View source for Silicate

{{Short description|Any polyatomic anion containing silicon and oxygen}}
[[File:Orthosilicate-2D-dimensions.png|thumb|Structure of the [[orthosilicate (ion)|orthosilicate]] anion {{chem|SiO|4|4−}}|250px]]

A '''silicate''' is any member of a family of [[polyatomic anion]]s consisting of [[silicon]] and [[oxygen]], usually with the general formula {{chem|[SiO|4−''x''|(4−2''x'')− |]|''n''}}, where {{nowrap|0 ≤ ''x'' < 2}}.  The family includes [[orthosilicate]] {{chem2|SiO4(4-)}} ({{nowrap|1=''x'' = 0}}), [[metasilicate]] {{chem2|SiO3(2-)}} ({{nowrap|1=''x'' = 1}}), and [[pyrosilicate]] {{chem2|Si2O7(6-)}} ({{nowrap|1=''x'' = 0.5}}, {{nowrap|1=''n'' = 2}}).  The name is also used for any [[salt (chemistry)|salt]] of such anions, such as [[sodium metasilicate]]; or any [[ester]] containing the corresponding [[Moiety (chemistry)|chemical group]], such as [[tetramethyl orthosilicate]].<ref name=green/> The name "silicate" is sometimes extended to any anions containing silicon, even if they do not fit the general formula or contain other atoms besides oxygen; such as [[hexafluorosilicic acid|hexafluorosilicate]] {{chem2|[SiF6](2-)}}. Most commonly, silicates are encountered as [[silicate minerals]].

For diverse manufacturing, technological, and artistic needs, silicates are versatile materials, both natural (such as [[granite]], [[gravel]], and [[garnet]]) and artificial (such as [[Portland cement]], [[ceramic]]s, [[glass]], and [[waterglass]]).

==Structural principles==
In most silicates, a silicon atom occupies the center of an idealized [[tetrahedron]] whose corners are four oxygen atoms, connected to it by single [[covalent bond]]s according to the [[octet rule]].<ref name=green>{{Greenwood&Earnshaw2nd}}</ref> The oxygen atoms, which bear some negative charge, link to other cations (M<sup>n+</sup>).  This Si-O-M-O-Si linkage is strong and rigid, which properties are manifested in the rock-like silicates.<ref>{{Cite book |title=The 23rd edition of the manual of mineral science. Buch |date=2008 |publisher=Wiley |isbn=978-0-471-72157-4 |location=Hoboken, N.J}}</ref>{{rp|435}}  The silicates can be classified according to the length and crosslinking of the silicate anions.

===Isolated silicates===
Isolated [[orthosilicate]] anions have the formula {{chem|SiO|4|4−}}. A common mineral in this group is [[olivine]] ({{chem2|(Mg,Fe)2SiO4}}).

Two or more silicon atoms can share oxygen atoms in various ways, to form more complex anions, such as [[pyrosilicate]] {{chem|Si|2|O|7|6−}}.

===Chains===
[[File:Silicate-single-chain-plan-view-2D.png|thumb|Depiction of a metasilicate chain, emphasizing the tetrahedral silicate subunits.]]
[[File:Na2SiO3idealized.png|thumb|right|220 px|Alternative depiction of a metasilicate chain emphasizing the Si-O bonds.]]
With two shared oxides bound to each silicon, cyclic or polymeric structures can result.  The cyclic [[metasilicate]] ring {{chem|Si|6|O|18|12−}} is a [[hexamer]] of SiO<sub>3</sub><sup>2-</sup>.  [[Polymer]]ic silicate anions of can exist also as long chains.

In single-chain silicates, which are a type of [[inosilicate]], tetrahedra link to form a chain by sharing two oxygen atoms each. A common mineral in this group is [[pyroxene]].

[[File:Silicate-double-chain-plan-view-2D.png|thumb|alt=Double chain tetrahedra.|Double chain tetrahedra.]]
Double-chain silicates, the other category of inosilicates, occur when tetrahedra form a double chain (not always but mostly) by sharing two or three oxygen atoms each. Common minerals for this group are [[amphiboles]].

=== Sheets ===
[[File:Silicate-sheet-3D-polyhedra.png|thumb|alt=Sheet Silicates.|Sheet silicates.]]
In this group, known as [[phyllosilicates]], tetrahedra all share three oxygen atoms each and in turn link to form two-dimensional sheets. This structure does lead to minerals in this group having one strong cleavage plane. [[Micas]] fall into this group. Both [[muscovite]] and [[biotite]] have very weak layers that can be peeled off in sheets.

