Silicate mineral

Silicate minerals are rock-forming minerals composed of silicate groups, representing the largest and most important class of minerals and constituting approximately 90 percent of Earth’s crust. In mineralogy, crystalline forms of silica are typically considered tectosilicates, classified as such in the Dana system, though the Nickel–Strunz classification categorizes them as oxide minerals. Silica occurs naturally as quartz and its polymorphs.

A wide variety of silicate minerals arise from processes that have formed and reworked the crust for billions of years, including partial melting, crystallization, fractionation, metamorphism, weathering, and diagenesis. Living organisms also contribute; for instance, diatoms construct their exoskeletons from silica extracted from seawater, and their frustules form a major part of deep ocean sediment and diatomaceous earth. Structurally, silicate minerals are generally inorganic compounds with subunits of the formula [SiO2+n]2n−, balanced by metal cations such as Mg2+, Fe2+, and Na+. The Si-O-M linkages are strong, polar-covalent bonds.

Silicate anions are colorless or white when powdered; mineral colors arise from the metal component, commonly iron. Silicon is tetrahedral, surrounded by four oxides, and some silicon centers may be replaced by other elements, altering the charge and requiring extra cations—for example, in orthoclase, aluminum substitution for silicon creates an anion neutralized by potassium. Silicate minerals are classified into seven major groups based on the silicate anion structure: tectosilicates, nesosilicates (isolated tetrahedra), sorosilicates (double tetrahedra), cyclosilicates (rings), inosilicates (chains), phyllosilicates (sheets), and others.