In chemistry, a silicate is any member of a family of anions consisting of silicon and oxygen, usually with the general formula [SiO(4−2x)−4−x]n, where 0 ≤ x < 2. The family includes orthosilicate SiO4−4 (x = 0), metasilicate SiO2−3 (x = 1), and pyrosilicate Si2O6−7 (x = 0.5, n = 2). The name is also used for any salt of such anions, such as sodium metasilicate; or any ester containing the corresponding chemical group, such as tetramethyl orthosilicate.Silicate anions are often large polymeric molecules with an extense variety of structures, including chains and rings (as in polymeric metasilicate [SiO2−3]n), double chains (as in [Si2O2−5]n, and sheets (as in [Si2O2−5]n.In geology and astronomy, the term silicate is used to mean silicate minerals, ionic solids with silicate anions; as well as rock types that consist predominantly of such minerals. In that context, the term also includes the non-ionic compound silicon dioxide SiO2 (silica, quartz), which would correspond to x = 2 in the general formula. The term also includes minerals where aluminium or other tetravalent atoms replace some of the silicon atoms, as in the aluminosilicates. Such silicates comprise most of Earth's crust and mantle, as well as the other terrestrial planets, rocky moons, and asteroids.Silicates are extremely important materials, both natural (such as granite, gravel, and garnet) and artificial (such as Portland cement, ceramics, glass, and waterglass), for all sorts of technological and artistic activities.
The name "silicate" is sometimes extended to any anions containing silicon, even if they do not fit the general formula or contain other atoms besides oxygen; such as the hexahydroxysilicate [Si(OH)6]2− or hexafluorosilicate [SiF6]2−.