Silicates
A silicate is a compound containing an anionic silicon compound. The great majority of the silicates are oxides, but hexafluorosilicate ([SiF6]2−) and other anions are also included.
"Orthosilicate" is the anion SiO4−
4 or its compounds. Related to orthosilicate are families of anions (and their compounds) with the formula [SiO2+n]2n−. Important members are the cyclic and single chain silicates {[SiO3]2−}n and the sheet-forming silicates {[SiO2.5]−}n.
Silicates constitute the majority of Earth's crust, as well as the other terrestrial planets, rocky moons, and asteroids. Sand, Portland cement, and thousands of minerals are examples of silicates. Silicate compounds, including the minerals, consist of silicate anions whose charge is balanced by various cations. Myriad silicate anions can exist, and each can form compounds with many different cations. Hence this class of compounds is very large. Both minerals and synthetic materials fit in this class.
In the vast majority of silicates, including silicate minerals, the Si occupies a tetrahedral environment, being surrounded by 4 oxygen centres. In these structures, the chemical bonds to silicon conform to the octet rule. These tetrahedra sometimes occur as isolated SiO4−
4 centres, but most commonly, the tetrahedra are joined together in various ways, such as pairs (Si
2O6−
7) and rings (Si
6O12−
18). Commonly the silicate anions are chains, double chains, sheets, and three-dimensional frameworks. All these such species have negligible solubility in water at normal conditions.
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