Jacobsite
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Jacobsite is a manganese iron oxide mineral. It is in the spinel group and forms a solid solution series with magnetite. The chemical formula is Fe₂O₄ or with oxidation states and substitutions:
(Fe³⁺, Mn³⁺)₂O₄. Wikipedia
Crystal class: Hexoctahedral (m3m); H-M symbol: (4/m 3 2/m)
Crystal habit: Disseminated to massive, rarely as octahedral crystals
Crystal system: Isometric
Mohs scale hardness: 5.5–6.5
Optical properties: Isotropic