Editing Silicate (section)

==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.