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a mineral, one of the three natural polymorphous modifications of titanium dioxide. The mineral crystallizes in a tetragonal system. Its hardness on the mineralogical scale is 5.5–6.0; its density is 3,820–3, 950 kg per cu m. Anatase contains impurities of iron, sometimes tin, and niobium (up to 2.16 percent Nb2O5). It forms small (up to 1 cm), sharp dipyramidal crystals, as well as crystals of plate and prismatic form. Its cleavage is good and in several directions. The color of anatase is greenish yellow to brownish (wiserine) or bluish gray to almost black. It has a bright, diamondlike luster. Upon heating to 620–650°C, anatase changes into rutile. Anatase is found in hydrothermal, crystal-bearing veins of a particular “alpine” type, in metamorphic rock, in placers, and as a product of ilmenite breakdown.