View source for Silicon dioxide

{{Short description|Oxide of silicon}}
{{Redirect|Silica}}
{{cs1 config|name-list-style=vanc}}

{{Chembox
|Verifiedfields = changed
|Watchedfields  = changed
|verifiedrevid  = 476993639
|ImageFile1 = Sample of silicon dioxide.jpg
|ImageCaption1 = A sample of silicon dioxide
|ImageFile1_Ref = {{Chemboximage|correct|??}}
|ImageNameR1 = A sample of silicon dioxide as sand in a dune
|ImageCaptionR1 = Silicon dioxide as sand
|IUPACName = Silicon dioxide
|OtherNames = {{unbulleted list |Quartz |Silica |Silicic oxide |Silicon(IV) oxide |Crystalline silica |Pure Silica |Silicea |Silica sand }}
|Section1 = {{Chembox Identifiers
|CASNo = 7631-86-9
|CASNo_Ref = {{cascite|correct|CAS}}
|PubChem = 24261
|ChemSpiderID = 22683
|ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
|UNII = ETJ7Z6XBU4
|UNII_Ref = {{fdacite|correct|FDA}}
|EINECS = 231-545-4
|KEGG = C16459
|KEGG_Ref = {{keggcite|correct|kegg}}
|MeSHName = Silicon+dioxide
|ChEBI_Ref = {{ebicite|correct|EBI}}
|ChEBI = 30563
|RTECS = VV7565000
|Gmelin = 200274
|StdInChI = 1S/O2Si/c1-3-2
|StdInChI_Ref = {{stdinchicite|correct|chemspider}}
|StdInChIKey = VYPSYNLAJGMNEJ-UHFFFAOYSA-N
|StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
}}
|Section2 = {{Chembox Properties
|Formula =SiO<sub>2</sub>
|MolarMass = 60.08 g/mol
|Appearance = Transparent or white
|Density = 2.648 (α-quartz), 2.196 (amorphous) g·cm<sup>−3</sup><ref name=b92>{{RubberBible92nd}}</ref>
|MeltingPtC = 1713
|MeltingPt_notes = (amorphous)<ref name=b92/>{{rp|page=4.88}}
|BoilingPtC = 2950
|BoilingPt_ref = <ref name=b92/>
|RefractIndex = 1.544 ([[Birefringence|o]]), 1.553 (e)<ref name=b92/>{{rp|page=4.143}}
|ThermalConductivity = 12 (<nowiki>||</nowiki> c-axis), 6.8 (⊥ c-axis), 1.4 (am.) W/(m⋅K)<ref name=b92/>{{rp|page=12.213}}
|MagSus = &minus;29.6·10<sup>−6</sup> cm<sup>3</sup>/mol
}}
|Section3 = {{Chembox Hazards
|NFPA-H = 0
|NFPA-F = 0
|NFPA-R = 0
|IDLH = 3000 mg/m<sup>3</sup> (amorphous)<ref name=PGCH0552>{{PGCH|0552}}</ref><br/>Ca [25 mg/m<sup>3</sup> (cristobalite, tridymite); 50 mg/m<sup>3</sup> (quartz)]<ref name=PGCH0682>{{PGCH|0682}}</ref>
|REL = TWA 6 mg/m<sup>3</sup> (amorphous)<ref name=PGCH0552/><br/>Ca TWA 0.05 mg/m<sup>3</sup><ref name=PGCH0682/>
|PEL = TWA 20 mppcf (80 mg/m<sup>3</sup>/%SiO<sub>2</sub>) (amorphous)<ref name=PGCH0552/>
}}
|Section4 = {{Chembox Related
|OtherFunction_label = diones
|OtherFunction = [[Carbon dioxide]]<br/>[[Germanium dioxide]]<br/>[[Tin dioxide]]<br/>[[Lead dioxide]]
|OtherCompounds = [[Silicon monoxide]]<br/>[[Silicon disulfide]]
}}
|Section5 = {{Chembox Thermochemistry
|DeltaHf = −911&nbsp;kJ·mol<sup>−1</sup><ref name=b1>{{cite book| author = Zumdahl, Steven S.|title =Chemical Principles 6th Ed.| publisher = Houghton Mifflin Company| year = 2009| isbn = 978-0-618-94690-7|page=A22}}</ref>
|Entropy = 42&nbsp;J·mol<sup>−1</sup>·K<sup>−1</sup><ref name=b1/>
}}
}}

'''Silicon dioxide''', also known as '''silica''', is an [[oxide]] of [[silicon]] with the [[chemical formula]] {{chem2|SiO2}}, commonly found in nature as [[quartz]].<ref>{{cite book|title=The Chemistry of Silica|vauthors=Iler RK|publisher=Wiley|year=1979|isbn=9780471024040|location=New York}}</ref><ref name="Fern">{{cite journal|vauthors=Fernández LD, Lara E, Mitchell EA|year=2015|title=Checklist, diversity and distribution of testate amoebae in Chile|url=http://doc.rero.ch/record/257075/files/Fernandez_L._D.-Checklist_diversity_distribution_of_testate_amoebae-20150922.pdf |archive-url=https://ghostarchive.org/archive/20221010/http://doc.rero.ch/record/257075/files/Fernandez_L._D.-Checklist_diversity_distribution_of_testate_amoebae-20150922.pdf |archive-date=2022-10-10 |url-status=live|journal=European Journal of Protistology|volume=51|issue=5|pages=409–24|doi=10.1016/j.ejop.2015.07.001|pmid=26340665}}</ref> In many parts of the world, silica is the major constituent of [[sand]]. Silica is one of the most complex and abundant families of [[material]]s, existing as a compound of several [[mineral]]s and as a synthetic product. Examples include [[fused quartz]], [[fumed silica]], [[opal]], and [[aerogel]]s. It is used in [[structural material]]s, [[microelectronics]], and as components in the food and pharmaceutical industries. All forms are white or colorless, although impure samples can be colored.

Silicon dioxide is a common fundamental constituent of [[glass]].

==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>

==Natural occurrence==
===Geology===
[[File:P-T Diagram for SiO2.svg|left|frameless|400x400px]]
{{chem2|SiO2}} is most commonly encountered in nature as [[quartz]], which comprises more than 10% by mass of the Earth's crust.<ref name="Ull">{{Ullmann|title=Silica|vauthors=Flörke OW, Graetsch HA, Brunk F, Benda L, Paschen S, Bergna HE, Roberts WO, Welsh WA, Libanati C, Ettlinger M, Kerner D, Maier M, Meon W, Schmoll R, Gies H, Schiffmann D|year=2018|doi=10.1002/14356007.a23_583.pub3|display-authors=3}}</ref> Quartz is the only polymorph of silica stable at the Earth's surface. Metastable occurrences of the high-pressure forms [[coesite]] and [[stishovite]] have been found around [[impact structure]]s and associated with [[eclogite]]s formed during [[ultra-high-pressure metamorphism]]. The high-temperature forms of [[tridymite]] and [[cristobalite]] are known from silica-rich [[volcanic rock]]s. In many parts of the world, silica is the major constituent of [[sand]].<ref>{{cite book|title=An Introduction to Forensic Geoscience|vauthors=Berslien E|publisher=Wiley & Sons|year=2012|isbn=9781405160544|pages=138}}</ref>

===Biology===
Even though it is poorly soluble, silica occurs in many plants such as [[rice]]. Plant materials with high silica [[phytolith]] content appear to be of importance to grazing animals, from chewing insects to [[ungulate]]s. Silica accelerates tooth wear, and high levels of silica in plants frequently eaten by [[herbivore]]s may have developed as a defense mechanism against predation.<ref>{{cite journal|vauthors=Massey FP, Ennos AR, Hartley SE|year=2006|title=Silica in grasses as a defence against insect herbivores: Contrasting effects on folivores and a phloem feeder|journal=[[Journal of Animal Ecology|J. Anim. Ecol.]]|volume=75|issue=2|pages=595–603|doi=10.1111/j.1365-2656.2006.01082.x|pmid=16638012|doi-access=free|bibcode=2006JAnEc..75..595M }}</ref><ref>{{cite journal|vauthors=Keeping MG, Kvedaras OL|year=2008|title=Silicon as a plant defence against insect herbivory: Response to Massey, Ennos and Hartley|journal=[[Journal of Animal Ecology|J. Anim. Ecol.]]|volume=77|issue=3|pages=631–3|doi=10.1111/j.1365-2656.2008.01380.x|pmid=18341561|doi-access=free|bibcode=2008JAnEc..77..631K }}</ref>

