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Cristobalite
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{{short description|Silica mineral, polymorph of quartz}} {{Infobox mineral | name = Cristobalite | category = [[Tectosilicates]], [[quartz]] [[Mineral group|group]] | boxwidth = | boxbgcolor = | image = Cristobalite-Fayalite-40048.jpg | caption = Cristobalite [[spherulite]]s formed by [[devitrification]] from the [[obsidian]] [[matrix (geology)|matrix]].<br />{{small|Specimen from California, US; size: {{cvt|5.9|×|3.8|×|3.8|cm|in}}.}} | formula = {{chem2|SiO2}} | IMAsymbol=Crs<ref>{{cite journal |last=Warr|first=L. N. |date=2021 |title=IMA–CNMNC approved mineral symbols |journal=[[Mineralogical Magazine]] |volume=85 |issue=3 |pages=291–320 |doi=10.1180/mgm.2021.43 |bibcode=2021MinM...85..291W |s2cid=235729616 |doi-access=free}}</ref> | molweight = | strunz = 4.DA.15 | dana = 75.1.1.1 | system = [[tetragonal crystal system|Tetragonal]] | class = [[trapezohedron|Trapezohedral]] (422) | symmetry = ''P''4<sub>1</sub>2<sub>1</sub>2, ''P''4<sub>3</sub>2<sub>1</sub>2 | unit cell = ''a'' = 4.9709(1) Å, <br />''c'' = 6.9278(2) Å; <br />''Z'' = 4 (α polytype) | color = Colorless, white | habit = [[octahedron|Octahedra]] or [[spherulite]]s up to several cm in diameter | twinning = on {111} | cleavage = | fracture = [[conchoidal fracture|Conchoidal]] | tenacity = [[brittleness|Brittle]] | mohs = 6–7 | luster = Vitreous | refractive = ''n''<sub>ω</sub> = 1.487 <br />''n''<sub>ε</sub> = 1.484 | opticalprop = Uniaxial (−) | birefringence = 0.003 | pleochroism = None | streak = White | gravity = 2.32–2.36 | density = | melt = {{cvt|1713|C|F}} (β)<ref name=Deer>{{cite book |last1=Deer|first1=W. A. |first2=R. A.|last2=Howie |first3=J.|last3=Zussman |title=An Introduction to the Rock Forming Minerals |publisher=[[Longman]] |year=1966 |pages=340–355 |isbn=0-582-44210-9}}</ref> | fusibility = | diagnostic = | solubility = | diaphaneity = Transparent | other = | references = <ref>[https://mineralienatlas.de/lexikon/index.php/MineralData?mineral=Cristobalite Mineralienatlas].</ref><ref name=Handbook>[https://rruff.info/doclib/hom/cristobalite.pdf Cristobalite] {{Webarchive|url=https://web.archive.org/web/20100715214929/http://rruff.geo.arizona.edu/doclib/hom/cristobalite.pdf |date=2010-07-15 }}. Handbook of Mineralogy.</ref><ref name=Mindat>[https://mindat.org/min-1155.html Cristobalite]. Mindat.</ref><ref name=Webmin>{{cite web|url=https://webmineral.com/data/Cristobalite.shtml|title=Cristobalite Mineral Data|website=Webmineral}}</ref> | SMILES = O[Si]3(O)O[Si](O)(O[Si]4(O)O)O[Si](O)(O)O[Si](O0)(O)O[Si]1(O)O[Si]5(O3)O[Si]2(O)O[Si](O4)(O)O[Si]0(O)O[Si](O)(O)O[Si](O)(O[Si](O)(O)O1)O[Si](O)(O2)O[Si](O)(O)O[Si](O)(O)O[Si](O)(O)O5 | Jmol = O[Si]3(O)O[Si](O)(O[Si]4(O)O)O[Si](O)(O)O[Si](O0)(O)O[Si]1(O)O[Si]5(O3)O[Si]2(O)O[Si](O4)(O)O[Si]0(O)O[Si](O)(O)O[Si](O)(O[Si]6(O)O1)O[Si](O)(O2)O[Si](O)(O)O[Si](O)(O[Si](O)(O)O6)O[Si](O)(O)O5 }} '''Cristobalite''' ({{IPAc-en|k|r|ɪ|ˈ|s|t|oʊ|b|ə|l|aɪ|t}} {{respell|krist|OH|bə|lyte}}) is a [[mineral]] [[polymorphism (materials science)|polymorph]] of [[silicon dioxide|silica]] that is formed at very high temperatures. It has the same chemical formula as [[quartz]], [[silicon|Si]][[oxygen|O]]<sub>2</sub>, but a distinct crystal structure. Both quartz and cristobalite are polymorphs with all the members of the quartz group, which also include [[coesite]], [[tridymite]] and [[stishovite]]. It is named after Cerro San Cristóbal in [[Pachuca Municipality]], [[Hidalgo (state)|Hidalgo]], [[Mexico]]. It is used in dentistry as a component of [[alginate]] impression materials as well as for making models of teeth.