Editing Titanium dioxide (section)

===Mineralogy and uncommon polymorphs===
Titanium dioxide occurs in nature as the minerals [[rutile]] and [[anatase]].  Additionally two high-pressure forms are known minerals: a [[monoclinic crystal system|monoclinic]] [[baddeleyite]]-like form known as [[akaogiite]], and the other has a slight monoclinic distortion of the [[orthorhombic crystal system|orthorhombic]] [[lead dioxide|α-PbO<sub>2</sub>]] structure and is known as riesite. Both of which can be found at the [[Nördlinger Ries|Ries crater]] in [[Bavaria]].<ref>{{cite journal |doi=10.1126/science.1062342|year=2001|title=An ultradense polymorph of rutile with seven-coordinated titanium from the Ries crater.|volume=293|issue=5534|pages=1467–70|pmid=11520981|journal=Science|author1=El, Goresy|author2=Chen, M|author3=Dubrovinsky, L|author4=Gillet, P|author5=Graup, G|bibcode=2001Sci...293.1467E|s2cid=24349901}}</ref><ref>{{cite journal |doi=10.1016/S0012-821X(01)00480-0|title=A natural shock-induced dense polymorph of rutile with α-PbO2 structure in the suevite from the Ries crater in Germany|year=2001|author=El Goresy, Ahmed|journal=Earth and Planetary Science Letters|volume=192|pages=485|last2=Chen|first2=Ming|last3=Gillet|first3=Philippe|last4=Dubrovinsky|first4=Leonid|last5=Graup|first5=GüNther|last6=Ahuja|first6=Rajeev|bibcode=2001E&PSL.192..485E|issue=4}}</ref><ref>[https://www.mindat.org/min-35912.html Akaogiite]. mindat.org</ref> It is mainly sourced from [[ilmenite]], which is the most widespread titanium dioxide-bearing ore around the world. Rutile is the next most abundant and contains around 98% titanium dioxide in the ore. The metastable anatase and brookite phases convert irreversibly to the equilibrium rutile phase upon heating above temperatures in the range {{convert|600|-|800|C|-1}}.<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.science/hal-02308408|doi-access=free }}</ref>

Titanium dioxide has twelve known polymorphs – in addition to rutile, anatase, brookite, akaogiite and riesite, three metastable phases can be produced synthetically ([[monoclinic crystal system|monoclinic]], [[tetragonal crystal system|tetragonal]], and orthorhombic ramsdellite-like), and four high-pressure forms (α-PbO<sub>2</sub>-like, [[cotunnite]]-like, orthorhombic OI, and cubic phases) also exist:

