Silicate
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In chemistry, a silicate is a compound containing an anion in which one or more central silicon atoms are surrounded by electronegative ligands. This definition is broad enough to include species such as hexafluorosilicate ("fluosilicate"), [SiF6]2−, but the silicate species that are enocuntered most often consist of silicon with oxygen as the ligand. Silicate anions, with a negative net electrical charge, must have that charge balanced by other cations to make an electrically neutral compound.
Silica, or silicon dioxide, SiO2, is sometimes considered a silicate, although it is the special case with no negative charge and no need for counter-ions. Silica is found in nature as the mineral quartz, and its polymorphs.
In the vast majority of silicates, including silicate minerals, the Si atom shows tetrahedral coordination by 4 oxygens. In different minerals the tetrahedra show different degrees of polymerization: they occur singly, joined together in pairs, in larger finite clusters including rings, in chains, double chains, sheets, and three-dimensional frameworks. The minerals are classified into groups based on these anion structures; a list is given below.
Silicon may adopt octahedral coordination by 6 oxygens at very high pressure, as in the dense stishovite polymorph of silica that is found in the lower mantle of the Earth, and which is also formed by shock during meteorite impacts. Lack of space around the oxygen atoms makes this coordination for Si very rare at normal pressure, but it is known in the hexahydroxysilicate anion, [Si(OH)6]2−, as found in the mineral thaumasite.
[edit] Silicate rock
In geology and astronomy, the term silicate is used to denote types of rock that consists predominantly of silicate minerals. Such rocks include a wide range of igneous, metamorphic and sedimentary types. Most of the Earth's mantle and crust are made up of silicate rocks. The same is true of other terrestrial planets.
On Earth, a wide variety of silicate minerals occur in an even wider range of combinations as a result of the processes that form and re-work the crust. These processes include partial melting, crystallization, fractionation, metamorphism, weathering and diagensis. Living things also contribute to the silicate budget near the Earth's surface. 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
[edit] Mineralogy
Mineralogically, silicate minerals are divided according to structure of their silicate anion into the following groups:
- Nesosilicates (lone tetrahedron) - [SiO4]4−, eg olivine.
- Sorosilicates (double tetrahedra) - [Si2O7]6−, eg epidote.
- Cyclosilicates (rings) - [SinO3n]2n−, eg tourmaline group.
- Inosilicates (single chain) - [SinO3n]2n−, eg pyroxene group.
- Inosilicates(double chain) - [Si4nO11n]6n−, eg amphibole group.
- Phyllosilicates (sheets) - [Si2nO5n]2n−, eg micas and clays.
- Tectosilicates (3D framework) - [AlxSiyO2(x+y)]x−, eg quartz, feldspars, zeolites.
Note that tectosilicates can only have additional cations if some of the silicon is replaced by a lower-charge cation such as aluminium, to give a negative charge overall. This substitution can also take place in other types of silicate.
Some rare minerals have more than one type of anion coexisting in their crystal structures, or complex-shaped anions that are intermediate between the simple types above.
