Anatase

Anatase
General
Category	Oxide minerals
Formula (repeating unit)	TiO2
IMA symbol	Ant
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

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.Glass coated with a thin film of TiO₂showsantifoggingandself-cleaningproperties underultravioletradiation.

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.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.

Crystal habit

Extended portion of the anatase lattice.

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.The (101) plane of anatase is the most thermodynamically stable surface and thus the most widely exposedfacetin natural and synthetic anatase.

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.

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.

Synthetic anatase

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.

Alternate and obsolete names

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.

See also

Black anatase crystals on smoked quartz

• Delustrant
• Titanium dioxide
• Rutile
• Brookite
• Akaogiite
• List of minerals