The Layer Silicates

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Abstract
The most commonly occurring layer silicates contain the planar hexagonal silicon-oxygen network found, for example, in micas, and their spectra have been studied in the greatest detail. Some studies have also been made on other planar or analogous structures, such as sepiolite, attapulgite, apophyllite, and some synthetic alkali and alkaline earth silicates with layer anions. Their spectra will also be briefly considered here, together with some species of uncertain structure. Within the main family of layer silicates it is useful to distinguish the 2: 1 layer silicates from 1:1 layer silicates. The first have mica-like structures, in which the hexagonal silicate anions are linked together by ions in octahedral coordination to give a structural unit resembling a sandwich (Fig. 15.1). These units are only weakly linked together in the crystal, either by Van Der Waals forces (talc and pyrophyllite) or by interlayer cations (micas and smectites). In the 1:1 layer silicates, (kaolinites and serpentines) one of the silicate anions is replaced by a sheet of hydroxyl groups, and the layer units are linked by hydrogen bonds between the hydroxyl surface of one layer and the oxygen surface of the next. In a third family, the 2:1:1 layer silicates represented by the chlorites, each mica-like layer is interleaved by a single layer of a mixed Mg–Al–Fe hydroxide with the brucite structure, so that here, as in the 1: 1 layer silicates, the structural units are linked by hydroxyl-oxygen interactions. Within each of these three families a further distinction can