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anatase (octahedrite) Mineral, TiO2; sp. gr. 3.9, hardness 5.5–6.0; tetragonal; yellow, or shades of brown, blue, or black; white streak; vitreous lustre; crystals normally bipyramidal octahedra, but sometimes tabular; cleavage perfect, basal, and pyramidal; occurs as an accessory mineral in metamorphic and igneous rocks, also in hydrothermal veins and granite pegmatites.