layer hydrated vermiculites by de la Calle et al. (1975, 1978b), and for one-layer hydrated vermiculites by de la Calle et al. (1984, 1985). Only two-layer stacking sequences are three-dimensionally ordered and produce discrete hkl reflections. The other layer stacking sequences are semi-ordered, i.e., the layers are randomly displaced into two or three different positions without modifi- cation of the arrangement of the oxygens surrounding the interlayer cation. The translation faults are always parallel to the 0y axis or to one of the equivalent directions oriented at 120 ~ in the xy plane, without change in the projection of the structure on the x0z plane. This kind of structure in particular either leads or does not lead to discrete hkl reflections, according to the value of k. The simplest case corresponds to k = 0, wherein the reflections are discrete. A method of studying this kind of structure was recently described for one of the layer stacking sequences assumed by one-layer hydrated vermiculites (de la Calle et al., 1984).

The collapsed Na- and Ba-substituted minerals were examined by selected-area electron diffraction (SAD).

EXPERIMENTAL RESULTS

Layer stacking types o f saponites and vermiculites

Layer stacking types corresponding to homogeneous swelling (rational 00l reflections) were studied. All possible layer stacking sequences for saponites and vermiculites saturated with eight different cations (Li, Na, K, Mg, Ni, Ca, Sr, and Ba) are listed in Tables 2, 3,

353

354 Suquet and Pezerat Clays and Clay Minerals

Table 1. Origin, structural formula, and charge of the natural minerals studied.

Charge per Minerals Structural formula 1/2 cell

Vermiculite A (Madagascar, transformed phlogopite)

Vermiculite, Santa Ollala, Spain

Vermiculite, Kenya t Vermiculite, Malawi,

Nyasaland 2 Vermiculite, Prayssac,

France 3 Vermiculite, Benahavis,

Spain Saponite, Kozakov,

Czechoslovakia

Nao.86(Si2.72Alt.zs)(Mgz.soAlo.~7Fe 3+o.I3Fez+o.I4Tio.o6)O~o(OH)2 0.86

igo.39Cao.o2(Siz 72A1 Lzs)(Mg2.59Alo.06Fe3+o.24FeZ+o.03Tio.os)Olo(OH)2 0.82 Mgo.3sCao.02(Si2.76A1 t.24)(Mg2.47Fe3+o.4Fe2+o.osTio.o~)Olo(OH)2 0.74

Cao.3oKo. os( S i 2.s9Al L ooFea +o A 2)( Mg2.s 3 T io.o6Mno.o ~ Fe3+o. a t)O ~o( O H )2 0.65

Mgo.25Ko.os~Nao.oo~Cao.ooos(Siz.s2A1L17)(Mg2.36Alo.zoFe3+o.43FeZ+o.oo4)Oto(OH)2 0.54

Mgo.24Cao.03(Si2.8 ~ml L ~oFea+o.09)(Mgz.46Tio.t iFe3+o.43)O lo(OH)2 0.53

Cao.22Ko.o~(Si33oAlo 68Fe3+0.o2)(Mg2.50Fe2+o zrFe3+o.24)O~o(OH)2 0.45

1 Besson et al. (1974a). 2 Norrish (1973). 3 Andr6 (1972).

and 4 for the two-water layer hydrates, the one-water layer hydrates, and the "zero-layer" states, respective- ly. Collapsed 2:1 phases are herein termed "zero-layer" materials, rather than anhydrous materials, because some of these phases still contain water molecules (Su- quet et aL, 1982). Only layer stacking sequences of hydrated minerals arising from transformed phlogopites having the 1M structure were listed (1M filiation). The study of minerals resulting from the hydrat ion of transformed phlogopites belonging to other polytypes (e.g., 2MI) would show layer rotation in excess of the translations observed here. Even so, these minerals would have the same structural relation between the interlayer space and the two adjacent tetrahedral sheets as the smectites and vermiculites belonging to the I M polytype. In Tables 2, 3, and 4, the following parameters are indicated for each layer stacking type: (1) the nature of the interlayer cation with which the layer stacking type is found; (2) the number of water molecules associated with each cation; (3) the components along 0x and 0y axes of the translation, which define the posit ion of one layer with respect to the next layer; and (4) the apparent basal distance and relative hu- midi ty (RH) range corresponding to the stability of the stacking sequence.

