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brookite Mineral, TiO2; sp. gr. 4.1; hardness 5.5–6.0; orthorhombic; reddish-brown to brownish-black; white streak; metallic lustre; crystals normally platy and tabular; cleavage imperfect prismatic; occurs as an accessory mineral in igneous and metamorphic rocks, and in hydrothermal veins; Brookite, anatase, and rutile are all polymorphs of titanium dioxide. It is named after the British mineralogist H. J. Brooke.