Silica is also the primary component of [[rice husk ash]], which is used, for example, in filtration and as supplementary cementitious material (SCM) in [[cement]] and [[concrete]] manufacturing.<ref>{{Cite journal |last1=Zain |first1=M. F. M. |last2=Islam |first2=M. N. |last3=Mahmud |first3=F. |last4=Jamil |first4=M. |date=2011 |title=Production of rice husk ash for use in concrete as a supplementary cementitious material |url=https://www.sciencedirect.com/science/article/pii/S0950061810003703 |journal=Construction and Building Materials |series=Composite Materials and Adhesive Bonding Technology |volume=25 |issue=2 |pages=798–805 |doi=10.1016/j.conbuildmat.2010.07.003 |issn=0950-0618}}</ref>

[[Silicification]] in and by cells has been common in the biological world and it occurs in bacteria, protists, plants, and animals (invertebrates and vertebrates).<ref>{{Cite journal |last=Perry |first=Carole C. |date=2003 |title=Silicification: The Processes by Which Organisms Capture and Mineralize Silica |url=https://pubs.geoscienceworld.org/msa/rimg/article-abstract/54/1/291/87497/Silicification-The-Processes-by-Which-Organisms?redirectedFrom=fulltext |journal=Reviews in Mineralogy and Geochemistry |volume=1 |issue=54 |pages=291–327|doi=10.2113/0540291 |bibcode=2003RvMG...54..291P }}</ref>

Prominent examples include:
*[[Test (biology)|Tests]] or [[frustule]]s (i.e. shells) of [[diatom]]s, [[Radiolaria]], and [[testate amoebae]].<ref name=Fern/>
*Silica [[phytolith]]s in the cells of many plants<ref>{{cite encyclopedia|last=Radini |first=Anita |title=Archaeobotany: Plant Microfossils |date=2024 |encyclopedia=Encyclopedia of Archaeology |edition=Second |pages=698–707 |editor-last=Nikita |editor-first=Efthymia |url=https://www.sciencedirect.com/science/article/pii/B9780323907996001142 |access-date=2024-06-20 |place=Oxford |publisher=Academic Press |doi=10.1016/b978-0-323-90799-6.00114-2 |isbn=978-0-323-91856-5 |editor2-last=Rehren |editor2-first=Thilo}}</ref> including [[Equisetaceae]],<ref>{{Cite journal |last1=Neumann |first1=Mike |last2=Wagner |first2=Sandra |last3=Noske |first3=Robert |last4=Tiersch |first4=Brigitte |last5=Strauch |first5=Peter |date=2010 |title=Morphology and Structure of Biomorphous Silica Isolated from Equisetum hyemale and Equisetum telmateia |journal=Zeitschrift für Naturforschung B |language=en |volume=65 |issue=9 |pages=1113–1120 |doi=10.1515/znb-2010-0910 |issn=1865-7117|doi-access=free }}</ref> many grasses, and a wide range of [[dicotyledon]]s.<ref>{{Citation |last1=Tubaña |first1=Brenda Servaz |title=Silicon in Soils and Plants |date=2015 |work=Silicon and Plant Diseases |pages=7–51 |editor-last=Rodrigues |editor-first=Fabrício A. |url=https://link.springer.com/10.1007/978-3-319-22930-0_2 |access-date=2024-07-19 |place=Cham |publisher=Springer International Publishing |language=en |doi=10.1007/978-3-319-22930-0_2 |isbn=978-3-319-22929-4 |last2=Heckman |first2=Joseph Raymond |editor2-last=Datnoff |editor2-first=Lawrence E.}}</ref><ref>{{Cite journal |last1=Irzaman |first1=Irzaman |last2=Yustaeni |first2=Diah |last3=Aminullah |first3=Aminullah |last4=Irmansyah |first4=Irmansyah |last5=Yuliarto |first5=Brian |date=2021-04-19 |title=Purity, Morphological, and Electrical Characterization of Silicon Dioxide from Cogon Grass (Imperata cylindrica) Using Different Ashing Temperatures |url=https://ejchem.journals.ekb.eg/article_165191.html |journal=Egyptian Journal of Chemistry |volume=64 |issue=8 |language=en |pages=4143–4149 |doi=10.21608/ejchem.2019.15430.1962 |issn=2357-0245}}</ref>
*The [[Sponge spicule|spicules]] forming the skeleton of many [[sponge]]s.<ref>{{Cite journal |last1=Uriz |first1=MJ |last2=Turon |first2=Xavier |last3=Becerro |first3=Mikel A. |last4=Agell |first4=Gemma |date=2003 |title=Siliceous spicules and skeleton frameworks in sponges: Origin, diversity, ultrastructural patterns, and biological functions |url=https://analyticalsciencejournals.onlinelibrary.wiley.com/doi/10.1002/jemt.10395 |journal=Microscopy Research and Technique |language=en |volume=62 |issue=4 |pages=279–299 |doi=10.1002/jemt.10395 |pmid=14534903 |issn=1059-910X}}</ref>

==Uses==
===Structural use===
About 95% of the commercial use of silicon dioxide (sand) is in the construction industry, e.g. in the production of concrete ([[Portland cement concrete]]).<ref name=Ull/>

Certain deposits of silica sand, with desirable particle size and shape and desirable [[clay]] and other mineral content, were important for [[sand casting]] of metallic products.<ref>{{cite book|title=Albany moulding sands of the Hudson Valley |first= 	Charles Merrick |last=Nevin|publisher= University of the State of New York at Albany|date= 1925}}</ref> The high melting point of silica enables it to be used in such applications such as iron casting; modern sand casting sometimes uses other minerals for other reasons.

Crystalline silica is used in [[hydraulic fracturing]] of formations which contain [[tight oil]] and [[shale gas]].<ref name="NYT82313">{{cite news|url=https://www.nytimes.com/2013/08/24/business/new-rules-would-cut-silica-dust-exposure.html|title=New Rules Would Cut Silica Dust Exposure|last=Greenhouse S|date=23 Aug 2013|newspaper=[[The New York Times]]|access-date=24 Aug 2013}}</ref>

===Precursor to glass and silicon===
Silica is the primary ingredient in the production of most [[glass]]. As other minerals are melted with silica, the principle of [[freezing point depression]] lowers the melting point of the mixture and increases fluidity. The [[glass transition]] temperature of pure SiO<sub>2</sub> is about 1475 K.<ref>{{cite journal|vauthors=Ojovan MI|year=2004|title=Glass formation in amorphous SiO<sub>2</sub> as a percolation phase transition in a system of network defects|journal=[[Journal of Experimental and Theoretical Physics Letters|JETP Lett.]]|volume=79|issue=12|pages=632–634|bibcode=2004JETPL..79..632O|doi=10.1134/1.1790021|s2cid=124299526}}</ref> When molten silicon dioxide SiO<sub>2</sub> is rapidly cooled, it does not crystallize, but solidifies as a glass.<ref>{{Cite book |last=Stachurski |first=Zbigniew H. |url=https://books.google.com/books?id=DBF1BgAAQBAJ&dq=silicon+dioxide+SiO2+is+rapidly+cooled+but+solidifies+as+a+glass&pg=PA176 |title=Fundamentals of Amorphous Solids: Structure and Properties |date=2015 |publisher=John Wiley & Sons |isbn=978-3-527-68219-5 |pages=176 |language=en}}</ref> Because of this, most [[ceramic glaze]]s have silica as the main ingredient.<ref>{{Cite book |url=https://books.google.com/books?id=qxRhA3MZg6AC&dq=ceramic+glazes+have+silica+as+the+main+ingredient&pg=PA563 |title=Advanced Inorganic Chemistry: Vollume II |publisher=Krishna Prakashan Media |pages=563 |language=en}}</ref>

The structural geometry of silicon and oxygen in glass is similar to that in quartz and most other crystalline forms of silicon and oxygen, with silicon surrounded by regular tetrahedra of oxygen centres. The difference between the glass and crystalline forms arises from the connectivity of the tetrahedral units: Although there is no long-range periodicity in the glassy network, ordering remains at length scales well beyond the SiO bond length. One example of this ordering is the preference to form rings of 6-tetrahedra.<ref>{{cite journal|vauthors=Elliott SR|year=1991|title=Medium-range structural order in covalent amorphous solids|journal=[[Nature (journal)|Nature]]|volume=354|issue=6353|pages=445–452|bibcode=1991Natur.354..445E|doi=10.1038/354445a0|s2cid=4344891}}</ref>

The majority of [[optical fiber]]s for [[telecommunications]] are also made from silica. It is a primary raw material for many ceramics such as [[earthenware]], [[stoneware]], and [[porcelain]].

