A2SiO4

A2SiO4 is a general chemical formula used to describe a class of orthosilicate compounds in which a divalent cation A2+ combines with the tetrahedral SiO4^4- unit. In natural systems the best-known representatives are the olivine group minerals, with end-members Mg2SiO4 (forsterite) and Fe2SiO4 (fayalite). The A2SiO4 family also accommodates solid-solution with other divalent cations, yielding compositions such as (Mg,Fe)2SiO4 and related substitutions.

Structure and bonding: The SiO4 tetrahedra in orthosilicates are discrete, not sharing oxygen with other SiO4 units. The negative charge of SiO4^4- is balanced by A2+ cations that occupy octahedral or similar coordination sites, linking the tetrahedra into a crystalline lattice. In olivine specifically, the arrangement is highly anisotropic and the mineral crystallizes in the orthorhombic system; Mg and Fe order within the lattice influences optical and physical properties.

Occurrence and significance: Mg-Fe olivines are among the most common minerals in the Earth's upper mantle and in ultramafic igneous rocks. They crystallize early from high-temperature magmas and persist to substantial depths, contributing to mantle rheology and seismic velocities. They can form phenocrysts in basalt and other inosilicate-bearing rocks and occur as megacrysts in certain igneous rocks.

Synthesis and applications: Synthetic A2SiO4 materials are produced by high-temperature solid-state reactions and are studied as model systems for silicate bonding. Magnesium-rich olivine and related compositions have been investigated for ceramic refractories and as host lattices for dopants in luminescent materials. Peridot, the green gemstone variety of Mg2SiO4, is a widely known commercial example.