SiO4

SiO4 is most commonly encountered as the orthosilicate tetrahedron, SiO4^4−, a fundamental structural unit in silicate minerals. In this unit, a silicon atom occupies the center of a tetrahedron formed by four oxide ions. Silicon is in the +4 oxidation state, and the four Si–O bonds are equivalent, giving the tetrahedron approximately Td symmetry.

These tetrahedra can exist as isolated building blocks or connect by sharing corner oxygens to create a wide variety of silicate structures. The degree of polymerization is used to classify silicates: isolated SiO4 tetrahedra (nesosilicates, charge-balanced by cations such as Mg2+ or Ca2+), pairs or dimers (disilicates, Si2O7^6−), single chains (inosilicates, -SiO3^- units), double chains, sheets, and three-dimensional frameworks (framework silicates such as SiO2).

In solution, silicate chemistry involves orthosilicic acid, H4SiO4, which can deprotonate and polymerize to form silicate species depending on pH and concentration. In minerals, the balance of acid-base properties and surrounding cations determines the exact formulae and structural arrangement of the SiO4 units.

SiO4 tetrahedra are the backbone of silicate minerals, the most abundant class of minerals in Earth’s crust. The SiO4 unit’s ability to link through shared oxygen atoms underpins the diversity of silicate materials, from simple olivine-type compounds to the extensive three-dimensional networks found in quartz and feldspars, as well as many synthetic silicates used in ceramics and glass.