Anatase is a metastable form of titanium dioxide (TiO2). The mineral in natural forms is mostly encountered as a black solid, although the pure material is colorless or white. Two other naturally occurring mineral forms of TiO2 are known, brookite.
Anatase is always found as small, isolated and sharply developed crystals, and like the thermodynamically stable rutile (the more commonly occurring polymorph of titanium dioxide), it crystallizes in the 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. 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 pyramid of anatase, parallel to the faces of which there are perfect cleavages, has an angle over the polar edge of 82°9', the corresponding angle of rutile being 56°52½'. Due to this steeper pyramid, in 1801 René Just Haüy named the mineral anatase — from the Greek anatasis ("extension"), the vertical axis of the crystals being longer than in rutile. Additional important differences exist between the physical characters of anatase and rutile: the former is less hard (5.5–6 vs. 6–6.5 Mohs) and dense (specific gravity about 3.9 vs. 4.2); anatase is optically negative whereas rutile is positive; and its luster is even more strongly adamantine or metallic-adamantine than that of rutile.