Silicon dioxide is used to produce elemental [[silicon]]. The process involves [[carbothermic reduction]] in an [[electric arc furnace]]:<ref>{{Cite book|title=Shriver & Atkins' inorganic chemistry|publisher=Oxford University Press|year=2010|isbn=9780199236176|veditors=Atkins PW, Overton T, Rourke J, Weller M, Armstrong F|edition=5th|location=Oxford|pages=354|oclc=430678988|display-editors=3}}</ref>
:<chem>SiO2 + 2 C -> Si + 2 CO</chem>

===Fumed silica===
[[Fumed silica]], also known as pyrogenic silica, is prepared by burning [[silicon tetrachloride|SiCl<sub>4</sub>]] in an oxygen-rich hydrogen flame to produce a "smoke" of SiO<sub>2</sub>.<ref name="Greenwood">{{Greenwood&Earnshaw1st|pages=393–99}}</ref> 
:<chem>SiCl4 + 2 H2 + O2 -> SiO2 + 4 HCl</chem>
It can also be produced by vaporizing quartz sand in a 3000&nbsp;°C electric arc. Both processes result in microscopic droplets of amorphous silica fused into branched, chainlike, three-dimensional secondary particles which then agglomerate into tertiary particles, a white powder with extremely low bulk density (0.03-0.15 g/cm<sup>3</sup>) and thus high surface area.<ref name=cabot>{{cite web |url=http://www.cabotcorp.com |title= Cab-O-Sil Fumed Metal Oxides}}</ref> The particles act as a [[Thixotropy|thixotropic]] thickening agent, or as an anti-caking agent, and can be treated to make them hydrophilic or hydrophobic for either water or organic liquid applications.
[[File:Kieselsaeure380m2prog.jpg|thumb|Manufactured fumed silica with maximum surface area of 380 m<sup>2</sup>/g]]

[[Silica fume]] is an ultrafine powder collected as a by-product of the silicon and [[ferrosilicon]] alloy production. It consists of [[amorphous]] (non-crystalline) spherical particles with an average particle diameter of 150&nbsp;nm, without the branching of the pyrogenic product. The main use is as [[pozzolanic]] material for high performance concrete. Fumed silica nanoparticles can be successfully used as an anti-aging agent in asphalt binders.<ref>{{cite journal |last1=Cheraghian |first1=Goshtasp |last2=Wistuba |first2=Michael P. |last3=Kiani |first3=Sajad |last4=Barron |first4=Andrew R. |last5=Behnood |first5=Ali |title=Rheological, physicochemical, and microstructural properties of asphalt binder modified by fumed silica nanoparticles |journal=Scientific Reports |date=December 2021 |volume=11 |issue=1 |pages=11455 |doi=10.1038/s41598-021-90620-w|pmid=34075083 |pmc=8169902 |bibcode=2021NatSR..1111455C }}</ref>

===Food, cosmetic, and pharmaceutical applications===
Silica, either colloidal, precipitated, or pyrogenic fumed, is a common additive in food production. It is used primarily as a flow or anti-[[caking]] agent in powdered foods such as spices and non-dairy coffee creamer, or powders to be formed into pharmaceutical tablets.<ref name=cabot/> It can [[adsorption|adsorb]] water in [[hygroscopy|hygroscopic]] applications. [[Colloidal silica]] is used as a [[fining agent]] for wine, beer, and juice, with the [[E number]] reference '''E551'''.<ref name=Ull/>

In cosmetics, silica is useful for its light-diffusing properties<ref>{{cite book|url=https://books.google.com/books?id=RIvOBQAAQBAJ&q=silica%20cosmetics%20light%20diffusing&pg=PA444|title=Handbook of Cosmetic Science and Technology|vauthors=Barel AO, Paye M, Maibach HI|publisher=CRC Press|year=2014|isbn=9781842145654|edition=4th|pages=444|quote=These soft-focus pigments, mainly composed of polymers, micas and talcs covered with rough or spherical particles of small diameters, such as silica or titanium dioxide, are used to optically reduce the appearance of wrinkles. These effects are obtained by optimizing outlines of wrinkles and reducing the difference of brightness due to diffuse reflection.}}</ref> and natural absorbency.<ref>{{cite book|url=https://books.google.com/books?id=RIvOBQAAQBAJ&q=silica%20cosmetics%20light%20diffusing&pg=PA444|title=Handbook of Cosmetic Science and Technology|vauthors=Barel AO, Paye M, Maibach HI|publisher=CRC Press|year=2014|isbn=9781842145654|edition=4th|pages=442|quote=The silica is a multiporous ingredient, which absorbs the oil and sebum.}}</ref>

[[Diatomaceous earth]], a mined product, has been used in food and cosmetics for centuries.  It consists of the silica shells of microscopic [[diatoms]]; in a less processed form it was sold as [[Dentifrice#Tooth powder|tooth powder]].<ref>{{Cite journal |last=Gardner |first=J. Starkie |date=1882 |title=On the Causes of Elevation and Subsidence |url=https://www.cambridge.org/core/product/identifier/S0016756800172474/type/journal_article |journal=Geological Magazine |language=en |volume=9 |issue=10 |pages=479–480 |doi=10.1017/S0016756800172474 |bibcode=1882GeoM....9..479G |issn=0016-7568}}</ref><ref>{{Cite book |last=Mann |first=Albert |title=The Economic Importance of the Diatoms |publisher=Smithsonian |year=1917 |location=Washington DC, United States of America}}</ref> Manufactured or mined [[hydrated silica]] is used as the hard abrasive in [[toothpaste]].

===Semiconductors===
{{See also|Surface passivation|Thermal oxidation|Planar process|MOSFET}}

Silicon dioxide is widely used in the semiconductor technology:

* for the primary passivation (directly on the semiconductor surface),
* as an original [[gate dielectric]] in [[MOS technology]]. Today when scaling (dimension of the gate length of the MOS transistor) has progressed below 10&nbsp;nm, silicon dioxide has been replaced by other [[dielectric materials]] like [[hafnium oxide]] or similar with higher dielectric constant compared to silicon dioxide,
* as a dielectric layer between metal (wiring) layers (sometimes up to 8–10) connecting elements and
* as a second passivation layer (for protecting semiconductor elements and the metallization layers) typically today layered with some other dielectrics like [[silicon nitride]].

Because silicon dioxide is a native oxide of silicon it is more widely used compared to other semiconductors like [[gallium arsenide]] or [[indium phosphide]].

Silicon dioxide could be grown on a silicon [[semiconductor]] surface.<ref name="Bassett22">{{cite book |last1=Bassett |first1=Ross Knox |title=To the Digital Age: Research Labs, Start-up Companies, and the Rise of MOS Technology |date=2007 |publisher=[[Johns Hopkins University Press]] |isbn=9780801886393 |pages=22–23 |url=https://books.google.com/books?id=UUbB3d2UnaAC&pg=PA22}}</ref> Silicon oxide layers could protect silicon surfaces during [[diffusion processes]], and could be used for diffusion masking.<ref name="Lecuyer">{{cite book |last1=Lécuyer |first1=Christophe |last2=Brock |first2=David C. |title=Makers of the Microchip: A Documentary History of Fairchild Semiconductor |date=2010 |publisher=[[MIT Press]] |isbn=9780262294324 |page=111 |url=https://books.google.com/books?id=LaZpUpkG70QC&pg=PA111}}</ref><ref name="Saxena">{{cite book |last= Saxena|first= A |title = Invention of integrated circuits: untold important facts |url = https://books.google.com/books?id=z7738Wq-j-8C |publisher = [[World Scientific]] |series = International series on advances in solid state electronics and technology |year = 2009 |isbn = 9789812814456 |pages = 96–97}}</ref>

[[Surface passivation]] is the process by which a semiconductor surface is rendered inert, and does not change semiconductor properties as a result of interaction with air or other materials in contact with the surface or edge of the crystal.<ref name="atalla">{{cite web|title=Martin Atalla in Inventors Hall of Fame, 2009|url=https://www.invent.org/inductees/martin-john-m-atalla|access-date=21 June 2013}}</ref><ref name="Black">{{cite book |last1=Black |first1=Lachlan E. |title=New Perspectives on Surface Passivation: Understanding the Si-Al2O3 Interface |date=2016 |publisher=[[Springer Science+Business Media|Springer]] |isbn=9783319325217 |page=17 |url=https://books.google.com/books?id=laYFDAAAQBAJ&pg=PA17}}</ref> The formation of a [[Thermal oxidation|thermally]] grown silicon dioxide layer greatly reduces the concentration of [[surface states|electronic states at the silicon surface]].<ref name="Black"/> SiO<sub>2</sub> [[Thin film|films]] preserve the electrical characteristics of [[p–n junction]]s and prevent these electrical characteristics from deteriorating by the gaseous ambient environment.<ref name="Saxena"/> Silicon oxide layers could be used to electrically stabilize silicon surfaces.<ref name="Lecuyer"/> The surface passivation process is an important method of [[semiconductor device fabrication]] that involves coating a [[silicon wafer]] with an insulating layer of silicon oxide so that electricity could reliably penetrate to the conducting silicon below. Growing a layer of silicon dioxide on top of a silicon wafer enables it to overcome the [[surface states]] that otherwise prevent electricity from reaching the semiconducting layer.<ref name="atalla"/><ref name="kahng">{{cite web |title=Dawon Kahng |url=https://www.invent.org/inductees/dawon-kahng |website=[[National Inventors Hall of Fame]] |access-date=27 June 2019}}</ref>

