Jacobsite is a manganese oxide mineral group and forms a solid solution. The chemical formula is MnFe2O4 or with oxidation states and substitutions: (Mn2+,Fe2+,Mg)(Fe3+,Mn3+)2O4.
It occurs as a primary phase or as alteration of other manganese minerals during metamorphism Typical associated minerals include hausmannite, pyrolusite and magnetite. It is a ferrimagnetic substance, which is weakly attracted by a magnet.
It was first described in 1869 and named for the Jakobsberg Mine, Nordmark, Sweden