13.6.2: Chemical Trends

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The chemistries of silicates correlate, in a general way, with the subclass to which they belong (table below). This correlation reflects silicon:oxygen ratios, and it also reflects the way in which silica polymerization controls the number and nature of cation sites between anions. There are many variables, but we can make some generalizations. Isolated tetrahedral silicates and chain silicates include minerals rich in Fe²⁺ and Mg²⁺, but framework silicates do not. The three-dimensional polymerization of framework silicates generally lacks sufficient anionic charge and the small crystallographic sites necessary for small highly charged cations. Even if Al³⁺ substitutes for Si⁴⁺, little charge is left over to allow other cations to be present. So small, highly charged cations are absent from framework silicates. For opposite reasons, Na⁺and K⁺ enjoy highly polymerized structures because of the large sites that easily accommodate monovalent cations. The alkalis are absent from island silicates and uncommon in chain silicates.