Nature of Layer Silicates Clay Minerals

Clays are by definition fine grained solids with particle size generally < 2 |im and many of their properties result from their small particle size. The layers of clays are formed by condensation of sheets of linked Si(O,OH)4 tetrahedra with those of linked M2_3(OH)4 octahedra, where M is a divalent or trivalent cation. A 1 : 1 condensation gives two sheet minerals such as kaolinite with a general layer formula of M2_3Si2O5(OH)4. A 2: 1 condensation results in the octahedral sheet being sandwiched between two sheets of tetrahedra giving the mica type layer structure with a layer formula of M2_3Si4O10(OH)2. In both cases the tetrahedral sheets are linked in the unit structure to octahedral sheets and to groups of coordinated cations or individual cations. The apical oxygen at the fourth corner of the tetrahedron, which is directed normal or nearly normal to the sheet, forms part of an immediately adjacent octahedral sheet in which octahedra are linked by sharing edges. The junction plane between tetrahedral and octahedral sheets consists of the shared apical oxygens of the tetrahedra and unshared OH groups that lie in projection at the centre of each sixfold ring of tetrahedra. Figure 4 shows a three-dimensional schematic illustration of layer silicates. Also possible, for example in chlorite, are four sheet clays in which the trimorphic units alternate with M(OH)2_3 sheets of octahedrally coordinated M2 + or M3 + ions.

Smectite clays, which exhibit the property of intercalation, are made up of negatively charged layers and therefore possess an ion exchange capacity which distinguishes them from the mica and pyrophillite-talc groups of minerals (see below). The layer charge arises generally from isomorphous substitution of Si4 + by Al3 + in the tetrahedral sheet and/or Al3 + by Mg2 +, Fe2 + in the octahedral sheet. Some charge may also arise from broken bonds at edges of the clay crystal. Following below are ideal structural formulae of some clay silicates showing, where appropriate, isomorphous substitution:

1. Dioctahedral smectites (two-thirds of octahedral sites are occupied by trivalent cations)

(a) Pyrophyllite

(b) Montmorillonite

[M,(Si8)[Al4-,MgJO20(OH)4• «H2O] Octahedral substitution.

(c) Beidellite

[M,Si8-*Al,(Al4)O20(OH)4 • «H2O] Tetrahedral substitution.

Figure 4 Three-dimensional illustration of the structure of silicate clays.

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