orthosilicate

Orthosilicate refers to a silicate unit in which silicon is coordinated by four oxygen atoms in a tetrahedral geometry, and the tetrahedra do not share oxygen with other SiO4 units. The fundamental anion is [SiO4]4−, in which silicon is in the +4 oxidation state. In compounds this unit is balanced by metal cations, giving formulas such as M2SiO4 for divalent cations (for example Mg2SiO4, Fe2SiO4) or Na4SiO4 and K4SiO4 for monovalent cations.

In mineralogy, orthosilicates are part of the nesosilicates, a class of silicates with isolated SiO4 tetrahedra. The SiO4 units do not polymerize into chains, sheets, or frameworks; instead, they are linked to a surrounding cation lattice. Olivine, with the formula (Mg,Fe)2SiO4, is a canonical example of a natural orthosilicate and illustrates how isolated tetrahedra are dispersed within a magnesium- and iron-rich matrix.

Occurrence and relevance: Orthosilicates occur widely in the Earth's mantle and crust, where minerals such as olivine are among the most abundant. In synthetic chemistry and materials science, orthosilicate salts serve as precursors to more complex silicate materials and can be used in ceramic and glass-ceramic syntheses. The term orthosilicate is often used interchangeably with nesosilicate in reference to isolated silicon tetrahedra.

See also: Silicate minerals, Nesosilicates, Olivine.