The process of silicon surface passivation by [[thermal oxidation]] (silicon dioxide) is critical to the [[semiconductor industry]]. It is commonly used to manufacture [[metal–oxide–semiconductor field-effect transistor]]s (MOSFETs) and silicon [[integrated circuit]] chips (with the [[planar process]]).<ref name="atalla"/><ref name="kahng"/>

===Other===
[[Hydrophobic silica]] is used as a [[anti-foaming agent|defoamer component]].

In its capacity as a [[refractory]], it is useful in fiber form as a high-temperature [[thermal protection]] fabric.<ref>{{Cite journal |last1=Liu |first1=Guoyi |last2=Liu |first2=Yuanjun |last3=Zhao |first3=Xiaoming |date=2017 |title=The Influence of Spherical Nano-SiO 2 Content on the Thermal Protection Performance of Thermal Insulation Ablation Resistant Coated Fabrics |journal=Journal of Nanomaterials |language=en |volume=2017 |pages=1–11 |doi=10.1155/2017/2176795 |doi-access=free |issn=1687-4110}}</ref>

Silica is used in the [[DNA separation by silica adsorption|extraction of DNA]] and [[RNA]] due to its ability to bind to the nucleic acids under the presence of [[chaotropic agent|chaotropes]].<ref>{{cite book|title=An Introduction to Forensic Genetics|vauthors=Goodwin W, Linacre A, Hadi S|publisher=Wiley & Sons|year=2007|isbn=9780470010259|pages=29}}</ref>

[[Silica aerogel]] was used in the [[Stardust (spacecraft)|Stardust spacecraft]] to collect extraterrestrial particles.<ref>{{Cite news|url=https://www.businessinsider.com/aerogel-science-history-kistler-new-applications-2015-8?IR=T|title=This cloud-like, futuristic material has been sneaking its way into your life since 1931|last=Calderone J|date=20 Aug 2015|work=[[Business Insider]]|access-date=11 Feb 2019}}</ref>

Pure silica (silicon dioxide), when cooled as fused quartz into a glass with no true melting point, can be used as a glass fibre for fibreglass.

==Production==
Silicon dioxide is mostly obtained by mining, including [[sand mining]] and purification of [[quartz]]. 
Quartz is suitable for many purposes, while chemical processing is required to make a purer or otherwise more suitable (e.g. more reactive or fine-grained) product.<ref>{{Cite book |url=http://link.springer.com/10.1007/978-3-642-22161-3 |title=Quartz: Deposits, Mineralogy and Analytics |date=2012 |publisher=Springer Berlin Heidelberg |isbn=978-3-642-22160-6 |editor-last=Götze |editor-first=Jens |series=Springer Geology |location=Berlin, Heidelberg |language=en |doi=10.1007/978-3-642-22161-3 |bibcode=2012qdma.book.....G |editor-last2=Möckel |editor-first2=Robert}}</ref><ref>{{Cite journal |last1=Pan |first1=Xiaodong |last2=Li |first2=Suqin |last3=Li |first3=Yongkui |last4=Guo |first4=Penghui |last5=Zhao |first5=Xin |last6=Cai |first6=Yinshi |date=2022 |title=Resource, characteristic, purification and application of quartz: a review |url=https://linkinghub.elsevier.com/retrieve/pii/S0892687522002102 |journal=Minerals Engineering |language=en |volume=183 |pages=107600 |doi=10.1016/j.mineng.2022.107600|bibcode=2022MiEng.18307600P }}</ref>

===Precipitated silica===
Precipitated silica or amorphous silica is produced by the acidification of solutions of [[sodium silicate]]. The gelatinous precipitate or [[silica gel]], is first washed and then dehydrated to produce colorless microporous silica.<ref name="Greenwood"/>  The idealized equation involving a trisilicate and [[sulfuric acid]] is:
:<chem>Na2Si3O7 + H2SO4 -> 3 SiO2 + Na2SO4 + H2O</chem>

Approximately one billion kilograms/year (1999) of silica were produced in this manner, mainly for use for polymer composites – tires and shoe soles.<ref name=Ull/>

===On microchips===
Thin films of silica grow spontaneously on [[silicon wafer]]s via [[thermal oxidation]], producing a very shallow layer of about 1 [[nanometre|nm]] or 10 [[angstrom|Å]] of so-called native oxide.<ref>{{cite book|url=https://books.google.com/books?id=Qi98H-iTgLEC|title=Handbook of Semiconductor Manufacturing Technology|publisher=CRC Press|year=2007|isbn=9781574446753|veditors=Doering R, Nishi Y}}</ref>
Higher temperatures and alternative environments are used to grow well-controlled layers of silicon dioxide on silicon, for example at temperatures between 600 and 1200&nbsp;°C, using so-called dry  oxidation with [[oxygen|O<sub>2</sub>]]
:<chem>Si + O2 -> SiO2</chem>

or wet oxidation with H<sub>2</sub>O.<ref name="Sunggyu Lee">{{cite book|title=Encyclopedia of chemical processing|author=Lee S|publisher=CRC Press|year=2006|isbn=9780824755638}}</ref><ref name="Morgan&Board">{{cite book|title=An Introduction To Semiconductor Microtechnology|vauthors=Morgan DV, Board K|publisher=John Wiley & Sons|year=1991|isbn=9780471924784|edition=2nd|location=Chichester, West Sussex, England|pages=72}}</ref>
:<chem>Si + 2 H2O -> SiO2 + 2 H2</chem>

The native oxide layer is beneficial in [[microelectronics]], where it acts as [[electric insulator]] with high chemical stability. It can protect the silicon, store charge, block current, and even act as a controlled pathway to limit current flow.<ref>{{Cite news|url=https://spectrum.ieee.org/the-silicon-dioxide-solution|title=The Silicon Dioxide Solution: How physicist Jean Hoerni built the bridge from the transistor to the integrated circuit|last=Riordan M|date=2007|work=[[IEEE Spectrum]]|access-date=11 Feb 2019}}</ref>

===Laboratory or special methods===
==== From organosilicon compounds ====
Many routes to silicon dioxide start with an organosilicon compound, e.g., HMDSO,<ref>{{Cite journal|last1=Chrystie|first1=Robin S. M.|last2=Ebertz|first2=Felix L.|last3=Dreier|first3=Thomas|last4=Schulz|first4=Christof|date=2019-01-28|title=Absolute SiO concentration imaging in low-pressure nanoparticle-synthesis flames via laser-induced fluorescence|journal=Applied Physics B|language=en|volume=125|issue=2|pages=29|doi=10.1007/s00340-019-7137-8|issn=1432-0649|bibcode=2019ApPhB.125...29C|s2cid=127735545}}</ref> TEOS. Synthesis of silica is illustrated below using [[tetraethyl orthosilicate]] (TEOS).<ref>{{Cite journal|last1=Romero-Jaime|first1=A. K.|last2=Acosta-Enríquez|first2=M. C.|last3=Vargas-Hernández|first3=D.|last4=Tánori-Córdova|first4=J. C.|last5=Pineda León|first5=H. A.|last6=Castillo|first6=S. J.|date=August 2021|title=Synthesis and characterization of silica–lead sulfide core–shell nanospheres for applications in optoelectronic devices|url=https://link.springer.com/10.1007/s10854-021-06648-1|journal=Journal of Materials Science: Materials in Electronics|language=en|volume=32|issue=16|pages=21425–21431|doi=10.1007/s10854-021-06648-1|s2cid=236182027|issn=0957-4522}}</ref>  Simply heating TEOS at 680–730&nbsp;°C results in the oxide:

:<chem>Si(OC2H5)4 -> SiO2 + 2 O(C2H5)2</chem>

Similarly TEOS combusts around 400&nbsp;°C:

:<chem>Si(OC2H5)4 + 12 O2 -> SiO2 + 10 H2O + 8 CO2</chem>

TEOS undergoes [[hydrolysis]] via the so-called [[sol-gel process]].  The course of the reaction and nature of the product are affected by catalysts, but the idealized equation is:<ref>{{Cite journal|display-authors=3|vauthors=Nandiyanto AB, Kim SG, Iskandar F, Okuyama K|year=2009|title=Synthesis of spherical mesoporous silica nanoparticles with nanometer-size controllable pores and outer diameters|journal=Microporous and Mesoporous Materials|volume=120|issue=3|pages=447–453|doi=10.1016/j.micromeso.2008.12.019|bibcode=2009MicMM.120..447N }}</ref>

:<chem>Si(OC2H5)4 + 2 H2O -> SiO2 + 4 HOCH2CH3</chem>

====Other methods====
Being highly stable, silicon dioxide arises from many methods. Conceptually simple, but of little practical value, combustion of [[silane]] gives silicon dioxide.  This reaction is analogous to the combustion of methane:

:<chem>SiH4 + 2 O2 -> SiO2 + 2 H2O</chem>

However the [[chemical vapor deposition]] of silicon dioxide onto crystal surface from silane had been used using nitrogen as a carrier gas at 200–500&nbsp;°C.<ref name="Morgan&Board2">{{cite book|title=An Introduction To Semiconductor Microtechnology|vauthors=Morgan DV, Board K|publisher=John Wiley & Sons|year=1991|isbn=9780471924784|edition=2nd|location=Chichester, West Sussex, England|pages=27}}</ref>

==Chemical reactions==
Silicon dioxide is a relatively inert material (hence its widespread occurrence as a mineral). Silica is often used as inert containers for chemical reactions. At high temperatures, it is  converted to silicon by reduction with carbon.

[[Fluorine]] reacts with silicon dioxide to form [[Silicon tetrafluoride|SiF<sub>4</sub>]] and O<sub>2</sub> whereas the other halogen gases (Cl<sub>2</sub>, Br<sub>2</sub>, I<sub>2</sub>) are unreactive.<ref name="Greenwood"/>

Most forms of silicon dioxide are attacked ("etched") by [[hydrofluoric acid]] (HF) to produce [[hexafluorosilicic acid]]:<ref name="Wiberg&Holleman"/>
:{{chem2|SiO2 + 6 HF -> H2SiF6 + 2 H2O}}
[[Stishovite]] does not react to HF  to any significant degree.<ref name="Fleischer1962">{{cite journal|last1=Fleischer|first1=Michael|year=1962|title=New mineral names|journal=American Mineralogist|volume=47|issue=2|pages=172–174|publisher=Mineralogical Society of America|url=http://rruff.info/uploads/AM47_805.pdf |archive-url=https://web.archive.org/web/20110722000427/http://rruff.info/uploads/AM47_805.pdf |archive-date=2011-07-22 |url-status=live}}</ref> 
HF is used to remove or pattern silicon dioxide in the semiconductor industry.

Silicon dioxide acts as a [[Acid–base reaction#Lux–Flood definition|Lux–Flood acid]], being able to react with bases under certain conditions. As it does not contain any hydrogen, non-hydrated silica cannot directly act as a [[Brønsted–Lowry acid–base theory|Brønsted–Lowry acid]]. While silicon dioxide is only poorly soluble in water at low or neutral [[pH]] (typically, 2 × 10<sup>−4</sup> [[Molar concentration|M]] for [[quartz]] up to 10<sup>−3</sup> [[Molar concentration|M]] for [[cryptocrystalline]] [[chalcedony]]), strong bases react with glass and easily dissolve it. Therefore, strong bases have to be stored in plastic bottles to avoid jamming the bottle cap, to preserve the integrity of the recipient, and to avoid undesirable contamination by silicate anions.<ref name="Rodgers2011">{{cite book|url=https://books.google.com/books?id=BY8IAAAAQBAJ&pg=PA421|title=Descriptive Inorganic, Coordination, and Solid State Chemistry|vauthors=Rodgers GE|publisher=Cengage Learning|year=2011|isbn=9781133172482|pages=421–2}}</ref>

Silicon dioxide dissolves in hot concentrated alkali or fused hydroxide, as described in this idealized equation:<ref name="Greenwood"/>
:<chem>SiO2 + 2 NaOH -> Na2SiO3 + H2O</chem>

Silicon dioxide will neutralise basic metal oxides (e.g. [[sodium oxide]], [[potassium oxide]], [[lead(II) oxide]], [[zinc oxide]], or mixtures of oxides, forming [[silicate]]s and glasses as the Si-O-Si bonds in silica are broken successively).<ref name="Wiberg&Holleman"/> As an example the reaction of sodium oxide and SiO<sub>2</sub> can produce sodium [[orthosilicate]], sodium silicate, and glasses, dependent on the proportions of reactants:<ref name = "Greenwood"/>
:<chem>2 Na2O + SiO2 -> Na4SiO4;</chem>

:<chem>Na2O + SiO2 -> Na2SiO3;</chem>

:<math>(0.25-0.8)</math> <chem>Na2O + SiO2 -> glass</chem>.

Examples of such glasses have commercial significance, e.g. [[soda–lime glass]], [[borosilicate glass]], [[lead glass]]. In these glasses, silica is termed the network former or lattice former.<ref name="Wiberg&Holleman"/> The reaction is also used in [[blast furnace]]s to remove sand impurities in the ore by neutralisation with [[calcium oxide]], forming [[calcium silicate]] [[slag]].
[[File:Fibreoptic.jpg|thumb|Bundle of [[optical fibre]]s composed of high purity silica]]

Silicon dioxide reacts in heated [[reflux]] under [[dinitrogen]] with [[ethylene glycol]] and an [[alkali metal]] base to produce highly reactive, [[Hypervalent molecule#Pentacoordinated silicon|pentacoordinate]] silicates which provide access to a wide variety of new silicon compounds.<ref name=Laine>{{cite journal|last1=Laine|first1=Richard M.|last2=Blohowiak|first2=Kay Youngdahl|last3=Robinson|first3=Timothy R.|last4=Hoppe|first4=Martin L.|last5=Nardi|first5=Paola|last6=Kampf|first6=Jeffrey|last7=Uhm|first7=Jackie|title=Synthesis of pentacoordinate silicon complexes from SiO<sub>2</sub>|journal=Nature|volume=353|date=17 October 1991|issue=6345|pages=642–644|doi=10.1038/353642a0|bibcode=1991Natur.353..642L|url=https://deepblue.lib.umich.edu/bitstream/2027.42/62810/1/353642a0.pdf |archive-url=https://web.archive.org/web/20170819150753/http://deepblue.lib.umich.edu/bitstream/2027.42/62810/1/353642a0.pdf |archive-date=2017-08-19 |url-status=live|hdl=2027.42/62810|s2cid=4310228|hdl-access=free}}</ref> The silicates are essentially insoluble in all [[Solvent|polar solvent]] except [[methanol]].

Silicon dioxide reacts with elemental silicon at high temperatures to produce SiO:<ref name="Wiberg&Holleman"/>
:<chem>SiO2 + Si -> 2 SiO</chem>

===Water solubility===
The solubility of silicon dioxide in water strongly depends on its crystalline form and is three to four times higher for amorphous silica than quartz; as a function of temperature, it peaks around {{convert|340|°C}}.<ref>{{cite journal|vauthors=Fournier RO, Rowe JJ|year=1977|title=The solubility of amorphous silica in water at high temperatures and high pressures|url=http://www.minsocam.org/ammin/AM62/AM62_1052.pdf |archive-url=https://ghostarchive.org/archive/20221010/http://www.minsocam.org/ammin/AM62/AM62_1052.pdf |archive-date=2022-10-10 |url-status=live|journal=[[American Mineralogist|Am. Mineral.]]|volume=62|pages=1052–1056}}</ref> This property is used to grow single crystals of quartz in a hydrothermal process where natural quartz is dissolved in superheated water in a pressure vessel that is cooler at the top. Crystals of 0.5–1&nbsp; kg can be grown for 1–2 months.<ref name="Wiberg&Holleman">{{Holleman&Wiberg}}</ref> These crystals are a source of very pure quartz for use in electronic applications.<ref name="Greenwood"/> Above the [[critical point (thermodynamics)|critical temperature]] of water {{convert|647.096|K}} and a pressure of {{convert|22.064|MPa}} or higher, water is a [[supercritical fluid]] and solubility is once again higher than at lower temperatures.<ref>{{cite journal | url=https://ui.adsabs.harvard.edu/abs/2019EGUGA..21.4614O/abstract | bibcode=2019EGUGA..21.4614O | title=Formation of silica particles from supercritical fluids and its impacts on the hydrological properties in the crust | last1=Okamoto | first1=Atsushi | journal=EGU General Assembly Conference Abstracts | year=2019 | page=4614 }}</ref>

