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4) structure
A silicate is a
containing an
compound. The great majority of the silicates are , but
([SiF6]2-) and other anions are also included.
"" is the anion SiO4-
4 or its compounds. Related to orthosilicate are families of anions (and their compounds) with the formula [SiO2+n]2n-. Important members are the cyclic and single chain silicates {[SiO3]2-}n and the sheet-forming silicates {[SiO2.5]-}n.
Silicates constitute the majority of , as well as the other , rocky , and . Sand, , and thousands of
are examples of silicates. Silicate compounds, including the minerals, consist of silicate anions whose charge is balanced by various . Myriad silicate anions can exist, and each can form compounds with many different cations. Hence this class of compounds is very large. Both minerals and synthetic materials fit in this class.
In the vast majority of silicates, including , the Si occupies a
environment, being surrounded by 4 oxygen centres. In these structures, the chemical bonds to silicon conform to the . These tetrahedra sometimes occur as , but most commonly, the tetrahedra are joined together in various ways, such as pairs (Si
7) and rings (Si
18). Commonly the silicate anions are chains, double chains, sheets, and three-dimensional frameworks. All these such species have negligible solubility in water at normal conditions.
Silicates are well characterized as solids, but are less commonly observed in solution. The anion SiO4-
4 is the conjugate base of silicic acid, Si(OH)4, and both are elusive as are all of the intermediate species. Instead, solutions of silicates are usually observed as mixtures of condensed and partially protonated silicate clusters. The nature of soluble silicates is relevant to understanding
and the synthesis of aluminosilicates, such as the industrially important
Although the tetrahedron is the common coordination geometry for silicon compounds, silicon is well known to also adopt higher coordination numbers. A well-known example of such a high coordination number is
coordination by 6 oxygen centres is observed. At very high pressure, even SiO2 adopts this geometry in the mineral , a dense polymorph of
found in the
of the Earth. This structure is also formed by shock during
impacts. Octahedral Si in the form of hexahydroxysilicate ([Si(OH)6]2-) is observed in [] a mineral occurring rarely in nature but sometimes observed amongst other
artificially formed in
submitted to a severe .
, as viewed under a microscope, is a soft, ,
made up of the
(shells) of single cell . Diatom cell walls are
silica synthesised in the diatom cell by the
of . This image of diatomaceous earth particles in
is at a scale of 6.236 pixels/, the entire image covers a region of approximately 1.13 by 0.69 mm.
and , the term silicate is used to denote types of
that consist predominantly of . 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 , , , ,
and . Living things also contribute to the silicate cycle near the Earth's surface. A type of
construct their , known as tests, from silica. The tests of dead diatoms are a major constituent of
Silica, or , 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
, and its .
Portland cement is produced by hydration of clinker, which consists of various calcium silicates, including
(Ca3SiO5, also written CaO·Ca2SiO4) and
(Ca2SiO4). These components are often generated in situ by heating various clays and limestone.
are divided according to structure of their silicate anion into the following groups:
(lone tetrahedron) – [SiO4]4-, e.g. .
(double tetrahedra) – [Si2O7]6-, e.g. ,
(rings) – [SinO3n]2n-, e.g.
(single chain) – [SinO3n]2n-, e.g.
(double chain) – [Si4nO11n]6n-, e.g.
(sheets) – [Si2nO5n]2n-, e.g.
(3D framework) – [AlxSiyO2x+2y)]x-, e.g. , , .
Note that tectosilicates can only have additional cations if some of the silicon is replaced by a lower-charge cation such as . Al for Si substitution is common.
; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). .  .
Knight, Christopher T. G.; Balec, Raymond J.; Kinrade, Stephen D. (2007). "The Structure of Silicate Anions in Aqueous Alkaline Solutions". Angewandte Chemie International Edition. 46: . :.
Siegbert Sprung "Cement" in Ullmann's Encyclopedia of Industrial Chemistry, 2012 Wiley-VCH, Weinheim. :
Deer, W.A.; Howie, R.A., & Zussman, J. (1992). An introduction to the rock forming minerals (2nd edition ed.). London: Longman  
Hurlbut, Cornelius S.; Klein, Cornelis (1985). Manual of Mineralogy, Wiley, (20th edition ed.).  
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