Table 2 shows that the double-layer hydrates assume an ordered triperiodic structure, VIII, having ditrigonal cavities of the adjacent layers that face each other and 6-8 water molecules in the first coordination sphere of Na and Ca. The VIII structure also exists with Ni and Mg, but only in the Kozakov saponite and in a l imited range of RH compared with the VI structure. Typically, Ni and Mg lead to VI structures in which the interlayer space contains less water than that of the Vln structure. Thus, for Ni- and Mg-saponites, the VI structure exists at lower RHs than the Vin type structure. The semi- ordered VI layer stacking type was described by de la Calle et al. (1975) from the model of Shirozu and Bailey

(1966). The Vv layer stacking sequence is specific for Ca- and Ba-minerals and corresponds to a less hydrated state than the VHI structure. The translations corresponding to the Vv layer stacking type were defined by de la Calle et al. (1978b). The Vvil layer stacking type exists only in a l imited range of RH. On the basis of observed basal spacings (13.9-13.8 ~), this structure appears to be a transitional state between the two-layer and the one-layer hydrates.

Table 3 shows that the one-layer hydrates adopt a specific layer stacking type i f the cation is relatively small (Li:V b structure) or i f the cation is relatively large (Ba:Vd structure). The most common layer stacking types are Va, having an average of three water molecules associated with each cation, and Ve, having an average of four water molecules per cation. Na minerals assume Vc or Va stacking sequences according to the layer charge density and the RH (Suquet, 1978). For high-charge Na-vermiculites, Vc is the most common structure (de la Calle et al., 1984).

The "zero-layer" state is characterized by the 1M layer stacking type ofphlogopite, or the V0 layer stacking type (Table 5). The 1M structure has a d(001) value of about 10 A, and it exists i f the radii of the interlayer cations > 1.3/~ (i.e., K, Rb, and Cs), or i f their ionic radii are small enough to enable them to occupy the ditrigonal cavities along with a water molecule (e.g., Li; Suquet et al., 1982). The 1Mlayer stacking sequence also exists for Na-saponite within a narrow range of RH (2 to 9%). Inasmuch as the Vo structure is semi- ordered, it probably corresponds to a translation of the layers such that the interlayer Na is closely coordinated by oxygen atoms in the adjacent layers. The layer stacking sequences adopted by the synthetic saponites are not listed in the tables. These samples exhibit the same layer stacking sequences described above for the natural minerals, but the number of random faults increases as the layer charge density decreases.

Vol. 35, No. 5, 1987 Layer stacking types in saponite and vermiculite

Table 2. Layer stacking types in two-water layer vermiculites (V) and saponites (S).

355

Number of Layer H20 per Number of H20 Translation s Relative

stacking Interlayer cation 1st total per cation Number of H20 d(001) humidity type cation sphere (a) (b) per cell (c) on 0x on 0y (A) (%)

V Mg 6 8 ~ 14.3-14.4 7-100 V Ni 14.4 2-100

V~ -aJ3 b/36 S Mg 14.5 9-60 S Ni 14.5 10-85

Vm

V Li 4.42 7.822 V Na 63 5.32 9.545 V Ca 6, 84 8.65 7.64: V Sr

S Na S Mg S Ni S Ca S Li S Ba

8.5-9.35 7.7-8.4 *5

165 7.2 *5

-a/3

14.6 95-100 14.85 52-100 14.92 45-100

14.9-15.0 35-100 0

15.1-15.2 52-100 15-15.2 60-98 15.0 85-100 15-15.4 32-100

15.2-15.5 57-100 15.7-15.9 42-100

V Ca -b/6 14.6 20-45 Vv -a/6 +b/6 14.7 65-100

V Ba +b/2 S Ca 14.7-14.8 20-35

V Mg Vw~ 0

S Mg

13.7 2-7

13.8 7-9

Numerical values of basal distance and relative humidity are given for powders of the Santa Olalla vermiculite (x = 0.82) and the Kozakov saponite (x = 0.45). Water of the first Coordination sphere (a) is obtained from the three-dimensional structure. Total numbers of water molecules per cation (b) and per cell (c) are obtained by one-dimensional Fourier projection. For saponites, the values (*) have been calculated assuming that interlayer water is equal to 17/22 of the total water measured by thermal gravimetric analysis and hydratation isotherm.