=== Framework ===

In a framework silicate, known as a [[tectosilicate]], each tetrahedron shares all 4 oxygen atoms with its neighbours, forming a 3D structure. [[Quartz]] and [[feldspar]]s are in this group.

==Silicates with non-tetrahedral silicon==
Although the tetrahedron is a common coordination geometry for silicon(IV) compounds, silicon may also occur with higher coordination numbers.  For example, in the anion [[hexafluorosilicate]] {{chem|SiF|6|2−}}, the silicon atom is surrounded by six [[fluorine]] atoms in an [[octahedron|octahedral]] arrangement.  This structure is also seen in the hexahydroxysilicate anion {{chem|Si(OH)|6|2-}} that occurs in [[thaumasite]], a mineral found rarely in nature but sometimes observed among other [[calcium silicate hydrate]]s artificially formed in [[cement]] and [[concrete]] structures submitted to a severe [[sulfate attack]] in [[clay|argillaceous]] grounds containing [[redox|oxidized]] [[pyrite]].<ref name="Crammond1995">{{Cite book |first1=N. J. |last1=Crammond |first2=M. A. |last2=Halliwell |date=1995 |title=The thaumasite form of sulfate attack in concretes containing a source of carbonate ions—A microstructural overview |url=https://www.concrete.org/publications/internationalconcreteabstractsportal.aspx?m=details&id=963 |language=en |publisher=American Concrete Institute |doi=10.14359/963 |isbn=978-0-87031-652-4}}</ref><ref name="Crammond2002">{{Cite journal |last1=Crammond |first1=Norah |date=2002-06-01 |title=The occurrence of thaumasite in modern construction – A review |url=https://www.sciencedirect.com/science/article/pii/S0958946501000920 |journal=Cement and Concrete Composites |volume=24 |issue=3 |pages=393–402 |doi=10.1016/S0958-9465(01)00092-0 |issn=0958-9465|url-access=subscription }}</ref><ref name="Crammond2003">{{Cite journal |last1=Crammond |first1=N. J |date=2003-12-01 |title=The thaumasite form of sulfate attack in the UK |url=https://www.sciencedirect.com/science/article/pii/S0958946503001069 |journal=Cement and Concrete Composites |series=Thaumasite in Cementitious Materials |volume=25 |issue=8 |pages=809–818 |doi=10.1016/S0958-9465(03)00106-9 |issn=0958-9465|url-access=subscription }}</ref><ref name="Longworth2003">{{Cite journal |last1=Longworth |first1=T. I |date=2003-12-01 |title=Contribution of construction activity to aggressive ground conditions causing the thaumasite form of sulfate attack to concrete in pyritic ground |url=https://www.sciencedirect.com/science/article/pii/S0958946503001240 |journal=Cement and Concrete Composites |series=Thaumasite in Cementitious Materials |volume=25 |issue=8 |pages=1005–1013 |doi=10.1016/S0958-9465(03)00124-0 |issn=0958-9465|url-access=subscription }}</ref><ref name="Sims2004">{{Cite journal |last1=Sims |first1=Ian |last2=Huntley (née Hartshorn) |first2=Sarah A |date=2004-10-01 |title=The thaumasite form of sulfate attack-breaking the rules |url=https://www.sciencedirect.com/science/article/pii/S0958946504000034 |journal=Cement and Concrete Composites |volume=26 |issue=7 |pages=837–844 |doi=10.1016/j.cemconcomp.2004.01.002 |issn=0958-9465|url-access=subscription }}</ref>

At very high pressure, such as exists in the majority of the Earth's rock, even SiO<sub>2</sub> adopts the six-coordinated octahedral geometry in the mineral [[stishovite]], a dense polymorph of [[silica]] found in the [[lower mantle]] of the Earth and also formed by shock during [[meteorite]] impacts.<ref>{{Cite journal |last1=Sinclair |first1=W. |last2=Ringwood |first2=A. E. |date=1978 |title=Single crystal analysis of the structure of stishovite |url=https://www.nature.com/articles/272714a0 |journal=Nature |language= |volume=272 |issue= 5655|pages=714–715 |doi=10.1038/272714a0 |bibcode=1978Natur.272..714S |issn=1476-4687|url-access=subscription }}</ref>

==Chemical properties==
<!--Solid silicates are generally stable and well characterized.-->

Silicates with [[alkali]] cations and small or chain-like anions, such as [[sodium orthosilicate|sodium ortho-]] and [[sodium metasilicate|metasilicate]], are fairly soluble in water.  They form several solid [[hydrate]]s when crystallized from solution.  Soluble [[sodium silicate]]s and mixtures thereof, known as [[waterglass]] are important industrial and household chemicals. Silicates of non-alkali cations, or with sheet and tridimensional polymeric anions, generally have negligible solubility in water at normal conditions.

