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braunite Oxide mineral Mn2O3, but with about 10% by weight of silica, so the mineral may be considered to have the composition 3Mn2O3.MnSiO3; sp. gr. 4.7–4.8; hardness 6.0–6.5; tetragonal; brownish-black to steel-grey; brown-black streak; sub-metallic lustre; crystals pyramidal, can also occur massive or granular; occurs in hydrothermal veins together with other manganese oxides, as a secondary mineral, and as a result of the metamorphism of manganese-bearing sediments. It is very similar to other manganese minerals, but is soluble in hydrochloric acid, when it leaves a residue of silica. It is an ore of manganese and is used in steel-making.