Alcover and Gatineau (1980b). 2 Le Renard and Mamy (1971). 3 Slade et al. (1985). 4 de le CaUe et al. (1977). 5 Suquet et aL (1980). 6 de la Calle et aL (1975). 7 de la Calle et al. (1978b).

Diffusions and extra spots between Bragg spots

Diffuse scattering, which links certain Bragg reflections together, was observed in Na- and Ba-saponites (Kozakov) (Figure 1). This abnormal diffusion seems to be independent of the interlayer cations. Figure 2 is a representation of another type of diffusion which results in the appearance of extra diffraction spots. Such extra diffraction spots were noted by XRD for an anhydrous Ba-vermiculite prepared from an Ontario phlogopite. This sample has a 1M layer stacking type, and the Ba atoms are located in the ditrigonal cavities. The extra diffraction spots indicate a superstructured organization of the Ba in the interlayer space. This superstructure corresponds to a planar centered lattice of 2a,2b parameters. Only about 10% of the particles in the Ontario sample showed this superstructure.

Six samples of Ba-vermiculite and one sample of Ba- saponite having a 1M structure were examined by SAD. The abnormal diffusions appear as honeycomb-like

patterns (Figure 3), previously described by Besson et al. (1974a) and Alcover and Gatineau (1975). Among the examined samples, the honeycomb-like patterns were observed in three Ba-vermiculites having layer charges of 0.74, 0.65, and 0.53.

DISCUSSION

Principal factors determining the structure o f the interlayer space

For a given RH, two principal factors determine the arrangement of the interlayer space: the nature of the interlayer cation, and the layer charge density.

Role o f the interlayer cation. Table 5 shows that for each balancing cation, a given succession of layer stacking types appears as the RH increases. From left to fight, the structures contain more and more water. To the extreme right, a "forbidden area" corresponds to structures which are never assumed by high-charge ver-


Table 3. Layer stacking types in two-water layer vermiculites (V) and saponites (S).

Layer slacking

type

Translation ~ Relative Interlayer Number of H20 Number of H20 humidity

cation per cation per cell on 0x on 0y d(001) (/~) (%)

Va

V Na 12.21 43-47 V Sr 3.31,2 3 11.85 0-2

+0.315 b S Na 4.1-4.83 3.7-4.33 - a / 3 -0 .315 b 12.4 9-42 S K 12.4-12.6 37-100 S Ni 11.5 0-2 S Ca 3.3-3.53 1.5-1.63 i 1.7 0-4

Vb V Li 2.44 4.284 12.2 2-95

- a / 6 +0.311 b S Li* -0.311 b 12.2 �9 2-45

Vc V Na 2.04,5 3.644 0 +0.307 b 11.85 2-43

0 -0 .307 b

V Ba 5.62 52 12.20 2-55 V~ -0.78 A +0.270 b

-0 .270 b S Ba 12.40 2-42

v Mg V Ni 11.60 0-2 V Ca 44 3.64 11.45 0-2 V Sr 44 3.64 +0.294 b 11.90 0-13

Ve 0 -0 .294 b 12.15 2-18 S Mg 11.5 0-7 S Ni 11.6 5-10 S Ca 11.9 4-13

Numerical values of basal distance and relative humidity are given for powders of the Santa Olalla vermiculite (x = 0182) and the Kozakov saponite (x = 0.45). The Kozakov Li-saponite (*) gives a stable one-layer hydrate having a disordered structure between 2 and 45% relative humidity. The synthetic Li-saponite having a high layer charge occurs with the stacking type V~.

de la Calle et al, (1985). 2 RausseI-Colom et al. (1980). 3 Suquet et al. (1980). 4 Le Renard and Mamy (1971). 5 de la Calle et al. (1984).

micul i tes , e v e n i f t hey are i m m e r s e d in water . Ba-ver - micu l i tes are an except ion: they h a v e the V.~ s t ruc tu re i f they are i m m e r s e d in water.

