Anatase

Anatase
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 scale hardness	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]

Anatase is a metastable mineral form of titanium dioxide (TiO₂) with a tetragonal crystal structure. Although colorless or white when pure, anatase in nature is usually a black solid due to impurities. Three other polymorphs (or mineral forms) of titanium dioxide are known to occur naturally: brookite, akaogiite, and rutile, with rutile being the most common and most stable of the bunch. Anatase is formed at relatively low temperatures and found in minor concentrations in igneous and metamorphic rocks.^[5] Glass coated with a thin film of TiO₂ shows antifogging and self-cleaning properties under ultraviolet radiation.^[6]

Anatase is always found as small, isolated, and sharply developed crystals, and like rutile, it crystallizes in a 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.^[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 common octahedral crystal habit of anatase, with four perfect 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) and less dense (specific gravity about 3.9 vs. 4.2) than rutile. Anatase is also optically negative, whereas rutile is optically positive. Anatase has a more strongly adamantine or metallic-adamantine luster than that of rutile as well.^[8]