Editing Anatase

{{Short description|Mineral form of titanium dioxide}}
{{Infobox mineral
| name        = Anatase
| category    = [[Oxide minerals]]
| boxwidth    = 
| boxbgcolor  =#4c6794 
| boxtextcolor = #fff
| image       = Anatase Oisans.jpg
| alt         = 
| caption     = 
| formula     = TiO<sub>2</sub>
|IMAsymbol=Ant<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   = 79.88 g/mol
| strunz      = 4.DD.05
| system      = [[Tetragonal crystal system|Tetragonal]]
| class       = Ditetragonal dipyramidal (4/mmm) <br/>[[H-M symbol]]: (4/m 2/m 2/m)
| symmetry    = ''I''4<sub>1</sub>/amd
| unit cell   = a = 3.7845, c = 9.5143&nbsp;[Å]; Z&nbsp;=&nbsp;4
| color       = Black, reddish to yellowish brown, dark blue, gray 
| habit       = Pyramidal (crystals are shaped like pyramids), tabular (form dimensions are thin in one direction). 
| twinning    = Rare on {112}
| cleavage    = Perfect on [001] and [011] 
| fracture    = Subconchoidal 
| tenacity    = Brittle 
| mohs        = 5.5–6
| luster      = Adamantine to splendent, metallic 
| refractive  = n<sub>ω</sub> = 2.561, n<sub>ε</sub> = 2.488
| opticalprop = Uniaxial (−), anomalously biaxial in deeply colored crystals	
| birefringence = δ = 0.073
| pleochroism = Weak
| streak      = Pale yellowish white
| gravity     = 3.79–3.97
| melt        = 
| fusibility  = 
| diagnostic  = 
| solubility  = 
| diaphaneity = Transparent to nearly opaque
| other       = 
| references  = <ref name=HBM>{{cite web| url= http://rruff.geo.arizona.edu/doclib/hom/anatase.pdf |title= Anatase| work= Handbook of Mineralogy| via= geo.arizona.edu}}</ref><ref name=Mindat>{{cite web| url= http://www.mindat.org/min-213.html |title= Anatase| website= Mindat.org}}</ref><ref name=Webmin>{{cite web| url = http://webmineral.com/data/Anatase.shtml| title = Anatase| access-date = 2009-06-06| website= Webmineral.com}}</ref>
}}

'''Anatase''' is a [[metastable]] [[mineral]] form of [[titanium dioxide]] (TiO<sub>2</sub>) with a [[Tetragonal crystal system|tetragonal]] crystal structure.  Although colorless or white when pure, anatase in nature is usually a black solid due to impurities.  Three other [[Polymorphism (materials science)|polymorphs]] (or mineral forms) of titanium dioxide are known to occur naturally: [[brookite]], [[akaogiite]], and [[rutile]], with rutile being the most common and most [[Chemical stability|stable]] of the bunch. Anatase is formed at relatively low temperatures and found in minor concentrations in [[Igneous rock|igneous]] and [[Metamorphic rock|metamorphic]] rocks.<ref>Page 419 Deer, Howie and Zussman "An Introduction to the Rock Forming Minerals" ISBN 0 582 44210 9</ref> Glass coated with a thin film of TiO<sub>2</sub> shows [[Anti-fog|antifogging]] and [[Self-cleaning surfaces|self-cleaning]] properties under [[ultraviolet]] radiation.<ref>{{cite journal|last1=Wang|first1=Rong|last2=Hashimoto|first2=Kazuhito|last3=Fujishima|first3=Akira|last4=Chikuni|first4=Makota|last5=Kojima|first5=Eiichi|last6=Kitamura|first6=Atsushi|last7=Shimohigoshi|first7=Mitsuhide|last8=Watanabe|first8=Toshiya|date=July 1997|title=Light-induced amphiphilic surfaces|journal=Nature|volume=388|issue=6641|pages=431–432|bibcode=1997Natur.388..431W|doi=10.1038/41233|s2cid=4417645|doi-access=free}}</ref>

