Silicate

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In chemistry, a silicate is a compound consisting of silicon and oxygen (Si_xO_y), one or more metals, and possibly hydrogen. It is also used to denote the salts of silica or of one of the silicic acids.

In common conditions, the most stable form is silicon dioxide, SiO₂, the mineral quartz, and similar species. This always has, in equilibrium, a minute amount of silicic acid, H₄SiO₄. Chemists consider quartz as insoluble under standard atmospheric conditions. Also, in basic conditions, we find H₂SiO₄²⁻.

Silicate minerals are noted for their tetrahedral form. In different minerals the tetrahedra show different degrees of polymerization, and may be joined together in chains, double chains, sheets, and three-dimensional frameworks. They are subclassified into groups based on these structures; a list is given below.

In geology and astronomy, the term silicate is used to denote a type of rock that consists predominately of silicate minerals. Such rocks range from granite to gabbro. Most of the Earth's crust is made up of silicate rocks, as are the crusts of other terrestrial planets. A type of plankton known as diatoms construct their exoskeletons, known as tests, from silica. The tests of dead diatoms are a major constituent of deep ocean sediment.

Mineralogically, silicate minerals are divided according to their molecular structure into the following groups:

Olivine (single tetrahedron) - Nesosilicates
Epidote (double tetrahedra) - Sorosilicates
Tourmaline (rings of tetrahedra) - Cyclosilicates
Pyroxene (single chain) - Inosilicates
Amphibole (double chain) - Inosilicates
Micas and clays (sheet) - Phyllosilicates
Feldspars (framework) - Tectosilicates
Quartz (SiO₂ framework)

Silicate

See also