Editing Tridymite

{{short description|Silica mineral, polymorph of quartz}}
{{Infobox mineral
| name = Tridymite
| category = [[Tectosilicates]], [[quartz]] [[Mineral group|group]]
| boxwidth = 
| boxbgcolor = 
| image = Tridymite tabulars - Ochtendung, Eifel, Germany.jpg
| imagesize   = 260px
| caption = Tabular tridymite crystals from Ochtendung, Eifel, Germany
| formula = SiO<sub>2</sub>
| molweight = 60.08 g/mol
| strunz = 4.DA.10
| IMAsymbol   = Trd<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>
| system   = [[Orthorhombic]] <br/>(α-tridymite)
| class    = Disphenoidal (222) <br/>[[H–M symbol]]: (222)
| symmetry = ''C''222<sub>1</sub>
| color    = Colorless, white
| habit    = Platy – sheet forms
| cleavage = {0001} indistinct, {1010} imperfect
| fracture = Brittle – conchoidal
| mohs     = 7
| luster   = Vitreous 
| refractive = ''n''<sub>α</sub>=1.468–1.482 <br> ''n''<sub>β</sub>=1.470–1.484 <br> ''n''<sub>γ</sub>=1.474–1.486
| opticalprop = Biaxial (+), <br> 2''V'' = 40–86°
| birefringence = δ < 0.004 
| pleochroism = Colorless
| streak = 	white
| gravity = 2.25–2.28
| melt = 
| fusibility = 
| diagnostic = 
| solubility = 
| diaphaneity = 
| other = non-radioactive, non-magnetic; fluorescent, short UV=dark red
| references =<ref name=handbook>{{cite book|editor1=Anthony, John W. |editor2=Bideaux, Richard A. |editor3=Bladh, Kenneth W. |editor4=Nichols, Monte C. |title= Handbook of Mineralogy|publisher= Mineralogical Society of America|place= Chantilly, VA, US|url=http://rruff.geo.arizona.edu/doclib/hom/tridymite.pdf|chapter=Tridymite |accessdate=December 5, 2011|isbn=0-9622097-2-4 |volume=III (Halides, Hydroxides, Oxides)}}</ref><ref>[http://www.mindat.org/min-4015.html Mindat]</ref>}}

'''Tridymite''' is a high-temperature [[polymorphism (materials science)|polymorph]] of [[silica]] and usually occurs as minute tabular white or colorless pseudo-hexagonal crystals, or scales, in cavities in [[felsic]] [[volcanic rock]]s. Its [[chemical formula]] is [[silicon|Si]][[oxygen|O]]<sub>2</sub>. Tridymite was first described in 1868 and the [[Type locality (geology)|type location]] is in Hidalgo, [[Mexico]]. The name is from the [[Greek (language)|Greek]] ''tridymos'' for ''triplet'' as tridymite commonly occurs as [[Crystal twinning|twinned]] crystal ''[[wikt:trilling|trillings]]''<ref name=handbook/> (compound crystals comprising three twinned crystal components).

==Structure==
[[File:OC-Tridymite.svg|100px|thumb|left|Crystal structure of α-tridymite]]
[[File:HP-Tridymite.svg|100px|thumb|left|β-tridymite]]
Tridymite can occur in seven crystalline forms. Two of the most common at standard pressure are known as α and β. The α-tridymite phase is favored at elevated temperatures (above 870&nbsp;°C) and it converts to β-[[cristobalite]] at 1,470&nbsp;°C.<ref name=trid/><ref name=b1/> However, tridymite does usually not form from pure β-quartz, one needs to add trace amounts of certain compounds to achieve this.<ref name=hean/> Otherwise the β-quartz-tridymite transition is skipped and β-quartz transitions directly to cristobalite at 1,050&nbsp;°C without occurrence of the tridymite phase.

:{|class="wikitable" style="text-align:center"
|+Crystal phases of tridymite<ref name=b1/>
!Name!!Symmetry!![[Space group]]!!{{abbr|Temp.|Temperature}} (°C)
|-
| HP (β) ||Hexagonal||''P''6<sub>3</sub>/mmc|| 460
|-
| LHP||Hexagonal||''P''6<sub>3</sub>22|| 400
|-
| OC (α)||Orthorhombic||''C''222<sub>1</sub>|| 220
|-
|OS ||Orthorhombic|||| 100–200
|-
|OP ||Orthorhombic||''P''2<sub>1</sub>2<sub>1</sub>2<sub>1</sub>|| 155
|-
| MC||Monoclinic||''Cc''|| 22
|-
| MX||Monoclinic||''C1''|| 22
|}
In the table, M, O, H, C, P, L and S stand for [[monoclinic]], [[orthorhombic]], [[hexagonal]], centered, primitive, low (temperature) and superlattice. T indicates the temperature, at which the corresponding phase is relatively stable, though the interconversions between phases are complex and sample dependent, and all these forms can coexist at ambient conditions.<ref name=b1/> Mineralogy handbooks often arbitrarily assign tridymite to the [[triclinic]] crystal system, yet use hexagonal [[Miller index|Miller indices]] because of the hexagonal crystal shape (see image).<ref name=handbook/>

