Editing Silicon dioxide (section)

==Structure==
[[File:SiO2repeat.png|thumb|left|upright=0.7|Structural motif found in α-quartz, but also found in almost all forms of silicon dioxide]]
[[File:Si-OCage.svg|thumb|left|Typical subunit for low pressure silicon dioxide]]
[[File:Quartzrn.PNG|thumb|left|Relationship between refractive index and density for some SiO<sub>2</sub> forms<ref name=mel/>]]
In the majority of silicon dioxides, the silicon atom shows [[Tetrahedral molecular geometry|tetrahedral coordination]], with four oxygen atoms surrounding a central Si atom ([http://www.mindat.org/min-3337.html see 3-D Unit Cell]). Thus, SiO<sub>2</sub> forms 3-dimensional network solids in which each silicon atom is covalently bonded in a tetrahedral manner to 4 oxygen atoms.<ref>{{Citation |title=Crystal Structures of Silica and Metal Silicates |date=2006 |url=https://doi.org/10.1007/0-387-36687-3_10 |work=Structure and Chemistry of Crystalline Solids |pages=233–278 |editor-last=Douglas |editor-first=Bodie E. |access-date=2023-10-08 |place=New York, NY |publisher=Springer |language=en |doi=10.1007/0-387-36687-3_10 |isbn=978-0-387-36687-6 |editor2-last=Ho |editor2-first=Shih-Ming}}</ref><ref>{{Cite journal |last1=Nekrashevich |first1=S. S. |last2=Gritsenko |first2=V. A. |date=2014-02-01 |title=Electronic structure of silicon dioxide (a review) |url=https://www.researchgate.net/publication/262894199 |journal=Physics of the Solid State |language=en |volume=56 |issue=2 |pages=207–222 |doi=10.1134/S106378341402022X |bibcode=2014PhSS...56..207N |s2cid=255234311 |issn=1090-6460}}</ref> In contrast, CO<sub>2</sub> is a linear molecule. The starkly different structures of the dioxides of carbon and silicon are a manifestation of the [[double bond rule]].<ref name="Norman">{{ cite book | title = Periodicity and the s- and p-Block Elements | author = N. C. Norman | publisher = Oxford University Press | year = 1997 | isbn = 978-0-19-855961-0 | pages = 50–52, 65–67 }}</ref>

Based on the crystal structural differences, silicon dioxide can be divided into two categories: crystalline and non-crystalline (amorphous). In crystalline form, this substance can be found naturally occurring as [[quartz]], [[tridymite]] (high-temperature form), [[cristobalite]] (high-temperature form), [[stishovite]] (high-pressure form), and [[coesite]] (high-pressure form). On the other hand, amorphous silica can be found in nature as [[opal]] and [[diatomaceous earth]]. Quartz glass is a form of intermediate state between these structures.<ref>{{Citation |title=Chapter 1 General chemistry of silica |date=1979 |url=https://www.sciencedirect.com/science/article/pii/S0301477008608052 |series=Journal of Chromatography Library |volume=16 |pages=1–14 |editor-last=Unger |editor-first=K. K. |access-date=2023-09-12 |publisher=Elsevier |doi=10.1016/s0301-4770(08)60805-2|isbn=978-0-444-41683-4 }}</ref>

All of these [[Polymorphism (materials science)|distinct crystalline forms]]  always have the same local structure around Si and O. In α-quartz the Si–O bond length is 161 pm, whereas in α-tridymite it is in the range 154–171 pm. The [[Silicon–oxygen bond#Bond angles|Si–O–Si angle]] also varies between a low value of 140° in α-tridymite, up to 180° in β-tridymite. In α-quartz, the Si–O–Si angle is 144°.<ref name="Wiberg&Holleman" />

===Polymorphism===
[[Quartz inversion|Alpha quartz]] is the most stable form of solid SiO<sub>2</sub> at room temperature. The high-temperature minerals, cristobalite and tridymite, have both lower densities and indices of refraction than quartz. The transformation from α-quartz to [[beta-quartz]] takes place abruptly at 573&nbsp;°C. Since the transformation is accompanied by a significant change in volume, it can easily induce fracturing of ceramics or rocks passing through this temperature limit.<ref>{{cite book|url=https://books.google.com/books?id=cUwwoR-RuJ0C&pg=PA93|title=Ceramic Technology for Potters and Sculptors|vauthors=Cuff YH|publisher=University of Pennsylvania|year=1996|isbn=9780812213775|location=Philadelphia|pages=93–95}}</ref> The high-pressure minerals, [[seifertite]], stishovite, and coesite, though, have higher densities and indices of refraction than quartz.<ref>{{cite book|title=Silica Stories|vauthors=De La Rocha C, Conley DJ|publisher=Springer|year=2017|isbn=9783319540542|location=Cham|pages=50–55|chapter=Mystical Crystals of Silica|doi=10.1007/978-3-319-54054-2_4}}</ref> Stishovite has a [[rutile]]-like structure where silicon is 6-coordinate. The density of stishovite is 4.287&nbsp;g/cm<sup>3</sup>, which compares to α-quartz, the densest of the low-pressure forms, which has a density of 2.648&nbsp;g/cm<sup>3</sup>.<ref name = "Greenwood"/> The difference in density can be ascribed to the increase in coordination as the six shortest Si–O bond lengths in stishovite (four Si–O bond lengths of 176 pm and two others of 181 pm) are greater than the Si–O bond length (161 pm) in α-quartz.<ref>{{cite book|title=Structural Inorganic Chemistry|vauthors=Wells AF|publisher=Oxford Science Publications|year=1984|isbn=9780198553700}}</ref>
The change in the coordination increases the ionicity of the Si–O bond.<ref>{{cite journal|display-authors=3|vauthors=Kirfel A, Krane HG, Blaha P, Schwarz K, Lippmann T|year=2001|title=Electron-density distribution in stishovite, SiO<sub>2</sub>: a new high-energy synchrotron-radiation study|journal=[[Acta Crystallographica Section A]]|volume=57|issue=6|pages=663–77|doi=10.1107/S0108767301010698|pmid=11679696|doi-access=free|bibcode=2001AcCrA..57..663K }}</ref>