Tab le 5 also shows: (1) Fo r p o t a s s i u m minera l s , the one- layer hydra t e exists on ly i f the layer charge is small . T h e one- layer hydra t e is cha rac t e r i zed by a h igh degree o f d isorder , l inked to the weakness o f the K - H 2 0 bond . (2) M a g n e s i u m mine ra l s p r o b a b l y a s s u m e a less hy- d ra t ed phase t h a n V~. F e r n a n d e z G o n z a l e z (1977) no t - ed two h o m o g e n e o u s phases at 150~ one h a v i n g a n 11.63-/~ basal spacing (Ve) a n d the o the r h a v i n g a n 11.53-/~ basa l spac ing (poss ib ly Va). (3) Fo r the ca l c ium mine ra l s and p e r h a p s the b a r i u m minera l s , s o m e re- sui ts suggest the ex is tence o f a t r ans i t i on phase be tween the two- layer a n d one- l aye r hydra tes . T h e basa l spac ing o f such a t r ans i t i on phase shou ld be 13 .6-13 .8 /k , w h i c h co r r e sponds to the basal spac ing o f the V v . t r an s i t i ona l phase o f Mg-mine ra l s . T h i s phase is e i the r an in ter - stratified s t ructure or a well-defined s t ructure tha t could exist in a par t icu la r ly n a r r o w range o f RH. (4) Fo r the

Table 4. Layer stacking types in "zero-water layer states" of vermiculites (V) and saponites (S).

Total number

Layer Inter- of H20 Translation Relative stacking layer per d(Q01) humidity

type cation cation on 0x on 0y (A) (%)

V Ba 0 9.9 0-2 V K 0 10.0 0-100 VLi 1 ~ 10.1 0-2 V Ni 10.0 0-2

1M - a / 3 0 S Li 1 ~ 10.0 0-2 S Ba 9.9 0 S K 10-10.1 0-27 S Na 9.9 2-9

V Na 9.8 0 V~ S Na 9.8 0

The numerical values are given for powders of the Santa Olalla vermiculite (x = 0.82) and the Kozakov saponite (x = 0.45).

Suquet et aL (1982).

Vol. 35, No. 5, 1987 Layer stacking types in saponite and vermiculite 357

Figure 1. Selected-area electron diffraction pattern ofa crys- Figure 3. Electron diffraction pattern of a single crystal of tal of Kozakov Ba-saponite. Benahavis Ba-vermiculite.

calcium minerals, a transition phase between the layer stacking type V m and the three-layer hydrate may exist when the material is submerged in water.

Role of the layer charge density. The layer charge density modifies the range of RH at which specific structures exist. The succession of the layer stacking types with increasing RH (to the point of submersion in water) is listed in Table 6 for high-charge (x = 0.7-0.9) and low-charge (x = 0.3-0.5) 2:1 phyllosilicates, with ref- erence to the same scale of RH.

To explain the effect of the layer charge density on the transitions between the different states of hydration, the interlayer structure of the two-layer and one- layer hydrates must be described. In the one-layer hydrate, the number of water molecules present is not

04.: �9

13 u

0 2 ~ �9

11

~t

02

~t

" r

2 o

u w

X*

Figure 2. Projection on the x*0y* plane of the reciprocal lattice of an anhydrous Ontario Ba-vermiculite. ~t projection of Bragg spots; �9 projection of linear diffusions normal to the layer.

sufficient to assure a continuous network of water molecules in the interlayer space. This interlayer space is therefore composed of islands of hydrated cations, pos- sibly linked together by water molecules belonging to the second hydrat ion sphere of the cations. The degree of order in the layer stacking is a function of the number of islands in the interlayer space. The lower the layer charge density, the fewer are the islands which bridge the layers and the greater is the layer stacking disorder. The interlayer space of the two-layer hydrates is a continuous framework o f water molecules linked together by hydrogen bonds. Some o f these hydrogen a toms interact with surface oxygen atoms that carry an excess negative charge. It is in the polyhedra of this framework that the interlayer cations are located.

