The second structural unit is the silica tetrahedral layer in which thesilicon atom is equidistant from four oxygens or possibly hydroxls arrangedin the form of a tetrahedron with the silicon atom in the center.These tetrahedrons are arranged to form a hexagonal network repeatedinfinitely in two horizontal directions to form what is called the silicatetrahedral sheet (Fig. 2). The silica tetrahedral sheet and the octahedralsheet are joined by sharing the apical oxygens or hydroxyls to form whatis termed the 1:1 clay mineral layer (e.g. kaolinite) or the 2:1 clay minerallayer (e.g. illite) as discussed in the following sections. The structure andcomposition of the major industrial clays, i.e. kaolins, smectites, andpalygorskite–sepiolite, are very different even though they are each
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