{| class="wikitable"
|-
! Form
! Crystal system
! Synthesis
|-
| [[Rutile]]
| [[Tetragonal crystal system|Tetragonal]]
|
|-
| [[Anatase]]
|[[Tetragonal crystal system|Tetragonal]]
|
|-
| [[Brookite]]
| [[Orthorhombic crystal system|Orthorhombic]]
|
|-
| TiO<sub>2</sub>(B)<ref>{{cite journal |author1=Marchand R. |author2=Brohan L. |author3=Tournoux M. |title= A new form of titanium dioxide and the potassium octatitanate K<sub>2</sub>Ti<sub>8</sub>O<sub>17</sub> |year= 1980|journal= Materials Research Bulletin|volume= 15|issue= 8|pages= 1129–1133|doi= 10.1016/0025-5408(80)90076-8}}</ref>
| [[Monoclinic crystal system|Monoclinic]]
| Hydrolysis of K<sub>2</sub>Ti<sub>4</sub>O<sub>9</sub> followed by heating
|-
| TiO<sub>2</sub>(H), [[hollandite]]-like form<ref>{{cite journal |title= New hollandite oxides: TiO<sub>2</sub>(H) and K<sub>0.06</sub>TiO<sub>2</sub>|year= 1989|journal= Journal of Solid State Chemistry|volume= 81|issue= 1|pages= 78–82 |doi= 10.1016/0022-4596(89)90204-1|author1= Latroche, M|author2= Brohan, L|author3= Marchand, R|author4= Tournoux|bibcode= 1989JSSCh..81...78L}}</ref>
|[[Tetragonal crystal system|Tetragonal]]
| Oxidation of the related potassium titanate bronze, K<sub>0.25</sub>TiO<sub>2</sub>
|-
| TiO<sub>2</sub>(R), [[ramsdellite]]-like form<ref>{{cite journal |title= Topotactic Oxidation of Ramsdellite-Type Li<sub>0.5</sub>TiO<sub>2</sub>, a New Polymorph of Titanium Dioxide: TiO<sub>2</sub>(R)|year= 1994|journal= Journal of Solid State Chemistry|volume= 113|issue= 1|pages= 27–36 |doi= 10.1006/jssc.1994.1337|bibcode= 1994JSSCh.113...27A|last1= Akimoto|first1= J.|last2= Gotoh|first2= Y.|last3= Oosawa|first3= Y.|last4= Nonose|first4= N.|last5= Kumagai|first5= T.|last6= Aoki|first6= K.|last7= Takei|first7= H.}}</ref>
|[[Orthorhombic crystal system|Orthorhombic]]
| Oxidation of the related lithium titanate bronze Li<sub>0.5</sub>TiO<sub>2</sub>
|-
| TiO<sub>2</sub>(II)-([[lead dioxide|α-PbO<sub>2</sub>]]-like form)<ref>{{cite journal |title= The structure of TiO<sub>2</sub>II, a high-pressure phase of TiO<sub>2</sub>|year= 1967|journal= Acta Crystallographica|volume= 23 |issue= 2|pages= 334–336|doi= 10.1107/S0365110X67002713|last1= Simons|first1= P. Y.|last2= Dachille|first2= F.|bibcode= 1967AcCry..23..334S}}</ref>
|[[Orthorhombic crystal system|Orthorhombic]]
|
|-
| [[Akaogiite]] ([[baddeleyite]]-like form, 7 coordinated Ti)<ref>{{cite journal |author1=Sato H |author2=Endo S |author3=Sugiyama M |author4=Kikegawa T |author5=Shimomura O |author6=Kusaba K |title= Baddeleyite-Type High-Pressure Phase of TiO<sub>2</sub>|year= 1991|journal= Science|volume= 251|issue= 4995|pages= 786–788|doi= 10.1126/science.251.4995.786|pmid= 17775458|bibcode= 1991Sci...251..786S|s2cid=28241170 }}</ref>
|[[Monoclinic crystal system|Monoclinic]]
|
|-
| TiO<sub>2</sub> -OI<ref>{{cite journal |author1=Dubrovinskaia N. A. |author2=Dubrovinsky L. S. |author3=Ahuja R. |author4=Prokopenko V. B. |author5=Dmitriev V. |author6=Weber H.-P. |author7=Osorio-Guillen J. M. |author8=Johansson B. |title= Experimental and Theoretical Identification of a New High-Pressure TiO<sub>2</sub> Polymorph|year= 2001|journal= Phys. Rev. Lett.|volume= 87|pages= 275501|doi= 10.1103/PhysRevLett.87.275501|pmid= 11800890|issue= 27 Pt 1|bibcode=2001PhRvL..87A5501D}}</ref>
|[[Orthorhombic crystal system|Orthorhombic]]
|
|-
| [[Cubic crystal system|Cubic]] form<ref>{{cite journal |author1=Mattesini M. |author2=de Almeida J. S. |author3=Dubrovinsky L. |author4=Dubrovinskaia L. |author5=Johansson B. |author6=Ahuja R. |title= High-pressure and high-temperature synthesis of the cubic TiO<sub>2</sub> polymorph|year= 2004|journal= Phys. Rev. B|volume= 70|pages= 212101|doi= 10.1103/PhysRevB.70.212101|issue= 21|bibcode= 2004PhRvB..70u2101M |title-link=cubic crystal system}}</ref>
| [[Cubic crystal system|Cubic]]
| P > 40&nbsp;GPa, T > 1600&nbsp;°C
|-
| TiO<sub>2</sub> -OII, [[cotunnite]]([[lead(II) chloride|PbCl<sub>2</sub>]])-like<ref name=du1>{{cite journal |title= Materials science: The hardest known oxide|year= 2001|journal= Nature|volume= 410|pages= 653–654|doi= 10.1038/35070650|pmid= 11287944|last1= Dubrovinsky|first1= LS|last2= Dubrovinskaia|first2= NA|last3= Swamy|first3= V|last4= Muscat|first4= J|last5= Harrison|first5= NM|last6= Ahuja|first6= R|last7= Holm|first7= B|last8= Johansson|first8= B|issue= 6829|bibcode= 2001Natur.410..653D |hdl= 10044/1/11018|s2cid= 4365291|hdl-access= free}}</ref>
|[[Orthorhombic crystal system|Orthorhombic]]
| P > 40&nbsp;GPa, T > 700&nbsp;°C
|}