In Table 6, for Li and Ba, the transit ion between the one-layer hydrate and the two-layer hydrate takes place at a RH as low as the layer charge is weak. Li best illustrates this phenomenon. The stability of the layer stacking of the one-layer hydrate of Li- and Ba-minerals (Vb and Vo) clearly increases with the number of interlayer cations. This delay in the hydrat ion of the high-charge minerals is similar to those described above concerning the forbidden area of Table 5. The greater the charge density, the stronger is the strength of the cohesion between the layers and the more difficult is the swelling.

I f two possible structures exist, e.g., V~ and VH~ for Mg two-layer hydrates (Tables 5 and 6), the more hydrated structure, Vm, can be detected only if the layer charge is weak. The low-charge phyllosilicates, however, exhibit the V~H type of strncture only i f they have a high degree of crystallinity (never the case for the synthetic saponites studied).

Two structures are possible for Ca one-layer hydrates, V~ and Va (Table 5). The V a layer stacking type exists only for low-charge minerals, i.e., if the cohesion energy of V e structure is small. Thus, for Ca and Mg,


Table 5. Succession of layer stacking types with increasing relative humidity.

Interlayer cation Layer stacking type "Forbidden area"

Li 1M -~ Vb ~ VIII ~ nl (n > 3)

Na Vo ~ 1M ~ Vc ~ Va ~ VIII

K 1M ~ Va ~- 2l (disorder)

Mg V~ �9 VVII ~ VI I~ VIII ~ 3l

Ca Va ~ We ~ Vv I~ VIII ~ 31

Ba 1M �9 Vd ~l VV-.-- ~ ~ VIII ~ 3l

The "forbidden area" represents structures which never appear in the high-charge minerals (l water layer hydrate).

the transit ion to the less hydrated state (Va) is easier as the layer charge decreases.

The transition between the one-layer hydrate and the "zero-layer" state is not consistently affected by the charge density for Li-, Na-, and Ba-minerals. For K-minerals, the cohesion energy of the 1M structure is so great that hydrat ion is only possible for the low- charge minerals.

For the tetrahedrally-charged 2:1 phyllosilicates, other factors, in addi t ion to the layer charge density, can modify the range of RH in which a layer stacking type corresponding to a given interlayer cation exists.

Other factors influencing the interlayer space structure

Effect o f bidimensional size. The size of the mineral particle directly influences the interlayer space hydration and, thus, the layer stacking type. de la Calle et aL (I 978a) explained this phenomenon by comparing the Li-mineral particles to circular plates of radius r. Only the external part of the interlayer particle of Ar depth is in equil ibrium with the surrounding medium. For a given cation and RH, de la Calle et al. (1978a) postulated that Ar is nearly constant, whatever the val-

Table 6. Succession of layer stacking types with increasing relative humidity and in water for low-charge (x = 0.3-0.5) and high-charge (x = 0.7-0.9) 2:1 phyllosilicates. ~

Li

Na

low-charge

high-charge

low-charge

high-charge

lO

1M 1l (disordered)

1M

Vo 1M l v.

Vo Vc

1M

30 , 7 8 , 90 Water

Relative humidity (%)

50 '

Vb

II,+ !l .... vi[i ; 5,

1 V.

VIII

VIII

VIII

Iv::: nl (n > 3)

Vlll

Vlll

VIII

Va low-charge K

high-charge

low-charge Mg

high-charge

low-charge Ca

high-charge

low-charge Ba

high-charge

Ve Vvll

Vo Vvli

V. V~

vo

IM

1M

13.8 l VIII

13.8 Vv "1 VIII

IM 1M

Vl [ VII I 2l + 31

VI VI

Vv 2 l+ 3l

Vd 1 VIII

l i;: il Vv

21 + 3l

VIII

21 + 3l

VIII

l = water layer hydrate.


ue of r. Two cases concerning the radius value r must be considered: In the first, r is small and the value of Ar/r is large (e.g., smectite or vermiculite in powder form). The interlayer spaces of these particles easily reach equilibrium with the surrounding medium. In the second, r is large and Ar/r is small (e.g., large crystals of vermiculites or large particles produced by lateral associations). In these conditions, the crystal cohesion energy is considerable, and the particles are less influ- enced by the surrounding humidity.

