Orthosilicate

In chemistry, orthosilicate is the anion SiO⁴⁻
₄, or any of its salts and esters. It is one of the silicate anions. It is occasionally called the silicon tetroxide anion or group.

Orthosilicate salts, like sodium orthosilicate, are stable, and occur widely in nature as silicate minerals, being the defining feature of the nesosilicates. Olivine, a magnesium or iron(II) orthosilicate, is the most abundant mineral in the upper mantle.

The orthosilicate anion is a strong base, the conjugate base of the extremely weak orthosilicic acid H
₄SiO
₄ (pK_a2 = 13.2 at 25 °C). This equilibrium is difficult to study since the acid tends to decompose into a hydrated silica condensate.