Bixbyite
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Bixbyite is a manganese iron oxide mineral with chemical formula: (Mn, Fe)₂O₃. The iron/manganese ratio is quite variable and many specimens have almost no iron. It is a metallic dark black with a Mohs hardness of 6.0 – 6.5. Wikipedia
Space group: Ia3
Unit cell: a = 9.411 Å; Z = 16
Cleavage: Imperfect on, in traces
Color: Black
Crystal class: Diploidal (m3); H-M symbol: (2/m 3)
Crystal habit: Massive to crystalline
Crystal system: Cubic