==Health effects==
[[File:Piasek kwarcowy.jpg|thumb|Quartz sand (silica) as main raw material for commercial glass production]]
Silica ingested orally is essentially nontoxic, with an {{LD50}} of 5000&nbsp;mg/kg (5&nbsp;g/kg).<ref name=Ull/> A 2008 study following subjects for 15 years found that higher levels of silica in water appeared to decrease the risk of [[dementia]]. An increase of 10&nbsp;mg/day of silica in drinking water was associated with a reduced risk of dementia of 11%.<ref>{{cite journal|display-authors=3|vauthors=Rondeau V, Jacqmin-Gadda H, Commenges D, Helmer C, Dartigues JF|year=2008|title=Aluminum and Silica in Drinking Water and the Risk of Alzheimer's Disease or Cognitive Decline: Findings from 15-Year Follow-up of the PAQUID Cohort|journal=[[American Journal of Epidemiology]]|volume=169|issue=4|pages=489–96|doi=10.1093/aje/kwn348|pmc=2809081|pmid=19064650}}</ref>

Inhaling finely divided crystalline silica dust can lead to [[silicosis]], [[bronchitis]], or [[lung cancer]], as the dust becomes lodged in the lungs and continuously irritates the tissue, reducing lung capacities.<ref>{{Cite web|url=https://www.silica-safe.org/|title=Work Safely with Silica|publisher=CPWR - The Center for Construction Research and Training|access-date=11 Feb 2019}}</ref> When fine silica particles are inhaled in large enough quantities (such as through occupational exposure), it increases the risk of [[systemic autoimmune disease]]s such as [[lupus]]<ref>{{Cite web|url=https://www.niams.nih.gov/about/working-groups/lupus-federal/action-plan|title=Action Plan for Lupus Research|date=2017|website=[[National Institute of Arthritis and Musculoskeletal and Skin Diseases]]|publisher=[[National Institutes of Health]]|access-date=11 Feb 2019}}</ref> and [[rheumatoid arthritis]] compared to expected rates in the general population.<ref name="Meyer-2017">{{Cite journal|display-authors=3|vauthors=Meyer A, Sandler DP, Beane Freeman LE, Hofmann JN, Parks CG|date=2017|title=Pesticide Exposure and Risk of Rheumatoid Arthritis among Licensed Male Pesticide Applicators in the Agricultural Health Study|journal=[[Environmental Health Perspectives]]|volume=125|issue=7|pages=077010-1-077010-7|doi=10.1289/EHP1013|pmid=28718769|pmc=5744649|doi-access=free}}</ref>

===Occupational hazard===
Silica is an occupational hazard for people who do [[Abrasive blasting|sandblasting]] or work with powdered crystalline silica products. Amorphous silica, such as fumed silica, may cause irreversible lung damage in some cases but is not associated with the development of silicosis. Children, asthmatics of any age, those with [[Allergy|allergies]], and the elderly (all of whom have reduced [[Lung volumes|lung capacity]]) can be affected in less time.<ref>{{cite journal|display-authors=3|vauthors=Reuzel PG, Bruijntjes JP, Feron VJ, Woutersen RA|year=1991|title=Subchronic inhalation toxicity of amorphous silica and quartz dust in rats|journal=[[Food and Chemical Toxicology|Food Chem. Toxicol.]]|volume=29|issue=5|pages=341–54|doi=10.1016/0278-6915(91)90205-L|pmid=1648030}}</ref>

Crystalline silica is an [[occupational hazard]] for those working with stone [[countertop]]s because the process of cutting and installing the countertops creates large amounts of airborne silica.<ref name="Hazard">{{cite web|url=https://www.cdc.gov/niosh/docs/2015-106/pdfs/2015-106.pdf |archive-url=https://ghostarchive.org/archive/20221010/https://www.cdc.gov/niosh/docs/2015-106/pdfs/2015-106.pdf |archive-date=2022-10-10 |url-status=live|title=Worker Exposure to Silica during Countertop Manufacturing, Finishing and Installation|date=2015|publisher=[[National Institute for Occupational Safety and Health]] and [[Occupational Safety and Health Administration]]|access-date=26 Feb 2015}}</ref> Crystalline silica used in [[hydraulic fracturing]] presents a health hazard to workers.<ref name=NYT82313/>

===Pathophysiology===
In the body, crystalline silica particles do not dissolve over clinically relevant periods. Silica crystals inside the lungs can activate the NLRP3 [[inflammasome]] inside macrophages and dendritic cells and thereby result in production of [[interleukin]], a highly [[pro-inflammatory cytokine]] in the immune system.<ref>{{cite journal|display-authors=3|vauthors=Hornung V, Bauernfeind F, Halle A, Samstad EO, Kono H, Rock KL, Fitzgerald KA, Latz E|year=2008|title=Silica crystals and aluminum salts activate the NALP3 inflammasome through phagosomal destabilization|journal=[[Nature Immunology|Nat. Immunol.]]|volume=9|issue=8|pages=847–856|doi=10.1038/ni.1631|pmc=2834784|pmid=18604214}}</ref><ref>{{Cite book|url=https://www.cdc.gov/niosh/docs/86-102/86-102.pdf?id=10.26616/NIOSHPUB86102|title=Occupational Respiratory Diseases|publisher=US Department of Health and Human Services, NIOSH|year=1986|veditors=Merchant JA|location=Cincinnati, OH|id=DHHS (NIOSH) Publication Number 86-102|doi=10.26616/NIOSHPUB86102|hdl=2027/uc1.31210023588922}}</ref><ref>NIOSH (2002) Hazard Review, Health Effects of Occupational Exposure to Respirable Crystalline Silica. Cincinnati, OH: U.S. Department of Health and Human Services, U.S. Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, [https://www.cdc.gov/niosh/docs/2002-129/ DHHS (NIOSH) Publication No. 2002-129].</ref>

===Regulation===
Regulations restricting silica exposure 'with respect to the silicosis hazard' specify that they are concerned only with silica, which is both crystalline and dust-forming.<ref>{{cite web|url=https://www.osha.gov/OshDoc/data_General_Facts/crystalline-factsheet.pdf|title=Crystalline Factsheet|access-date=3 August 2017|archive-date=22 December 2017|archive-url=https://web.archive.org/web/20171222125021/https://www.osha.gov/OshDoc/data_General_Facts/crystalline-factsheet.pdf|url-status=dead}}</ref><ref>{{cite web|url=https://www.osha.gov/dsg/topics/silicacrystalline/|access-date=3 August 2017|title=Silica, Crystalline}}</ref><ref>{{cite web|url=http://www.silica-safe.org/ask-a-question/faq|access-date=3 August 2017|title=Frequently Asked Questions}}</ref><ref>{{cite web|url=http://www.ehs.uconn.edu/Word%20Docs/Silica%20fact%20sheet.pdf |archive-url=https://ghostarchive.org/archive/20221010/http://www.ehs.uconn.edu/Word%20Docs/Silica%20fact%20sheet.pdf |archive-date=2022-10-10 |url-status=live|access-date=3 August 2017|title=If It's Silica, It's Not Just Dust!}}</ref><ref>{{cite web|url=http://osha.oregon.gov/OSHAPubs/3301.pdf |archive-url=https://ghostarchive.org/archive/20221010/http://osha.oregon.gov/OSHAPubs/3301.pdf |archive-date=2022-10-10 |url-status=live|access-date=3 August 2017|title=What you should know about crystalline silica, silicosis, and Oregon OSHA silica rules}}</ref><ref>{{cite book|last1=Szymendera|first1=Scott D.|title=Respirable Crystalline Silica in the Workplace: New Occupational Safety and Health Administration (OSHA) Standards|date=January 16, 2018|publisher=Congressional Research Service|location=Washington, DC|url=https://fas.org/sgp/crs/misc/R44476.pdf |archive-url=https://ghostarchive.org/archive/20221010/https://fas.org/sgp/crs/misc/R44476.pdf |archive-date=2022-10-10 |url-status=live|access-date=27 January 2018}}</ref>

In 2013, the U.S. [[Occupational Safety and Health Administration]] reduced the exposure limit to 50 [[micrograms|μg]]/m<sup>3</sup> of air. Prior to 2013, it had allowed 100&nbsp;μg/m<sup>3</sup> and in construction workers even 250&nbsp;μg/m<sup>3</sup>.<ref name=NYT82313 />
In 2013, OSHA also required the "green completion" of fracked wells to reduce exposure to crystalline silica and restrict the exposure limit.<ref name=NYT82313/>

