Isomorphic substitution tetrahedral sheet

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.

Two cases of isomorphic substitution can be distinguished In the tetrahedral sheet, or in the octahedral sheet (Sposito, 1984). 

If isomorphic substitution of Si(IV) by AI(III) occurs in the tetrahedral sheet, the resulting negative charge can distribute itself over the three oxygen atoms of the tetrahedron (in which the Si has been substituted) the charge is localized and relatively strong inner-sphere surface complexes (Fig. 3.10a) can be formed. 

The material from the Hector area of California is believed to have formed by the action of hot spring waters containing Li and F on clinoptiolite. The Mg was obtained from the alkaline lake waters (Ames and Goldich, 1958). The material from Morocco is associated with marls and is believed to be authigenic. These two types of trioctahedral smectite appear to be the only ones with a relatively pure Si tetrahedral sheet. No analyses were found which indicated tetrahedral Al values between 0.02 and 0.30. Analyses of saponite indicate there is complete isomorphous substitution between the range Si3.70 Al0.3o and Si3.0s Al0.92 (Table XXXIX). Caillere and Henin (1951) reported an analysis of a fibrous expanded clay (diabantite) which had a tetrahedral composition of Si3.i7 Alo.49 Fe3+0.34. There is some question as to whether this should be classified as a smectite regardless, it indicates the possibility of Fe3+ substitution in the tetrahedral sheets of the trioctahedral 2 1 clays. 

It is assumed that some exchange is due to isomorphous substitution but this has not been proven. Schofield and Samson (1953) calculated that only one Al3+ need replace one Si4+ in 400 unit cells to afford an exchange capacity of 2 mequiv./lOOg. There is enough excess Al3+ in most kaolinites to account for 10 times this exchange capacity. Thus it appears likely that most of the excess Al3+ does not substitute in the tetrahedral sheet. The iron-rich kaolinite described by Kunze and Bradley (1964)has an exchange capacity of 60 mequiv./lOO g however, it is likely that much of this is due to the presence of iron oxides. 

Ruotsala et al. (1964) reported a partial analysis of a chamosite which contained 18.7% MgO and only 9.70% FeO. This is enough Mg to fill approximately half the octahedral positions and Fe2+ to fill one-seventh. If this analysis is valid, the composition of the octahedral sheet would be similar to that of Mg-biotites (Fig.24), considerably extending the range of isomorphous substitution. If chamosite behaves as the trioctahedral micas (Foster, 1960), then as octahedral Mg increases at the expense of Fe2+, octahedral Al and tetrahedral Al both tend to decrease and the composition... 

As indicated in Table 1, the three 2 1 groups differ from one another in two principal ways. The layer charge decreases in the order illite > vermiculite > smectite, and the vermiculite group is further distinguished from the smectite group by the extent of isomorphic substitution in the tetrahedral sheets. Among the smectites, those in which substitution of Al for Si exceeds that of Fe2+ or Mg for Al are called beidellite, and those in which the reverse is true are called montmorillonite. The sample chemical formula in Table 1 for smectite thus represents montmorillonite. In any of these 2 1 clay... 

An important feature of the smectites, vermiculites and other 2 1 layer silicates is that isomorphous substitutions can occur in both the tetrahedral and octahedral sheets. Thus, substitution of Si by A1 occurs in the tetrahedral sheet, together with replacement of A1 by Mg, Fe, Li or other small atoms in the octahedral sheet. The substitutions lead to a deficit of positive charge, which is compensated by the presence of exchangeable, interlayer cations. 

Montmorillonite is a 2 1 clay with isomorphic substitutions mainly in the octahedral sheet and some substitutions in the tetrahedral sheets. When the clay is exchanged with monovalent ions, water and electrolyte ions can enter the interlayer spacing and delaminate the system. With Li+ or Na+ as the exchanging cations the delamination is almost complete, whereas with K+ or Cs+ the delamination is less effective.4849 At low pH, edge-to-face interactions can lead to the formation of aggregates. 

Within the sheets, Si or A1 may be replaced by a different element by the process of isomorphous substitution. Common replacements in the clay minerals are AP for Si in the tetrahedral sheet and Mg or... 

Vermiculite, a 2 1 clay mineral, is also a hydrous mica, with isomorphous substitution in the tetrahedral sheet, resulting in a charge of between 1.2 and 1.9 mol negative charge per unit cell. In this case the... 

Now the stoicheiometry of all mineral phases is complex and beset by problems of isomorphous substitution. In the sheet silicates such as talc, the incorporation of material between the layers, as occurs in intercalation compounds, is also common. Beneath this complexity, though, lies the fact that the tetrahedral-octahedral framework of these materials is of a fixed and inflexible metal to oxygen stoicheiometry. 

Smectites, which are based on either the trioctahedral 2 1 (talc) or dioctahedral 2 1 (pyrophyllite) structure, differ from these neutral structures by the presence of isomorphous substitution in the octahedral or tetrahedral sheet. For example, the dioctahedral smectite, montmorillonite, has the general formula... 

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