Isomorphic substitution octahedral 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). 

Robertson et al. (1954) analyzed two kaolinites in detail and concluded that Fe was present in the octahedral sheet and that there was sufficient isomorphous substitution to account for the cation exchange capacity (Table LXIV). These clays were not pure and it was necessary to make a number of assumptions in order to obtain these results. 

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... 

The clay minerals usually are classified into layer types, distinguished by the number of tetrahedral and octahedral sheets combined to form a layer, and groups, differentiated by the kinds of isomorphic cation substitutions that occur [2]. Layer types are shown in Fig. 1 and some of the groups are described... 

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. 

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... 

The silicate layer is an electrically neutral structure. If part of tetrahedral Si4+ or octahedral Al3+ is isomorphously substituted by lower valency cations the sheet becomes negative. In this case the clay will have exchangeable cations between every layer to compensate for the negative charge of the layers. The cation exchange capacity (CEC) of a clay is equivalent to the layer charge and is dependent on the degree of isomorphous substitution. 

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