==Crystalline forms==
SiO<sub>2</sub>, more so than almost any material, exists in many crystalline forms. These forms are called [[polymorphism (materials science)|polymorphs]].
{|class="wikitable"
|+ Crystalline forms of SiO<sub>2</sub><ref name="Wiberg&Holleman"/>
! Form
! Crystal symmetry <br/>[[Pearson symbol]], group no.
! ρ <br />(g/cm<sup>3</sup>)
! width=350|Notes
! Structure
|-
|α-quartz
|[[rhombohedral]] (trigonal)<br/>hP9, P3<sub>1</sub>21 No.152<ref>{{cite journal|journal=[[Journal of Applied Physics]] |year=1982|volume=53|pages=6751–6756|title=Crystal structure and thermal expansion of a-quartz SiO<sub>2</sub> at low temperature|author1=Lager G. A. |author2=Jorgensen J. D. |author3=Rotella F.J. |doi=10.1063/1.330062|issue=10|bibcode = 1982JAP....53.6751L }}</ref>
| 2.648
| Helical chains making individual single crystals optically active; α-quartz converts to β-quartz at 846 K
|[[File:a-quartz.png|100px]]
|-
|β-quartz
|[[Hexagonal crystal system|hexagonal]]<br/>hP18, P6<sub>2</sub>22, No. 180<ref>{{cite journal |doi=10.1016/0022-4596(81)90449-7 |title=The structure of quartz at 25 and 590&nbsp;°C determined by neutron diffraction |year=1981 |last1=Wright |first1=A. F. |last2=Lehmann |first2=M. S. |journal=Journal of Solid State Chemistry |volume=36 |issue=3 |pages=371–80|bibcode = 1981JSSCh..36..371W }}</ref>
| 2.533
| Closely related to α-quartz (with an Si-O-Si angle of 155°) and optically active; β-quartz converts to β-tridymite at 1140 K
|[[File:b-quartz.png|100px]]
|-
|α-tridymite
|[[orthorhombic]]<br/>oS24, C222<sub>1</sub>, No.20<ref name=trid>{{cite journal |doi=10.1524/zkri.1986.177.1-2.27 |title=Structural change of orthorhombic-Itridymite with temperature: A study based on second-order thermal-vibrational parameters |year=1986 |last1=Kihara |first1=Kuniaki |last2=Matsumoto |first2=Takeo |last3=Imamura |first3=Moritaka |journal=Zeitschrift für Kristallographie |volume=177 |issue=1–2 |pages=27–38|bibcode = 1986ZK....177...27K }}</ref>
| 2.265
| Metastable form under normal pressure
|[[File:a-tridymite.png|100px]]
|-
|β-tridymite
| hexagonal<br/>hP12, P6<sub>3</sub>/mmc, No. 194<ref name=trid/>
|
| Closely related to α-tridymite; β-tridymite converts to β-cristobalite at 2010 K
|[[File:b-tridymite.png|100px]]
|-
|α-cristobalite
|[[tetragonal]]<br/>tP12, P4<sub>1</sub>2<sub>1</sub>2, No. 92<ref>{{cite journal|journal=[[American Mineralogist]]|year=1994 |volume=79|pages=9–14|title=The pressure behavior of a cristobalite|url=http://www.geo.arizona.edu/xtal/group/pdf/AM79_9.pdf |archive-url=https://ghostarchive.org/archive/20221010/http://www.geo.arizona.edu/xtal/group/pdf/AM79_9.pdf |archive-date=2022-10-10 |url-status=live|author1=Downs R. T. |author2=Palmer D. C. }}</ref>
| 2.334
| Metastable form under normal pressure
|[[File:a-cristobalite.png|100px]]
|-
|β-cristobalite
|[[Cubic crystal system|cubic]]<br/>cF104, Fd<u style="text-decoration:overline">3</u>m, No.227<ref>{{cite journal |doi=10.1080/00318087508228690 |title=The structures of the β-cristobalite phases of SiO<sub>2</sub> and AlPO<sub>4</sub> |year=1975 |last1=Wright |first1=A. F. |last2=Leadbetter |first2=A. J. |journal=[[Philosophical Magazine]] |volume=31 |issue=6 |pages=1391–401 |bibcode=1975PMag...31.1391W}}</ref>
|
| Closely related to α-cristobalite; melts at 1978 K
|[[File:b-cristobalite.png|100px]]
|-
|[[keatite]]
| tetragonal<br/>tP36, P4<sub>1</sub>2<sub>1</sub>2, No. 92<ref>{{cite journal |doi=10.1524/zkri.1959.112.1-6.409 |title=The crystal structure of keatite, a new form of silica |year=1959 |last1=Shropshire |first1=Joseph |last2=Keat |first2=Paul P. |last3=Vaughan |first3=Philip A. |journal=Zeitschrift für Kristallographie |volume=112 |issue=1–6 |pages=409–13|bibcode = 1959ZK....112..409S }}</ref>
| 3.011
| Si<sub>5</sub>O<sub>10</sub>, Si<sub>4</sub>O<sub>8</sub>, Si<sub>8</sub>O<sub>16</sub> rings; synthesised from glassy silica and alkali at 600–900 K and 40–400 MPa
|[[File:keatite.png|100px]]
|-
|[[moganite]]
|[[monoclinic]]<br/>mS46, C2/c, No.15<ref>{{cite journal |journal=European Journal of Mineralogy |year=1992 |volume=4 |issue=4 |pages=693–706 |title=Crystal structure of moganite: a new structure type for silica |first1=Gerhard |last1=Miehe |first2=Heribert |last2=Graetsch |doi=10.1127/ejm/4/4/0693|bibcode=1992EJMin...4..693M }}</ref>
|
| Si<sub>4</sub>O<sub>8</sub> and Si<sub>6</sub>O<sub>12</sub> rings
|[[File:Moganite.png|100px]]
|-
|[[coesite]]
| monoclinic<br/>mS48, C2/c, No.15<ref>{{cite journal|journal=[[American Mineralogist]] |year=1981|volume=66|pages=324–333|title=High-pressure crystal structure and compressibility of coesite|url=http://www.minsocam.org/ammin/AM66/AM66_324.pdf |archive-url=https://ghostarchive.org/archive/20221010/http://www.minsocam.org/ammin/AM66/AM66_324.pdf |archive-date=2022-10-10 |url-status=live|author1=Levien L. |author2=Prewitt C. T. }}</ref>
| 2.911
| Si<sub>4</sub>O<sub>8</sub> and Si<sub>8</sub>O<sub>16</sub> rings; 900 K and 3–3.5 GPa
|[[File:coesite.png|100px]]
|-
|[[stishovite]]
| tetragonal<br/>tP6, P4<sub>2</sub>/mnm, No.136<ref>{{cite journal|journal=[[American Mineralogist]] |year=1995|volume=80|issue=5–6|pages=454–456|title=H in rutile-type compounds: II. Crystal chemistry of Al substitution in H-bearing stishovite |author1=Smyth J. R. |author2=Swope R. J. |author3=Pawley A. R. |url=http://rruff.geo.arizona.edu/doclib/am/vol80/AM80_454.pdf |archive-url=https://ghostarchive.org/archive/20221010/http://rruff.geo.arizona.edu/doclib/am/vol80/AM80_454.pdf |archive-date=2022-10-10 |url-status=live|doi=10.2138/am-1995-5-605|bibcode=1995AmMin..80..454S|s2cid=196903109}}</ref>
| 4.287
| One of the densest (together with seifertite) polymorphs of silica; [[rutile]]-like with 6-fold coordinated Si; 7.5–8.5 GPa
|[[File:stishovite.png|100px]]
|-
|[[seifertite]]
| orthorhombic<br/>oP, Pbcn<ref>{{cite journal|author1=Dera P. |author2=Prewitt C. T. |author3=Boctor N. Z. |author4=Hemley R. J. |journal=[[American Mineralogist]] |volume=87|issue=7 |year=2002|page=1018|title=Characterization of a high-pressure phase of silica from the Martian meteorite Shergotty|url=http://rruff.geo.arizona.edu/AMS/authors/Boctor%20N%20Z|doi=10.2138/am-2002-0728 |bibcode=2002AmMin..87.1018D |s2cid=129400258 }}</ref>
| 4.294
| One of the densest (together with stishovite) polymorphs of silica; is produced at pressures above 40 GPa.<ref>[http://www.mindat.org/min-26715.html Seifertite]. Mindat.org.</ref>
|[[File:SeifertiteStructure.png|100px]]
|-
|[[melanophlogite]]
| cubic (cP*, P4<sub>2</sub>32, No.208)<ref name="mel">{{cite journal|vauthors=Skinner BJ, Appleman DE|year=1963|title=Melanophlogite, a cubic polymorph of silica|url=http://www.minsocam.org/ammin/AM48/AM48_854.pdf |archive-url=https://ghostarchive.org/archive/20221010/http://www.minsocam.org/ammin/AM48/AM48_854.pdf |archive-date=2022-10-10 |url-status=live|journal=[[American Mineralogist|Am. Mineral.]]|volume=48|pages=854–867}}</ref> or<br>tetragonal (P4<sub>2</sub>/nbc)<ref>{{cite journal |author1=Nakagawa T. |author2=Kihara K. |author3=Harada K. |journal=[[American Mineralogist]] |volume=86|issue=11–12 |year=2001|page=1506|title=The crystal structure of low melanophlogite|url=http://rruff.geo.arizona.edu/AMS/minerals/Melanophlogite|doi=10.2138/am-2001-11-1219 |bibcode=2001AmMin..86.1506N |s2cid=53525827 }}</ref>
| 2.04
| Si<sub>5</sub>O<sub>10</sub>, Si<sub>6</sub>O<sub>12</sub> rings; mineral always found with hydrocarbons in interstitial spaces - a [[clathrasil]] (silica [[Clathrate compound|clathrate]])<ref>{{cite book|author=Rosemarie Szostak|year=1998|title=Molecular sieves: Principles of Synthesis and Identification|publisher=Springer|isbn=978-0-7514-0480-7|url=https://books.google.com/books?id=lteintjA2-MC}}</ref>
|[[File:MelanophlogiteStucture.png|100px]]
|-
| fibrous<br/> W-silica<ref name="Greenwood"/>
| orthorhombic<br/>oI12, Ibam, No.72<ref>{{cite journal |doi=10.1002/zaac.19542760110 |title=Über Siliciumchalkogenide. VI. Zur Kenntnis der faserigen Siliciumdioxyd-Modifikation |year=1954 |last1=Weiss |first1=Alarich |last2=Weiss |first2=Armin |journal=Zeitschrift für Anorganische und Allgemeine Chemie |volume=276 |issue=1–2 |pages=95–112}}</ref>
| 1.97
| Like [[silicon sulfide|SiS<sub>2</sub>]] consisting of edge-sharing chains, melts at ~1700 K
|[[File:SiS2typeSilica.png|100px]]
|-
| [[2D silica]]<ref>{{cite journal|doi=10.1038/srep03482|pmid=24336488|pmc=3863822|title=Defects in bilayer silica and graphene: common trends in diverse hexagonal two-dimensional systems|journal=Scientific Reports|volume=3|pages=3482|year=2013|last1=Björkman|first1=T|last2=Kurasch|first2=S|last3=Lehtinen|first3=O|last4=Kotakoski|first4=J|last5=Yazyev|first5=O. V.|last6=Srivastava|first6=A|last7=Skakalova|first7=V|last8=Smet|first8=J. H.|last9=Kaiser|first9=U|last10=Krasheninnikov|first10=A. V.|bibcode=2013NatSR...3E3482B}}</ref>
| hexagonal
|
| Sheet-like bilayer structure
|[[File:2D silica structure.png|100px]]
|}

