Anatase
Anatase | |
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General | |
Category | Oxide minerals |
Formula (repeating unit) | TiO2 |
IMA symbol | Ant[1] |
Strunz classification | 4.DD.05 |
Crystal system | Tetragonal |
Crystal class | Ditetragonal dipyramidal (4/mmm) H-M symbol:(4/m 2/m 2/m) |
Space group | I41/amd |
Unit cell | a = 3.7845, c = 9.5143 [Å]; Z = 4 |
Identification | |
Formula mass | 79.88 g/mol |
Color | Black, reddish to yellowish brown, dark blue, gray |
Crystal 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 scalehardness | 5.5–6 |
Luster | Adamantine to splendent, metallic |
Streak | Pale yellowish white |
Diaphaneity | Transparent to nearly opaque |
Specific gravity | 3.79–3.97 |
Optical properties | Uniaxial (−), anomalously biaxial in deeply colored crystals |
Refractive index | nω= 2.561, nε= 2.488 |
Birefringence | δ = 0.073 |
Pleochroism | Weak |
References | [2][3][4] |
Anataseis ametastablemineralform oftitanium dioxide(TiO2) with atetragonalcrystal structure. Although colorless or white when pure, anatase in nature is usually a black solid due to impurities. Three otherpolymorphs(or mineral forms) of titanium dioxide are known to occur naturally:brookite,akaogiite,andrutile,with rutile being the most common and moststableof the bunch. Anatase is formed at relatively low temperatures and found in minor concentrations inigneousandmetamorphicrocks.[5]Glass coated with a thin film of TiO2showsantifoggingandself-cleaningproperties underultravioletradiation.[6]
Anatase is always found as small, isolated, and sharply developedcrystals,and like rutile, it crystallizes in atetragonal 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 lowersurface energy,with a transformation to rutile taking place at elevated temperatures.[7]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 commonoctahedralcrystal habitof anatase, with four perfectcleavageplanes, 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, which led French mineralogistRené Just Haüyto name the mineralanatasein 1801, from theGreekanatasis( "extension" ). 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 theMohs scale) and less dense (specific gravityabout 3.9 vs. 4.2) than rutile. Anatase is alsooptically negative,whereas rutile is optically positive. Anatase has a more stronglyadamantineormetallic-adamantinelusterthan that of rutile as well.[8]
Crystal habit
[edit]![A ball-and-stick chemical model of an anatase crystal](https://upload.wikimedia.org/wikipedia/commons/thumb/3/36/Anatase_crystal_structure.png/220px-Anatase_crystal_structure.png)
Two growthhabitsof anatase crystals may be distinguished. The more common occurs as simple acuteoctahedrawith an indigo-blue to black color and steely luster. Crystals of this kind are abundant atLe Bourg-d'OisansinDauphiné, France,where they are associated with rock-crystal,feldspar,andaxinitein crevices ingraniteandmicaschist.Similar crystals of microscopic size are widely distributed insedimentary rockssuch assandstones,clays,andslates,from which they may be separated by washing away the lighter constituents of the powdered rock.[8]The (101) plane of anatase is the most thermodynamically stable surface and thus the most widely exposedfacetin natural and synthetic anatase.[9]
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 mineralxenotimein appearance and were historically thought to be a special form of xenotime, termedwiserine.They occur attached to the walls of crevices ingneissesin theAlps,a well-known locality being theBinnenthalnearBrigin cantonValais,Switzerland.[8]
While anatase is not an equilibrium phase of TiO2,it is metastable near room temperature. At temperatures between 550 and about 1000 °C, anatase converts to rutile. The temperature of this transformation strongly depends on impurities, ordopants,as well as the morphology of the sample.[10]
Synthetic anatase
[edit]Due to its potential application as asemiconductor,anatase is often prepared synthetically. Crystalline anatase can be prepared in laboratories by chemical methods such as thesol-gel process.This might be done through controlledhydrolysisoftitanium tetrachloride(TiCl4) ortitanium ethoxide.Often dopants are included in such synthesis processes to control the morphology, electronic structure, and surface chemistry of an anatase sample.[11]
Alternate and obsolete names
[edit]Another name commonly in use for anatase isoctahedrite,which is earlier than anatase and given because of the common (acute) octahedral habit of the crystals. Other names, now obsolete, areoisaniteanddauphinite,from the well-known French locality ofLe Bourg-d'OisansinDauphiné.[8]
See also
[edit]![](https://upload.wikimedia.org/wikipedia/commons/8/8d/Anatase-Quartz-221342.jpg)
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References
[edit]- ^Warr, L.N. (2021)."IMA–CNMNC approved mineral symbols".Mineralogical Magazine.85(3): 291–320.Bibcode:2021MinM...85..291W.doi:10.1180/mgm.2021.43.S2CID235729616.
- ^"Anatase"(PDF).Handbook of Mineralogy– via geo.arizona.edu.
- ^"Anatase".Mindat.org.
- ^"Anatase".Webmineral.com.Retrieved2009-06-06.
- ^Page 419 Deer, Howie and Zussman "An Introduction to the Rock Forming Minerals" ISBN 0 582 44210 9
- ^Wang, Rong; Hashimoto, Kazuhito; Fujishima, Akira; Chikuni, Makota; Kojima, Eiichi; Kitamura, Atsushi; Shimohigoshi, Mitsuhide; Watanabe, Toshiya (July 1997)."Light-induced amphiphilic surfaces".Nature.388(6641): 431–432.Bibcode:1997Natur.388..431W.doi:10.1038/41233.S2CID4417645.
- ^Hanaor, Dorian A. H.; Sorrell, Charles C. (2011)."Review of the anatase to rutile phase transformation".Journal of Materials Science.46(4): 855–874.Bibcode:2011JMatS..46..855H.doi:10.1007/s10853-010-5113-0.S2CID97190202.
- ^abcdpublic domain:Spencer, Leonard James(1911). "Anatase".InChisholm, Hugh(ed.).Encyclopædia Britannica.Vol. 1 (11th ed.). Cambridge University Press. pp. 919–920. One or more of the preceding sentences incorporates text from a publication now in the
- ^Assadi, MHN; Hanaor, DAH (2016)."The effects of copper doping on photocatalytic activity at (101) planes of anatase TiO 2: A theoretical study".Applied Surface Science.387:682–689.arXiv:1811.09157.Bibcode:2016ApSS..387..682A.doi:10.1016/j.apsusc.2016.06.178.S2CID99834042.
- ^Hanaor, Dorian A. H.; Sorrell, Charles C. (February 2011)."Review of the anatase to rutile phase transformation"(PDF).Journal of Materials Science.46(4): 855–874.Bibcode:2011JMatS..46..855H.doi:10.1007/s10853-010-5113-0.S2CID97190202.
- ^Jeantelot, Gabriel; Ould-Chikh, Samy; Sofack-Kreutzer, Julien; Abou-Hamad, Edy; Anjum, Dalaver H.; Lopatin, Sergei; Harb, Moussab; Cavallo, Luigi; Basset, Jean-Marie (2018)."Morphology control of anatase TiO2 for well-defined surface chemistry".Physical Chemistry Chemical Physics.20(21): 14362–14373.Bibcode:2018PCCP...2014362J.doi:10.1039/C8CP01983E.hdl:10754/627938.PMID29767182.