<ref>{{Cite book |last=Anusavice, Kenneth J. |title=Phillips' science of dental materials |date=2013 |publisher=Elsevier/Saunders |isbn=978-1-4377-2418-9 |oclc=934359978}}</ref> == Properties == === Metastability === Cristobalite is stable only above 1470 °C, but can crystallize and persist [[metastable|metastably]] at lower temperatures. The persistence of cristobalite outside its thermodynamic stability range occurs because the transition from cristobalite to [[quartz]] or [[tridymite]] is "reconstructive", requiring the breaking up and reforming of the [[silica]] framework. These frameworks are composed of SiO<sub>4</sub> [[tetrahedra]] in which every oxygen atom is shared with a neighbouring tetrahedron, so that the [[chemical formula]] of silica is SiO<sub>2</sub>. The breaking of these bonds, required to convert cristobalite to tridymite and quartz, requires considerable [[activation energy]] and may not happen on a human time frame at room temperature. Framework silicates are also known as [[Silicate minerals#Tectosilicates|tectosilicates]]. When [[Devitrification|devitrifying]] silica, cristobalite is usually the first phase to form, even when well outside its thermodynamic stability range. This is an example of [[Ostwald's rule|Ostwald's step rule]]. The dynamically disordered nature of the β phase is partly responsible for the low enthalpy of fusion of silica. === Structures === There is more than one form of the cristobalite framework. At high temperatures, the structure is called β-cristobalite. It is in the [[cubic crystal system]], [[space group]] Fd{{overline|3}}m (No. 227, [[Pearson symbol]] ''cF104'').<ref>{{cite journal|journal=Philosophical Magazine|year=1975 |volume=31|pages=1391–1401 |title=The structures of the b-cristobalite phases of SiO<sub>2</sub> and AlPO<sub>4</sub> |doi=10.1080/00318087508228690 |author=Wright A. F., Leadbetter A. J.|issue=6 |bibcode=1975PMag...31.1391W }}</ref> It has the [[diamond structure]] but with linked tetrahedra of silicon and oxygen where the carbon atoms are in diamond. A [[chiral]] [[tetragonal crystal system|tetragonal]] form called α-cristobalite (space group either 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|author=Downs R. T., Palmer D. C.|url=http://www.geo.arizona.edu/xtal/group/pdf/AM79_9.pdf|access-date=2009-12-15|archive-url=https://web.archive.org/web/20190515215339/https://www.geo.arizona.edu/xtal/group/pdf/AM79_9.pdf|archive-date=2019-05-15}}</ref> or P4<sub>3</sub>2<sub>1</sub>2, No. 96, at random) occurs on cooling below about 250 °C at ambient pressure and is related to the cubic form by static tilting of the silica tetrahedra in the framework. This transition is variously called the low-high or <math>\alpha{-}\beta</math> transition. It may be termed "displacive"; i.e., it is not generally possible to prevent the cubic β form from becoming tetragonal by rapid cooling. Under rare circumstances the cubic form may be preserved if the crystal grain is pinned in a matrix that does not allow for the considerable spontaneous strain that is involved in the transition, which causes a change in shape of the crystal. This transition is highly discontinuous. Going from the α form to the β form causes an increase in volume of 3<ref>{{cite book |last1=R.E. Smallman & R.J. Bishop |title=Modern Physical Metallurgy and Materials Engineering |date=1999 |isbn=978-0-7506-4564-5 |edition=6 |url=https://www.sciencedirect.com/topics/engineering/cristobalite |chapter=2|publisher=Butterworth-Heinemann }}</ref> or 4<ref>{{cite journal |last1=A.J. Leadbetter & A.F. Wright |title=The α—β transition in the cristobalite phases of SiO{{sub|2}} and AIPO{{sub|4}} I. X-ray studies |journal=[[The Philosophical Magazine]] |date=1976 |volume=33 |issue=1 |pages=105–112 |doi=10.1080/14786437608221095|bibcode=1976PMag...33..105L }}</ref> percent. The exact transition temperature depends on the crystallinity of the cristobalite sample, which itself depends on factors such as how long it has been annealed at a particular temperature. The cubic β phase consists of dynamically disordered silica tetrahedra. The tetrahedra remain fairly regular and are displaced from their ideal static orientations due to the action of a class of low-frequency [[phonons]] called [[rigid unit mode]]s. It is the "freezing" of one of these rigid unit modes that is the soft mode for the α–β transition. In β-cristobalite, there are right-handed and left-handed helices of tetrahedra (or of silicon atoms) parallel to all three axes. In the α–β phase transition, however, only the right-handed or the left-handed helix in one direction is preserved (the other becoming a two-fold screw axis), so only one of the three degenerate cubic crystallographic axes retains a fourfold rotational axis (actually a [[screw axis]]) in the tetragonal form. (That axis becomes the "c" axis, and the new "a" axes are rotated 45° compared to the other two old axes. The new "a" lattice parameter is shorter by approximately the square root of 2, so the α unit cell contains only 4 silicon atoms rather than 8.) The choice of axis is arbitrary, so that various [[twins]] can form within the same grain. These different twin orientations coupled with the discontinuous nature of the transition (volume and slight shape change) can cause considerable mechanical damage to materials in which cristobalite is present and that pass repeatedly through the transition temperature, such as refractory bricks. <gallery widths="200px" heights="200px"> File:B-cristobal1.png|An idealized model of β-cristobalite, showing corner-bonded [[silicon|Si]][[Oxygen|O]]<sub>4</sub> tetrahedra. There is a right-handed four-fold screw axis at the centre of half the white squares, and a left-handed one at the centre of the others. In this projection we see glide planes parallel to the axes and mirrors on the diagonals. In reality the tetrahedra are constantly wobbling. File:B-cristobal2.png|β-cristobalite viewed along the 101 direction File:A-cristobal.png|The crumpled framework of α-cristobalite, related to the β form by static tilting of the tetrahedra. This view corresponds to the view along the 101 direction of the previous illustration, except that the "b" axis of that picture is now horizontal. The two-fold screw axes appear here as two-fold axes of rotation going through the middle of the white areas and between the pairs of almost superimposed oxygen atoms. File:Α-Cristobalite.svg|Unit cell of α-cristobalite; red spheres are oxygen atoms. We see here five silicon atoms in a helix (the first and the last are equivalent atoms in the lattice) going in the "c" direction (into the page). The horizontal and vertical axes are the "a" axes. File:Β-Cristobalite.svg|Unit cell of β-cristobalite; red spheres are oxygen atoms. </gallery> == Occurrence == Cristobalite occurs as white octahedra or [[spherulite]]s in acidic volcanic rocks and in converted diatomaceous deposits in the [[Monterey Formation]] of the US state of California and similar areas. The micrometre-scale spheres that make up precious [[opal]] exhibit some X-ray diffraction patterns that are similar to that of cristobalite, but lack any long-range order so they are not considered true cristobalite. In addition, the presence of structural water in opal makes it doubtful that opal consists of cristobalite.<ref>{{cite journal |author=Deane K. Smith |year=1998 |title=Opal, cristobalite, and tridymite: Noncrystallinity versus crystallinity, nomenclature of the silica minerals and bibliography |url=http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=8512817 |journal=Powder Diffraction |volume=13 |issue=1 |pages=2–19 |bibcode=1998PDiff..13....2S |doi=10.1017/S0885715600009696 |s2cid=97394861|url-access=subscription }}</ref><ref>{{cite web |title=Silica, Crystalline - Overview | Occupational Safety and Health Administration |url=https://www.osha.gov/dsg/topics/silicacrystalline/smithdk/pdf/nomenc.pdf |archive-url=https://web.archive.org/web/20160304134008/https://www.osha.gov/dsg/topics/silicacrystalline/smithdk/pdf/nomenc.pdf |archive-date=2016-03-04 |website=www.osha.gov}}</ref> Cristobalite is visible as the white inclusions in ''[[snowflake obsidian]]'', a volcanic glass. ==References== {{reflist}} ==Further reading== * ''American Geological Institute Dictionary of Geological Terms''. * Durham, D. L., "Monterey Formation: Diagenesis". in: ''Uranium in the Monterey Formation of California''. US Geological Survey Bulletin 1581-A, 1987. * ''Reviews in Mineralogy and Geochemistry'', vol. 29., ''Silica: behavior, geochemistry and physical applications''. Mineralogical Society of America, 1994. * R. B. Sosman, ''The Phases of Silica''. (Rutgers University Press, 1965) ==External links== * {{ICSC|0809|08}} {{Silica minerals}} [[Category:Polymorphism (materials science)]] [[Category:Tetragonal minerals]] [[Category:Minerals in space group 92]] [[Category:Minerals in space group 96]] [[Category:Silica polymorphs]] [[Category:Silicon compounds]] [[Category:Silicon dioxide]] [[Category:Minerals in space group 227]]
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