==Safety==
Inhaling finely divided crystalline silica can lead to severe inflammation of the [[lung]] tissue, [[silicosis]], [[bronchitis]], [[lung cancer]], and [[systemic autoimmune disease]]s, such as [[lupus]] and [[rheumatoid arthritis]]. [[Inhalation]] of [[amorphous]] silicon dioxide, in high doses, leads to non-permanent short-term inflammation, where all effects heal.<ref>{{Cite journal|display-authors=3|vauthors=Johnston CJ, Driscoll KE, Finkelstein JN, Baggs R, O'Reilly MA, Carter J, Gelein R, Oberdörster G|date=2000|title=Pulmonary Chemokine and Mutagenic Responses in Rats after Subchronic Inhalation of Amorphous and Crystalline Silica|journal=[[Toxicological Sciences]]|volume=56|issue=2|pages=405–413|doi=10.1093/toxsci/56.2.405|pmid=10911000|doi-access=free}}</ref>

== Other names ==
This extended list enumerates synonyms for silicon dioxide; all of these values are from a single source; values in the source were presented capitalized.<ref>{{Cite book|last=Lewis|first=Grace Ross|url=https://archive.org/details/1001ChemicalsInEverydayProducts/page/n261/mode/2up|title=1001 chemicals in everyday products|publisher=John Wiley & Sons (Wiley-Interscience)|year=1999|isbn=0-471-29212-5|edition=2nd|pages=250–1|via=Internet Archive}}</ref>

{{Columns-list |colwidth=20em |
* CAS 112945-52-5
* Acitcel
* Aerosil
* Amorphous silica dust
* Aquafil
* CAB-O-GRIP II
* CAB-O-SIL
* CAB-O-SPERSE
* Catalogue
* [[Colloidal silica]]<ref>{{Cite book |url=https://pubs.acs.org/doi/book/10.1021/ba-1994-0234 |title=The Colloid Chemistry of Silica |date=1994-05-05 |publisher=American Chemical Society |isbn=978-0-8412-2103-1 |editor-last=Bergna |editor-first=Horacio E. |series=Advances in Chemistry |volume=234 |pages=1–47 |location=Washington DC |language=en |doi=10.1021/ba-1994-0234.ch001}}</ref>
* Colloidal silicon dioxide
* Dicalite
* DRI-DIE Insecticide 67
* FLO-GARD
* Fossil flour
* [[Fumed silica]]
* Fumed silicon dioxide
* HI-SEL
* LO-VEL
* Ludox
* Nalcoag
* Nyacol
* Santocel
* Silica
* [[Silica aerogel]]
* Silica, amorphous
* Silicic anhydride
* Silikill
* Synthetic amorphous silica
* Vulkasil
}}

==See also==
*[[Mesoporous silica]]
*[[Orthosilicic acid]]
*[[Silicon carbide]]

==References==
{{Reflist|30em}}

==External links==
{{Commons category}}
* {{Cite EB1911|wstitle=Silica}}
* Tridymite, {{ICSC|0807|08}}
* Quartz, {{ICSC|0808|08}}
* Cristobalite, {{ICSC|0809|08}}
* Amorphous, [https://www.cdc.gov/niosh/npg/npgd0552.html NIOSH Pocket Guide to Chemical Hazards]
* Crystalline, as respirable dust, [https://www.cdc.gov/niosh/npg/npgd0684.html NIOSH Pocket Guide to Chemical Hazards]
* [http://crystec.com/klloxide.htm Formation of silicon oxide layers in the semiconductor industry]. LPCVD and PECVD method in comparison. Stress prevention.
* [https://web.archive.org/web/20110715083534/http://piezomaterials.com/Quartz-SiO2.htm Quartz (SiO<sub>2</sub>) piezoelectric properties]
* [http://water-chemistry.blogspot.com/2008/08/silica-sio2.html Silica (SiO<sub>2</sub>) and water]
* [https://web.archive.org/web/20120618153827/http://www.iom-world.org/pubs/IOM_TM9709.pdf Epidemiological evidence on the carcinogenicity of silica: factors in scientific judgement] by C. Soutar and others. [[Institute of Occupational Medicine]] Research Report TM/97/09
* [https://web.archive.org/web/20120618132318/http://www.iom-world.org/pubs/IOM_TM9508.pdf Scientific opinion on the health effects of airborne silica] by A Pilkington and others. [[Institute of Occupational Medicine]] Research Report TM/95/08
* [http://www.iom-world.org/pubs/IOM_TM8713.pdf The toxic effects of silica] {{Webarchive|url=https://web.archive.org/web/20160415111007/http://www.iom-world.org/pubs/IOM_TM8713.pdf |date=2016-04-15}} by A. Seaton and others. [[Institute of Occupational Medicine]] Research Report TM/87/13
* [http://www.antenchem.com/en/News/Silicas-Technology/Structureofprecipitatedsilica.html Structure of precipitated silica]

{{Silica minerals}}
{{Oxides}}
{{Silicon compounds}}
{{Authority control}}

[[Category:Silicon dioxide| ]]
[[Category:Ceramic materials]]
[[Category:Refractory materials]]
[[Category:IARC Group 1 carcinogens]]
[[Category:Excipients]]
[[Category:E-number additives]]
[[Category:Oxides]]
[[Category:Occupational safety and health]]