Anatase is always found as small, isolated, and sharply developed [[crystal]]s, and like rutile, it crystallizes in a [[Tetragonal crystal system|tetragonal system]]. Anatase is metastable at all temperatures and pressures, with rutile being the equilibrium polymorph. Nevertheless, anatase is often the first titanium dioxide phase to form in many processes due to its lower [[surface energy]], with a transformation to rutile taking place at elevated temperatures.<ref>{{cite journal |last1=Hanaor |first1=Dorian A. H. |last2=Sorrell |first2=Charles C. |title=Review of the anatase to rutile phase transformation |journal=Journal of Materials Science |year=2011 |volume=46 |issue=4 |pages=855–874 |doi=10.1007/s10853-010-5113-0 |bibcode=2011JMatS..46..855H |s2cid=97190202 |url=https://hal.science/hal-02308408|doi-access=free }}</ref> Although the degree of symmetry is the same for both anatase and rutile phases, there is no relation between the interfacial angles of the two minerals, except in the prism-zone of 45° and 90°. The common [[Octahedron|octahedral]] [[crystal habit]] of anatase, with four perfect [[Cleavage (crystal)|cleavage]] planes, has an angle over its polar edge of 82°9', whereas rutile octahedra only has a polar edge angle of 56°52½'. The steeper angle gives anatase crystals a longer vertical axis and skinnier appearance than rutile. Additional important differences exist between the physical characters of anatase and rutile. For example, anatase is less hard (5.5–6 vs. 6–6.5 on the [[Mohs scale of mineral hardness|Mohs scale]]) and less dense ([[specific gravity]] about 3.9 vs. 4.2) than rutile. Anatase is also [[Birefringence#Positive or negative|optically negative]], whereas rutile is optically positive. Anatase has a more strongly [[adamantine lustre|adamantine]] or [[Lustre (mineralogy)#Metallic lustre|metallic]]-adamantine [[Lustre (mineralogy)|luster]] than that of rutile as well.<ref name="EB1911">{{EB1911|inline=y|wstitle=Anatase|volume=1|pages=919–920|first=Leonard James|last=Spencer|authorlink=Leonard James Spencer}}</ref>

==Nomenclature==
The modern name was introduced by [[René Just Haüy]] in 1801, but the mineral was known and described before. It derives from {{Langx|grc|ἀνάτασις}} 'stretching out', because the crystals are stretched along an axis compared to other dipyramidal ones.<ref name=":0">{{Cite book |last=Dana |first=James Dwight |url=https://books.google.com/books?id=UOz4MXhQLMEC&q=anatase |title=A system of mineralogy |date=1868 |pages=XXXI & 162 |language=en}}</ref>

Another name commonly in use for anatase is ''octahedrite'', which is earlier than anatase and was given by [[Horace Bénédict de Saussure]]<ref name=":0" /> because of the common (acute) octahedral habit of the crystals. Other names, now obsolete, are ''oisanite'' (by [[Jean-Claude Delamétherie]]) and ''dauphinite'', from the well-known French locality of [[Le Bourg-d'Oisans]] in [[Dauphiné]].<ref name="EB1911" />

==Crystal habit==
[[File:Anatase crystal structure.png|alt=A ball-and-stick chemical model of an anatase crystal|left|thumb|Extended portion of the anatase lattice.]]
Two growth [[Crystal habit|habits]] of anatase crystals may be distinguished. The more common occurs as simple acute [[Octahedron|octahedra]] with an indigo-blue to black color and steely luster.  Crystals of this kind are abundant at [[Le Bourg-d'Oisans]] in [[Dauphiné|Dauphiné, France]], where they are associated with rock-crystal, [[feldspar]], and [[axinite]] in crevices in [[granite]] and [[mica]] [[schist]].  Similar crystals of microscopic size are widely distributed in [[sedimentary rock]]s such as [[sandstone]]s, [[clay]]s, and [[slate]]s, from which they may be separated by washing away the lighter constituents of the powdered rock.<ref name="EB1911"/> The (101) plane of anatase is the most thermodynamically stable surface and thus the most widely exposed [[facet]] in natural and synthetic anatase.<ref>{{cite journal |last1= Assadi | first1=MHN | last2=Hanaor | first2 = DAH | title= The effects of copper doping on photocatalytic activity at (101) planes of anatase TiO 2: A theoretical study| url=https://www.researchgate.net/publication/304714130 |journal= Applied Surface Science | volume= 387| pages=682–689|year=2016 | doi=10.1016/j.apsusc.2016.06.178| arxiv=1811.09157 | bibcode=2016ApSS..387..682A | s2cid=99834042 }}</ref>

