Silicate clays structural classification

In order to study in more detail the clay minerals, it is first helpful to review briefly the basic structural classification of the silicates in general. Although ultimately complicated, the general progression is logical, and is based on the degree of polymerization of the basic structural unit which is the Si04 tetrahedron (see below). The sequence runs as follows ... 

We have already learnt that the building unit of the silicates is the Si04 pyramid. By linking these pyramids in various ways the different kinds of silicate anions are formed. The classification of the silicates is based on these kinds of ions. The silicate anions of clay minerals have a layered structure, the top view of which is again represented in figure 8.5. 

Whereas soil scientists would classify clay by considering the particle size, a chemist describes it as part of the family of silicates and more precisely as hydrated silicates of aluminium. However, the classification of minerals with a stoichiometric formula does not reflect the complexity of their structure. In order to explain the properties of clay minerals in an adequate way, the coordination of silicon has to be described first, before looking at a long-range order in the crystals. 

Silicates are materials composed primarily of silicon and oxygen, the two most abundant elements in Earth s crust consequently, the bulk of soils, rocks, clays, and sand come under the silicate classification. Rather than characterizing the crystal structures of these materials in terms of unit eeUs, it is more convenient to use various arrangements of an SiOl tetrahedron (Figine 12.9). Eaeh atom of silicon is bonded to four oxygen atoms. 

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