13.6.1: Silicate Classification

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13.23.jpg — Figure 13.23: Some ways that silicon

The orderly way silica (or alumina) tetrahedra polymerize leads naturally to the division of silicate minerals into the subclasses introduced in Chapter 1 and Chapter 2, and discussed in Chapter 6. Figure 13.23 is the same as Figure 6.24 – it shows the different kinds of polymerization in different subclasses. A table below lists examples of minerals in each subclass.

We call silicates such as olivine, in which tetrahedra share no O^2–, island silicates or isolated tetrahedral silicates (also called nesosilicates or orthosilicates). Silicates in which pairs of tetrahedra share oxygen are paired tetrahedral silicates (sorosilicates). If two oxygen on each tetrahedron link to other tetrahedra, we get single-chain silicates (inosilicates) or ring silicates (cyclosilicates). If some oxygen are shared between two tetrahedra, and some between three, we get double-chain silicates (also considered inosilicates). If three oxygen on each tetrahedron link to other tetrahedra to form tetrahedral planes, we get sheet silicates (also called layered silicates or phyllosilicates), and if all oxygen are shared between tetrahedra we get framework silicates (also called network silicates or tectosilicates). The ratio of Si:O, then, indicates silicate subclass because different ratios result from different amounts of oxygen sharing.

In minerals containing tetrahedral aluminum, the ratio of (Al^IV+ Si^IV):O, which we can abbreviate T:O, reflects the silicate subclass. But if the only tetrahedral cation is silicon, island silicates are often characterized by SiO₄ in their formulas, paired tetrahedral silicates by Si₂O₇, single-chain silicates by SiO₃ or Si₂O₆_, ring silicates by Si₆O₁₈, double-chain silicates by Si₄O₁₁_, sheet silicates by Si₂O₅ or Si₄O₁₀, and framework silicates by SiO₂.