The [[cotunnite]]-type phase was claimed to be the hardest known oxide with the [[Vickers hardness]] of 38&nbsp;GPa and the [[bulk modulus]] of 431&nbsp;GPa (i.e. close to diamond's value of 446&nbsp;GPa) at atmospheric pressure.<ref name=du1/> However, later studies came to different conclusions with much lower values for both the hardness (7–20&nbsp;GPa, which makes it softer than common oxides like corundum Al<sub>2</sub>O<sub>3</sub> and rutile TiO<sub>2</sub>)<ref>{{cite journal |author1=Oganov A.R. |author2=Lyakhov A.O. |title= Towards the theory of hardness of materials |year= 2010|journal= Journal of Superhard Materials |volume= 32|pages= 143–147|doi= 10.3103/S1063457610030019|issue= 3|arxiv= 1009.5477|bibcode=2010JSMat..32..143O|s2cid=119280867 }}</ref> and bulk modulus (~300&nbsp;GPa).<ref>{{cite journal |author1=Al-Khatatbeh, Y. |author2=Lee, K. K. M. |author3=Kiefer, B. |title= High-pressure behavior of TiO<sub>2</sub> as determined by experiment and theory|year= 2009|journal= Phys. Rev. B |volume= 79|page= 134114|doi=10.1103/PhysRevB.79.134114|issue= 13|bibcode= 2009PhRvB..79m4114A}}</ref><ref>{{cite journal |author1=Nishio-Hamane D. |author2=Shimizu A. |author3=Nakahira R. |author4=Niwa K. |author5=Sano-Furukawa A. |author6=Okada T. |author7=Yagi T. |author8=Kikegawa T. |title= The stability and equation of state for the cotunnite phase of TiO<sub>2</sub> up to 70 GPa|year= 2010|journal= Phys. Chem. Miner. |volume= 37|pages= 129–136|doi=10.1007/s00269-009-0316-0|issue= 3|bibcode= 2010PCM....37..129N|s2cid=95463163 }}</ref>

Titanium dioxide (B) is found as a [[mineral]] in magmatic rocks and hydrothermal veins, as well as weathering rims on [[perovskite]]. TiO<sub>2</sub> also forms [[lamella (materials)|lamellae]] in other minerals.<ref>{{cite journal |author1= Banfield, J. F.|author2=Veblen, D. R.|author3=Smith, D. J. |title= The identification of naturally occurring TiO<sub>2</sub> (B) by structure determination using high-resolution electron microscopy, image simulation, and distance–least–squares refinement|journal= American Mineralogist|url=http://www.minsocam.org/ammin/AM76/AM76_343.pdf |year= 1991 |volume= 76|page= 343}}</ref>