Silicate

This article is about silicate in chemistry. For silicate rocks, see silicate mineral.

In chemistry, a silicate is an anion (negatively charged ion) made of silicon and oxygen. Silicate anions, with a negative net electrical charge, must have that charge balanced by other cations to make an electrically neutral compound. These compounds are also called silicates.

General structure

All silicates are made up of tetrahedral shapes, with a silicon atom at the centre and four oxygen atoms at the corners. In silicates, silicon is always in the +4 oxidation state, and oxygen is always in the -2 oxidation state. Different structures can form based on whether or not multiple silicon atoms share a single oxygen atom.

As silicates are normally large polymers, they are usually described by an empirical formula for a repeating unit, not the formula for a whole molecule.

Types of silicate

Tectosilicate

Tectosilicate is a neutral compound, not an ion, with the formula SiO₂. In tectosilicates, every oxygen atom is shared by two silicon atoms. Quartz is a common mineral form of tectosilicate.

Tectosilicate anions only exist when silicon is replaced by another element that has a lower oxidation state. Feldspar is a tectosilicate where one out of four silicon atoms is replaced by aluminium, giving an anion with the formula AlSi₃O−8. Zeolite is an important tectosilicate aluminosilicate in the chemical industry.

Phyllosilicate

Structure of the flat phyllosilicate sheet in muscovite, a mica.

Phyllosilicate has the formula Si₂O2−5. In phyllosilicates, each silicon tetrahedron shares three of its four oxygen atoms to make flat sheets.

Phyllosilicate minerals tend to have multiple layers. The oxygen atoms that aren't shared in the silicate sheet are shared with a layer of octahedral cations like magnesium or aluminium to form the whole mineral structure. Hydroxide, chloride, or fluoride fills out the octahedral layer. In some phyllosilicates, like the micas, there are two phyllosilicate layers, one on each side of the octahedral layer.

Phyllosilicates are an important part of clay. Kaolinite, the main mineral used to make porcelain, is a phyllosilicate with an aluminium octahedral layer.

Orthosilicate

Orthosilicate is the ion SiO−4. None of the oxygen atoms are shared between silicon atoms.

The orthosilicate ion is a strong base. Protonation of orthosilicate gives orthosilicic acid, a weak polyprotic acid. Orthosilicate, orthosilicic acid, and the various [SiO_x(OH)_4−x]^x− hydrogen orthosilicates are the main form of dissolved silica in water.