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a mineral of composition TiO2. Sometimes contains admixtures of iron and niobium. Layered structure, consisting of octahedral groups of TiO6, united by common ribs. Crystallizes in a rhombic system. The crystals are usually tabular, less frequently prismatic and dipyramidal. Their color is yellowish brown to reddish brown; their luster is adamantine or metallic. The hardness of brookite on the mineralogical scale is 5.5-6; density, 3,900-4,200 kg/m3. The crystals are found in gold placers and in alpine-type veins in the Urals (USSR); abroad the mineral is found in Switzerland, France, Austria, and the USA.