Two to one clay mineral structure

The other important structural arrangement is a 2 : 1 structure, comprising an octahedral layer, sandwiched between two tetrahedral sheets with apical oxygens pointing inward on each side of the octahedral sheet (Fig. 4.11). The mutual sharing of two layers of apical oxygens in the octahedral sheets implies a higher oxygen : OH ratio in the structure of the 2:1 vs. the 1: 1 octahedral sheets. All of the other clay mineral groups share this structure, the most important being the

OH OH

Octahedral sheet

OH OH

Tetrahedral sheet

Tetrahedral sheet

1:1 clay mineral structure

e

Silicon

e

Aluminium

Oxygen

Fig. 4.8 Schematic diagram to show how the octahedral and tetrahedral sheets, seen as separate entities in (a), can be merged to form 1:1 clay mineral structure shown in (b).

mica group, which includes the common mica minerals and illite, the smectite group and the chlorite group.

Illite is a term used to describe clay-sized mica-type minerals and is not a specific mineral name; however, in general, illite composition is similar to muscovite mica (Fig. 4.11). In the muscovite structure, one of every four tetrahedral silicons is replaced by aluminium. The regular replacement of tetravalent silicon by triva-lent aluminium means that the tetrahedral sheet in muscovite carries a strong net negative charge. Ideally, illite has dioctahedral structure, but some of the octahedral aluminium is substituted by Fe2+ and Mg2+ (Box 4.6), resulting in a net negative charge for the octahedral sheet. In total, the illite 2 : 1 unit has a strong net negative charge, known as the layer charge. This negative charge is neutralized by large cations, usually K+, which sit between the 2 : 1 units and bond ionically with basal oxygens of the opposing tetrahedral sheets in six-fold coordination. The radius ratio rule predicts that K+ should exist in eight-fold or

Fig. 4.10 Scanning electron microscope image of kaolinite showing plate-like crystals arranged in stacks. Scale bar = 1.36 mm. Photograph courtesy of S. Bennett.
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