2:1 clay structures octahedral sheet

Layer-silicate structure, as in other silicate minerals, is dominated by the strong Si-O bond, which accounts for the relative insolubility of these minerals. Other elements involved in the building of layer silicates are Al, Mg, or Fe coordinated with O and OH. The spatial arrangement of Si and these metals with O and OH results in the formation of tetrahedral and octahedral sheets (see Fig. 8-2). The combination of the tetrahedral and octahedral sheets in different groupings, and in conjunction with different metal oxide sheets, generates a number of different layer silicate clays (see Table 8-1). 

Figure 3.4. Two types of isomorphous substitution. The middle structures are two-dimensional representations of clay without isomorphous substitution. On the left is an isomorphous substitution of Mg for A1 in the aluminum octahedral sheet. On the right is isomorphous A1 substitution for Si in the silicon tetrahedral sheet. Clays are three-dimensional and -OH on the surface may be protonated or deprotonated depending on the pH of the surrounding soil solution. There will be additional water molecules and ions between many clay structures. Note that clay structures are three-dimensional and these representations are not intended to accurately represent the three-dimensional nature nor the actual bond lengths also, the brackets are not intended to represent crystal unit cells.

Figure 3.2. Structural units of clay minerals and clay colloids (a) Octahedral sheet, (b) tetrahedral sheet. Reprinted with permission from Grim, R. E. (1968). Clay Mineralogy, 2nd edition, McGraw-Hill, New York.

In general, when either Al or Fe3+ is the dominant (greater than 1.0) cation in the octahedral sheet of a 2 1 dioctahedral clay, the maximum Mg content the sheet can accommodate is 0.50-0.60 (0.5 if Fe3+ is dominant and 0.6 if Al is dominant). When the Mg content is larger than 0.6, as for most celadonites, seldom is any other cation present in amounts greater than 1.0. This suggests structural control of composition. 

The maximum amount of Al3+ tetrahedral substitution that 2 1 clays minerals formed at low temperatures can accommodate appears to be 0.80—0.90 per four tetrahedra. While this appears to place an upper limit on the amount of R3+ octahedral substitution, it is not clear why the limit should be such a low value. The dioctahedral smectites can accommodate more substitution (R2 + for R3+) in the octahedral sheet than can the dioctahedral micas. The reverse situation exists for trioctahedral equivalents. In the latter clays octahedral R3+ increases as tetrahedral Al increases. Thus, as one sheet increases its negative charge, the other tends to increase its positive charge. This is likely to introduce additional constraints on the structure. In the dioctahedral clays substitution in either sheet affords them a negative charge and substitution in one sheet is not predicted by substitution in the other sheet thus, one might expect more flexibility. 

The difference in the composition of these two size fractions is similar to the difference between the two types of clay vermiculite described by Barshad and Kishk. The two sets of data confirm the idea that clay vermiculites developed by mild leaching action of pre-existing sheet structures tend to inherit much of their octahedral and tetrahedral character. Clay vermiculites formed by relatively intense weathering and by diagenetic alterations and in approximate equilibrium with their soil environment will have little if any tetrahedral Al the octahedral sheet can be quite variable in composition and depends on the availability of Al, Fe, and Mg. 

Low-temperature sheet structure silicates with a high Fe2+ content seem to be restricted to the 1 1 minerals. The Fe2+ content of the octahedral sheet of the 2 1 clays is seldom larger than 0.6 per 3.0 positions (less than 0.3 for most samples). Few low-temperature 2 1 clays have enough A1 substitution in the tetrahedral sheet to adjust its size to that of the large octahedral sheet. Substitutions of this magnitude, 1 A1 per four tetrahedral positions, at low temperatures, are favored more by the 1 1 than the 2 1 arrangement. Presumably, the lack of a layer charge and, therefore, the need for the tetrahedra to rotate to accommodate an interlayer K, and the fact that the tetrahedral sheet is sandwiched between two octahedral sheets allow interlayer size adjustments to be made more easily in the 1 1 than in the 2 1 clays. 

The chlorite clays appear to have octahedral sheets with compositions that are largely intermediate between the 2 1 and 1 1 octahedral sheets. The chloritic structure allows for a wider range of substitution than the other clays. In part this is because most data on the octahedral composition are an average of two octahedral sheets, each of which could have relatively restricted compositions. 

The maximum amount of Fe2+ the octahedral sheets of the 2 1 dioctahedral clays generally contain is 12% (Fig.26). This is equivalent to approximately 0.25—0.30 octahedral positions. Due to the relatively large size of the Fe2 + ion, the structure can apparently adjust to only about half as much Fe2+ as Mg. 

Attapulgite and sepiolite data plot on both sides of the neutral line.. The calculated structural formulas of these two clays are more subject to error than those of the sheet structure clays. As is suggested by their low cation exchange capacity octahedral sheets tend to approach neutrality. The amount of R3+ required to maintain neutrality is less if OH substitues for O2. ... 

Most of the celadonite samples lie in the area where some mixed-layering is to be expected. Although celadonite is commonly considered to be non-mixed, the literature suggests that little effort has been made to establish this. Of the 15 analyses examined by Wise and Eugster (1964) six reported adsorbed water and in the others it was not determined. In any event, the sheet structure of the celadonite is distinctly different from that of the other 2 1 dioctahedral clays (Radoslovich,1963a). It has a very thick octahedral sheet all three octahedral positions are of equal size (in the other 2 1 dioctahedral clay the two filled positions are smaller than the vacant position) and the interlayer separation is larger than in other contracted 2 1 dioctahedral micas (Radoslovich,1963a). 

Using the structural formulas that were used to plot Fig.29, the oct./ tet. ratio was calculated for a number of clays and the amount of tetrahedral rotation estimated from the graphs in Fig.32. Some of these values are shown in Fig.31. The montmoril-lonites with a low tetrahedral A1 and low octahedral R3+ have high ratio values and presumably a low degree of tetrahedral rotation (0° —1.5°). As the amount of octahedral R3+ increases, Mg decreases, the octahedral sheet becomes smaller, and the amount of tetrahedral rotation increases (6.5°). 

Clay Structures. The 1 1 structures are the simplest since they contain only a single octahedral sheet and a single tetrahedral layer. The double layers are held together by hydrogen bonding. The most notable iron phyllosilicates with the 1 1 structure are berthierine and cronstedite. Very little iron... 

Teppen et al. [89] have used a flexible model for clay minerals that allows full movement of the M-O-M bonds in the clay structure, where M represents Si, Al, or other cations in the octahedral sheet. This model was used in MD simulations of interactions of hydrated clay minerals with trichloroethene [90, 91]. The simulations suggest that at least three distinct mechanisms coexist for trichloroethene sorption on clay minerals [90], The most stable interactions of trichloroethene with clay surfaces are by full molecular contact, coplanar with the basal surface. The second type more reversible, less stable is adsorption through single-atom contact between one chlorine atom and the surface. In a third mechanism, trichloroethene interacts with the first water layer and does not interact with clay surface directly. Using MC and MD simulation the structure and dynamics of methane in hydrated Na-smectite were studied [92], Methane particles are solvated by approximately 12-13 water molecules, with six oxygen atoms from the clay surface completing the coordination shell. 

Clay minerals are aluminosilicates that predominate in the clay fractions of earth materials at intermediate stages of weathering. These minerals, like the micas, are sandwiches of tetrahedral and octahedral sheet structures [1]. 

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