These observations may be illustrated by two ex- amples for the Kozakov saponites: (1) Ba-, Mg-, and Ca-saponites swell in water to as many as two and three water layers. I f these samples are ground in a mortar, they swell homogeneously to three water layers as a consequence of diminishing particle size. (2) Oriented films ofLi- , Na-, K-, and Ba-saponites do not collapse as easily as randomly oriented powders because the mean dimension of the particles (r) increases due to lateral association.

In millimeter-size crystals of vermiculites having different origins, the size of the crystalline domains also induces shifts in the transitions between hydrat ion states. Thus, crystals of Llano Li-vermiculi te measur- ing 1 m m on each side give a one-layer hydrate, where- as Santa Olalla Li-vermiculite crystals of the same size and similar layer charge form a two-layer hydrate between 10% and 100% RH. de la Calle et al. (1978a) explained this behavioral difference as s temming from a difference in the size of the ordered domains. The Llano vermiculite crystals contain a greater density of pores and fissures than the Santa Olalla vermiculite crystals. Conditions of mineral genesis may induce a greater density of defects, equivalent to fragmentation of the particle.

Other workers have emphasized the importance of particle size and the external surface (Pedro, 1976; Tes- sier and Pedro, 1976; Pedro and Tessier, 1984; Tessier, 1984). According to these researchers, the type of particle association conditions the external surface and consequently the degree of hydrat ion of the clay minerals.

Effect of the degree of occupation of the octahedral sheet. Trioctahedral minerals having tetrahedral substitutions were discussed above. Some comments may be added concerning the swelling ofdioctahedral minerals with tetrahedral substitutions (beidellites and nontronites). Most of the known beidellites are not true beidellites, i.e., they contain substitions in both the tetrahedral and the octahedral sheets. We verified that the type of triperiodic structure reported by Glaeser et al. (1967) for the two-layer hydrate of the Rupsroth R-beidellite corresponds to the Vm layer stacking type. As a result of the amount of negative charge in the octahedral sheet (50%), however, this structure is characterized by numerous random faults. The layer stack-

ing and hydration behaviors of nontronite, which is an iron-rich beidellite, are similar to those of montmo- rillonites. Three hypotheses have been offered to explain these phenomena: (1) Bonnin (1981) suggested that the bidimensional ordered domains are of very l imited extent. (2) Besson et al. (1983), on the other hand, suggested a segregation of R 3+ cations into py- roxene-type tetrahedral chains. (3) Suquet et al. (1987) advocated electron transfer from A104 tetrahedra to the coordination sphere o f F & § in the octahedral sheet.

Organization of the interlayer space by interpretation of diffusions and extra spots between Bragg spots

Interpretation of abnormal diffusions between Bragg spots allows a more complete evaluation of the factors that determine the interlayer space organization. Dif- fuse scattering, which links certain Bragg reflections, was observed in Na- and Ba-saponites (Kozakov) (Fig- ure 1). It was also observed in muscovite-2M, by Gatineau (1964), in Ba-montmori l loni te and synthetic Ba-beidellite by Besson et al. (1973, 1974a), and in natural Ba-beidellite by Giiven et al. (1977). Because this type of scattering occurs for unsubstituted phyllosilicates such as talc and pyrophyllite (Kodama, 1975, 1977), it can no longer be considered to be an indicator of order and disorder of cationic substitutions and of interlayer cations. The diffusions can be interpreted either as thermal diffusions or as the trace of planar diffusions that correspond to linear distortions in the direct structure perpendicular to the diffusion planes. In the latter, the distortions would appear as "chains" in the [ 10] direction and along the two other 120 ~ axes. Inasmuch as these three directions are in fact those of the surface oxygen atoms, and because their z value exhibits significant fluctuations, we postulate, as did Kodama (1975, 1977), that the diffuse scattering which links certain reflections is produced by fluctuations of the position o f the surface oxygen atoms around their equilibrium position. These fluctuations are static or dynamic.