[[Faujasite]] silica, another polymorph, is obtained by the [[wikt:dealumination|dealumination]] of a low-sodium, ultra-stable Y [[zeolite]] with combined acid and thermal treatment. The resulting product contains over 99% silica, and has high [[crystallinity]] and [[specific surface area]] (over 800 m<sup>2</sup>/g). Faujasite-silica has very high thermal and acid stability. For example, it maintains a high degree of long-range molecular order or [[crystallinity]] even after boiling in concentrated [[hydrochloric acid]].<ref name="fau">{{cite journal|vauthors=Scherzer J|year=1978|title=Dealuminated faujasite-type structures with SiO<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub> ratios over 100|journal=[[Journal of Catalysis|J. Catal.]]|volume=54|issue=2|page=285|doi=10.1016/0021-9517(78)90051-9}}</ref>

===Molten SiO<sub>2</sub>===
[[Molten silica]] exhibits several peculiar physical characteristics that are similar to those observed in liquid [[water (properties)|water]]: negative temperature expansion, density maximum at temperatures ~5000&nbsp;°C, and a heat capacity minimum.<ref>{{cite journal|vauthors=Shell SM, Debenedetti PG, Panagiotopoulos AZ|year=2002|title=Molecular structural order and anomalies in liquid silica|url=http://www.engr.ucsb.edu/~shell/papers/2002_PRE_silica.pdf|journal=[[Physical Review E|Phys. Rev. E]]|volume=66|issue=1|pages=011202|arxiv=cond-mat/0203383|bibcode=2002PhRvE..66a1202S|doi=10.1103/PhysRevE.66.011202|pmid=12241346|s2cid=6109212|access-date=2009-07-07|archive-date=2016-06-04|archive-url=https://web.archive.org/web/20160604062440/http://www.engr.ucsb.edu/~shell/papers/2002_PRE_silica.pdf|url-status=dead}}</ref> Its density decreases from 2.08&nbsp;g/cm<sup>3</sup> at 1950&nbsp;°C to 2.03&nbsp;g/cm<sup>3</sup> at 2200&nbsp;°C.<ref>{{cite journal|vauthors=Aksay IA, Pask JA, Davis RF|year=1979|title=Densities of SiO<sub>2</sub>-Al<sub>2</sub>O<sub>3</sub> Melts|url=http://www.princeton.edu/~cml/assets/pdf/7962aksay.pdf |archive-url=https://ghostarchive.org/archive/20221010/http://www.princeton.edu/~cml/assets/pdf/7962aksay.pdf |archive-date=2022-10-10 |url-status=live|journal=[[Journal of the American Ceramic Society|J. Am. Ceram. Soc.]]|volume=62|issue=7–8|pages=332–336|doi=10.1111/j.1151-2916.1979.tb19071.x}}</ref>

===Molecular SiO<sub>2</sub>===
The molecular SiO<sub>2</sub> has a linear structure like {{CO2}}. It has been produced by combining [[silicon monoxide]] (SiO) with oxygen in an [[argon]] matrix. 
The dimeric silicon dioxide, (SiO<sub>2</sub>)<sub>2</sub> has been obtained by reacting O<sub>2</sub> with matrix isolated dimeric silicon monoxide, (Si<sub>2</sub>O<sub>2</sub>). In dimeric silicon dioxide there are two oxygen atoms bridging between the silicon atoms with an Si–O–Si angle of 94° and bond length of 164.6 pm and the terminal Si–O bond length is 150.2 pm. The Si–O bond length is 148.3&nbsp;pm, which compares with the length of 161&nbsp;pm in α-quartz. The bond energy is estimated at 621.7 kJ/mol.<ref name="Jutzi">{{cite book|title=Silicon chemistry: from the atom to extended systems|vauthors=Jutzi P, Schubert U|publisher=Wiley-VCH|year=2003|isbn=9783527306473}}</ref>