18 Photos
Jacobsite
Chemical Formula	MnFe2O4
Species	Oxides & Hydroxides
Crystal System	Isometric
Mohs Scale	5-6
Specific Gravity	5.03
Color	Black to brownish black
Streak	reddish black to brown
Luster	Sub-Metallic
Refractive Index	~?2.3
Diaphaneity	Opaque
Cleavage	, probably a parting
Fracture	Conchoidal
Crystal Habit：Disseminated to massive, rarely as octahedral crystals

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 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 of manganese deposits. Typical associated minerals include hausmannite, galaxite, braunite, pyrolusite, coronadite, hematite and magnetite.

It was first described in 1869 and named for the Jakobsberg Mine, Nordmark, Filipstad, V?rmland, Sweden.