The other types of diffuse non-Bragg scattering result in the appearance of extra diffraction spots, indicating a particular superstructure organization of the interlayer cations. Four types of superstructures have been observed by means of single crystal X-ray diffraction or SAD.

Type-1 superstructure. Alcover and Gatineau (1980a) reported a type of superstructure for a one-layer hydrate o f Llano Ba-vermiculite (x = 0.9) in which the layer stacking type is Vd, and the Ba protrude partially into the ditrigonal cavities (z = 5.12 and 7.07 /~; the shortest distance between two Ba atoms is 7.34 /k). This superstructure corresponds to two planar lattices of 2a, b parameters having a configuration such that the nodes of one lattice are projected into the center of the other (Figure 4). This scheme of two parallel and planar


(3

(3

(3

�9

(3

(3

�9

(3

Figure 4. Projection on the x0y plane of the two 2a,b sublattices of the interlayer Ba cations in a high-charge vermiculite (Alcover and Gatineau, 1980). �9 Ba at a distance of 5.15

from the octahedral Mg. O Ba at a distance of 7.05 ~ from the octahedral Mg.

lattices separated by 1.95 A along the normal of their plane corresponds strictly to a layer charge of 1, which explains why this superstructure exists in smaller and smaller domains as the layer charge decreases. Alcover and Gatineau (1980c) reported that the extent of these ordered domains in the Llano vermiculite (x = 0.9) is > 90 Ik, but that it is too small in the Kenya vermiculite (x = 0.74) to be detected.

Type-2 superstructure. A second type of superstructure was reported by Alcover et al. (1973, 1980b) for the two-layer hydrates of Mg- and Ni-vermiculites. Here, the layer stacking type is V~, and the cations are located between the inverted bases of tetrahedra of two successive layers (Shirozu and Bailey, 1966). These cation positions result in a planar superstructure having a centered lattice of 3a, b parameters. In the interlayer space, each hydrated cation is surrounded by six others at a distance equal to b. This scheme of superstructure corresponds strictly to a layer charge of 0.66. Alcover and Gatineau (1980c) reported that the extent of ordered domains in which this type of diffuse scattering appears is about 90 /~ for a Kenya vermiculite (x = 0.74) and about 30 ~ for a Llano vermiculite (x = 0.90)�9 Domains in which the cations are ordered according to this bidimensional superstructure become smaller as the layer charge deviates from the theoretical value.

Type-3 superstructure. The type-3 superstructure has been observed in anhydrous Ba-vermiculite prepared

I

�9 . | - �9 . i - �9 z a

a ,

�9

. . Q . | . " \ .

Figure 5. Centered biperiodic lattice (2a,2b) showing the cations localization in the interlayer space of an anhydrous Ontario Ba-vermiculite. The right-hand part of the illustration shows an overlapping of two domains: one depicted by solid circles, the other by open circles.

from the Ontario phlogopite. This superstructure corresponds to a planar-centered lattice of 2a, 2b parameters, in which the distance between a Ba cation and its six nearest neighbors is equal to 2a (Figure 5). This superstructure scheme corresponds strictly to a layer charge of 0.5, seldom found in vermiculites.

Type-4 superstructure. The type-4 superstructure has been observed in anhydrous Ba-vermiculite having a 1M structure by us (Figure 3), by Besson et al. (1974a), and by Alcover and Gatineau (1975). Here, the abnormal diffusions appear as segments forming honeycomb-like patterns. This honeycomb-shape diffusion has also been observed for a microcrystalline Ba-muscovite (Kodama, 1975) and in two Ba-illites (Besson et al., 1974a). The type-4 superstructure was interpreted by Besson et al. (1974b) and Alcover and Ga- tineau (1975) in terms of probability of the presence of Ba. According to these workers, three values exist for the occupation probability of the ditrigonal cavities surrounding the cavity at the origin and occupied by a Ba (Figure 6): (1) A probability close to zero exists for the six cavities surrounding the origin cavity, i.e., in the 0x direction and in the two other equivalent directions at 120 ~ This zero probability suggests that the divalent cations are separated by a maximum distance. (2) A probability close to 0.5 (0.56) of occupying cavities exists in the 0y direction and in two other equivalent directions at 120 ~ to 0y. Among this group, all the cations are separated by b, as in the type-2 superstructure corresponding to x = 0.66. (3) A lower probability (0.47) of occupying the second-neighbor cavities exists in the 0x direction and in the two equivalent directions at 120 ~ to 0x. Here also, Ba is surrounded, with a probability close to. 5, by six other Ba atoms at a distance equal to 2a, i.e., at the same distance and in the same place as in the type-3 superstructure (corresponding to x = 0.5). Inasmuch as the Ba distribution must agree with the layer charge, this dis t r ibut ion-- in the domains where the honeycomb-