===Reactions===
{{Biomineralization sidebar|geologic forms}}
Silicates are generally inert chemically.  Hence they are common minerals. Their resiliency also recommends their use as building materials.

When treated with calcium oxides and water, silicate minerals form [[Portland cement]].

Equilibria involving hydrolysis of silicate minerals are difficult to study. The chief challenge is the very low solubility of SiO<sub>4</sub><sup>4-</sup> and its various protonated forms. Such equilibria are relevant to the processes occurring on geological time scales.<ref name=knight/><ref name=GBAlex>G. B. Alexander (1953): "The Reaction of Low Molecular Weight Silicic Acids with Molybdic Acid". ''Journal of the American Chemical Society, volume 75, issue 22, pages 5655–5657. {{doi|10.1021/ja01118a054}}</ref>  Some plants excrete ligands that dissolve silicates, a step in [[biomineralization]].

Catechols can depolymerize SiO<sub>2</sub>—a component of silicates with ionic structures like orthosilicate (SiO<sub>4</sub><sup>4-</sup>), metasilicate (SiO<sub>2</sub><sup>3-</sup>), and pyrosilicate (Si<sub>2</sub>O<sub>6</sub><sup>7-</sup>)—by forming bis- and tris(catecholate)silicate dianions through coordination.<ref>{{Cite journal |last=Mazaheri |first=Omid |date=14 November 2024 |title=Assembly of Silicate–Phenolic Network Coatings with Tunable Properties for Controlled Release of Small Molecules |url=https://onlinelibrary.wiley.com/doi/full/10.1002/adma.202413349 |journal=Advanced Materials |volume=36 |issue=52 |article-number=2413349 |doi=10.1002/adma.202413349|pmid=39535829 |bibcode=2024AdM....3613349M |url-access=subscription }}</ref> This complexes can be further coated on various substrates for applications such as drug delivery systems, antibacterial and antifouling applications.

===Detection===
Silicate anions in solution react with [[molybdate]] anions yielding yellow [[silicomolybdate]] complexes. In a typical preparation, [[monomer]]ic orthosilicate was found to react completely in 75 seconds; [[dimer (chemistry)|dimer]]ic pyrosilicate in 10 minutes; and higher [[oligomer]]s in considerably longer time.  In particular, the reaction is not observed with suspensions of [[colloidal silica]].<ref name=GBAlex/>

===Zeolite formation and geopolymers polymerisation===
The nature of soluble silicates is relevant to understanding [[biomineralization]] and the synthesis of [[aluminosilicate]]s, such as the industrially important [[catalyst]]s called [[zeolite]]s.<ref name=knight>{{cite journal | last1 = Knight | first1 = Christopher T. G. | last2 = Balec | first2 = Raymond J. | last3 = Kinrade | first3 = Stephen D. | year = 2007 | title = The Structure of Silicate Anions in Aqueous Alkaline Solutions | journal = Angewandte Chemie International Edition | volume = 46 | issue = 43 | pages = 8148–8152 | doi = 10.1002/anie.200702986 | pmid = 17886822 | bibcode = 2007ACIE...46.8148K }}</ref> Along with [[aluminate]] [[anion]]s, soluble silicate anions also play a major role in the polymerization mechanism of [[geopolymer]]s. Geopolymers are [[amorphous]] aluminosilicates whose production requires less energy than that of ordinary [[Portland cement]]. So, [[geopolymer cement]]s could contribute to limiting the {{CO2|link=carbon dioxide}} emissions in the Earth [[atmosphere]] and the [[global warming]] caused by this [[greenhouse gas]].

== See also ==
* 
* [[Alkali-silica reaction]]
* [[Carbon cycle]]
* [[Carbonate–silicate cycle|Carbonate-silicate cycle]]
* [[Ocean acidification]]

==References==
{{Reflist}}

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{{DEFAULTSORT:Silicon-oxygen tetrahedron}}
[[Category:Silicates]]
[[Category:Silicon oxyanions]]