==Mars==
[[Timeline of Mars Science Laboratory#2015 events|In December 2015]], the team behind [[NASA]]'s [[Mars Science Laboratory]] announced the discovery of large amounts of tridymite in Marias Pass on the slope of [[Mount Sharp|Aeolis Mons]], popularly known as Mount Sharp, on the planet [[Mars]].<ref name="NYT-20151217">{{cite news |last=Chang |first=Kenneth |title=Mars Rover Finds Changing Rocks, Surprising Scientists |url=https://www.nytimes.com/2015/12/18/science/mars-rover-finds-changing-rocks-surprising-scientists.html |date=December 17, 2015 |work=[[New York Times]] |accessdate=December 22, 2015 }}</ref> This discovery was unexpected given the rarity of the mineral on Earth and the apparent lack of volcanic activity where it was discovered, and at the time of discovery no explanation for how it was formed was forthcoming. Its discovery was serendipitous: two teams, responsible for two different instruments on the [[Curiosity rover|''Curiosity'' rover]], both happened to report what in isolation were relatively uninteresting findings related to their instruments: the [[ChemCam]] team reported a region of high silica while the [[Dynamic Albedo of Neutrons|DAN]] team reported high neutron readings in what turned out to be the same area. Neither team would have been aware of the other's findings had it not been for a fortuitous Mars [[conjunction (astronomy)|conjunction]] in July 2015, during which the various international teams took advantage of the downtime to meet in [[Paris]] and discuss their scientific findings. 

DAN's high neutron readings would normally have been interpreted as meaning the region was hydrogen-rich, and ChemCam's high-silica readings were not surprising given the ubiquity of silica-rich deposits on Mars, but taken together it was clear that further study of the region was needed. Following conjunction, NASA directed the ''Curiosity'' rover back to the area where the readings had been taken and discovered that large amounts of tridymite were present. How they were formed was unknown, {{As of|2015|December|lc=y}}.<ref>{{cite web |url=http://www.planetary.org/blogs/emily-lakdawalla/2015/12181522-curiosity-stories-from-agu.html |title=Curiosity stories from AGU: The fortuitous find of a puzzling mineral on Mars, and a gap in Gale's history |last=Lakdawalla |first=Emily |authorlink=Emily Lakdawalla|date=December 18, 2015 |website=[[The Planetary Society]] |access-date=December 21, 2015}}</ref>

==See also==
{{wikisourcepar|EB1911:Tridymite}}
*[[Cristobalite]]
*[[Coesite]]
*[[Stishovite]]

==References==
{{reflist|refs=
<ref name=b1>{{cite book|author1=William Alexander Deer|author2=R. A. Howie|author3=W. S. Wise|title=Rock-Forming Minerals: Framework Silicates: Silica Minerals, Feldspathoids and the Zeolites|url=https://books.google.com/books?id=c4H5TsJbUdsC&pg=PA22|accessdate=2 January 2012|year=2004|publisher=Geological Society|isbn=978-1-86239-144-4|page=22}}</ref>
<ref name=trid>{{cite journal|journal=Zeitschrift für Kristallographie|year=1986 |volume=177|pages=27–38|title=Structural change of orthorhombic-I tridymite with temperature: A study based on second-order thermal-vibrational parameters|author1=Kuniaki Kihara |author2=Matsumoto T. |author3=Imamura M. |issue=1–2 |doi=10.1524/zkri.1986.177.1-2.27|bibcode = 1986ZK....177...27K }}</ref>
<ref name=hean>{{cite journal|journal= Reviews in Mineralogy|year=1994 |volume=29|title=Structure and chemistry of the low-pressure silica polymorphs |author=Heaney, P. J.}}</ref>
}}

==External links==
{{Commons category|Tridymite}}
*[https://web.archive.org/web/20051105223454/http://mineral.galleries.com/minerals/silicate/tridymit/tridymit.htm Mineral galleries]
*[http://www.quartzpage.de/gen_mod.html Standard pressure polymorphs of SiO2]
*[https://www.npr.org/sections/thetwo-way/2015/12/18/460254072/curiouser-and-curiouser-nasas-curiosity-rover-finds-piles-of-silica-on-mars Curiouser And Curiouser: NASA's Curiosity Rover Finds Piles Of Silica On Mars]
*[https://news.rice.edu/news/2022/study-explosive-volcanic-eruption-produced-rare-mineral-mars Study: Explosive volcanic eruption produced rare mineral on Mars]

{{Silica minerals}}

[[Category:Orthorhombic minerals]]
[[Category:Minerals in space group 20]]
[[Category:Industrial minerals]]
[[Category:Silica polymorphs]]
[[Category:Silicon dioxide]]