Crystals of the second type have numerous pyramidal faces developed, and they are usually flatter or sometimes prismatic in habit. Their color is honey-yellow to brown. Such crystals closely resemble the mineral [[xenotime]] in appearance and were historically thought to be a special form of xenotime, termed ''wiserine''.  They occur attached to the walls of crevices in [[gneiss]]es in the [[Alps]], a well-known locality being the [[Binnenthal]] near [[Brig, Switzerland|Brig]] in canton [[Valais]], [[Switzerland]].<ref name="EB1911"/>

While anatase is not an equilibrium phase of TiO<sub>2</sub>, it is metastable near room temperature. At temperatures between 550 and about 1000&nbsp;°C, anatase converts to rutile. The temperature of this transformation strongly depends on impurities, or [[Dopant|dopants]], as well as the morphology of the sample.<ref>{{cite journal |last1=Hanaor |first1=Dorian A. H. |last2=Sorrell |first2=Charles C. |title=Review of the anatase to rutile phase transformation |journal=Journal of Materials Science |date=February 2011 |volume=46 |issue=4 |pages=855–874 |doi=10.1007/s10853-010-5113-0 |bibcode=2011JMatS..46..855H |s2cid=97190202 |url=https://hal.archives-ouvertes.fr/hal-02308408/file/Hanaor%20Sorrell%20-%20JMatSci-Review%20of%20the%20Anatase%20to%20Rutile%20Phase%20Transformation.pdf }}</ref>

==Synthetic anatase==
Due to its potential application as a [[semiconductor]], anatase is often prepared synthetically. Crystalline anatase can be prepared in laboratories by chemical methods such as the [[sol-gel|sol-gel process]]. This might be done through controlled [[hydrolysis]] of [[titanium tetrachloride]] (TiCl<sub>4</sub>) or [[titanium ethoxide]]. Often dopants are included in such synthesis processes to control the morphology, electronic structure, and surface chemistry of an anatase sample.<ref>{{cite journal |last1=Jeantelot |first1=Gabriel |last2=Ould-Chikh |first2=Samy |last3=Sofack-Kreutzer |first3=Julien |last4=Abou-Hamad |first4=Edy |last5=Anjum |first5=Dalaver H. |last6=Lopatin |first6=Sergei |last7=Harb |first7=Moussab |last8=Cavallo |first8=Luigi |last9=Basset |first9=Jean-Marie |title=Morphology control of anatase TiO2 for well-defined surface chemistry |journal=Physical Chemistry Chemical Physics |date=2018 |volume=20 |issue=21 |pages=14362–14373 |doi=10.1039/C8CP01983E |pmid=29767182 |bibcode=2018PCCP...2014362J |hdl=10754/627938 |hdl-access=free |doi-access=free }}</ref>

==See also==
[[File:Anatase-Quartz-221342.jpg|alt=|thumb|400x400px|Black anatase crystals on smoked quartz]]{{commons category|Anatase}}
*[[Delustrant]]
*[[Titanium dioxide]]
*[[Rutile]]
*[[Brookite]]
*[[Akaogiite]]
* [[List of minerals]]

==References==
{{Reflist}}

{{Titanium minerals}}

[[Category:Titanium minerals]]
[[Category:Oxide minerals]]
[[Category:Tetragonal minerals]]
[[Category:Minerals in space group 141]]
[[Category:Minerals described in 1801]]