'T

Figure 6. Existence probability of Ba-Ba vectors in the interlayer space of an anhydrous Ba-vermiculite presenting honeycomb-shape diffusion (Alcover and Gatineau, 1975).

shaped diffusions are observed- -mus t correspond to layers having a charge density between 0.66 and 0.55. Thus, among six samples o f vermiculite, only three exhibited this type of superstructure and then in only 10--20% of the examined particles. The mean layer charges of the three vermiculites exhibiting honeycomb-shaped diffusion are 0.74, 0.65, and 0.53, where- as the mean layer charges of the three vermiculites without this superstructure are 0.86, 0.82, and 0.54.

Not all possible layer stacking sequences, types of interlayer cations, and amounts of layer charge have been examined to determine whether or not a superstructure exists; however, based on the superstructures examined, the following conclusions may be drawn: (1) For three of the superstructures studied, the repulsion forces between the interlayer cations apparently induce them to locate at an equal distance from each other at the nodes of a lattice plane for which the parameters are a function of the nature of the cation, of their number, and of their degree of hydration. For the fourth superstructure (honeycomb-shaped diffusions), the short-range order is essentially conditioned by the forces of repulsion between cations. The experimental data (Alcover and Gatineau, 1975) indicate that the extent of the ordered domains decreases as the difference between their charge and that o f the mean layer charge increases. (2) In some samples, superstructures cannot be detected by the X-ray diffraction studies (e.g., as in the Vin structure displayed by Ca-vermiculites) either because the ordered domains are too small, or because

the presence of a superstructure is incompatible with the distribution of the cations on two kinds o f sites (octahedral and distorted cubic) (de la Calle et aL, 1977). Even in this latter case, the bidimensional distr ibution of the interlayer cations is probably controlled by the repulsion forces. (3) In at least one group of samples, a particular superstructure type appears to induce a specific layer stacking sequence. Indeed, for one-layer hydrates of Ba-saponites and Ba-verrniculites, the adopted structure (Va) corresponds to an unusual value of the vector that defines the translation of one layer with respect to the next one. The component of this vector along the 0x axis is not equal to an integral fraction of the a parameter. This type of stacking is the only one which allows the existence of the type-1 superstructure in which the cations are located at the nodes of two plane sublattices and in which these two sublattices are displayed in projection such that the cations o f the first are as far as possible from the cations of the second. Although the available experimental data do not allow generalizations to be made, the obvious trend of the cations to lie in an ordered distr ibution in the interlayer space may influence the observed layer stacking type.

CONCLUSION

Tetrahedrally substituted 2:1 phyllosilicates may adopt 11 ordered or semi-ordered layer stacking sequences. Thus, strong electrostatic bonds must exist between the hydrated interlayer cations and the surface oxygen of the substituted tetrahedra. In octahedrally substituted 2:1 phyllosilicates the layer stacking sequences are disordered because the distr ibution of the negative charges on the surface oxygens of the layers is highly disordered.

Inasmuch as saponites and low-charge vermiculites commonly assume the same layer stacking types, for a given interlayer cation and a given relative humidity, crystallochemical differences (e.g., order-disorder of the Si-A1 substitutions) must exist between these two phyllosilicates.

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

The authors thank Hideomi Kodama for providing the sample of Ontario vermiculite and for help with the selected-area electron diffraction study.

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(Received 4 August 1986; accepted 15 April 1987; Ms. 1600)

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