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Category Oxide minerals
Spinel group
Spinel structural group
(repeating unit)
iron(II,III) manganese oxide, MnFe2O4
Strunz classification 4.BB.05
Crystal system Isometric
Crystal class Hexoctahedral (m3m)
H-M symbol: (4/m 3 2/m)
Space group Fd3m
Unit cell a = 8.457 Å; Z = 8
Color Black to brownish black
Crystal habit Disseminated to massive, rarely as octahedral crystals
Twinning Spinel law, flattened on {111} or lamellar
Cleavage {111}, probably a parting
Fracture Conchoidal
Mohs scale hardness 5.5 - 6.5
Luster Metallic
Streak reddish black to brown
Diaphaneity Opaque
Specific gravity 4.76
Optical properties Isotropic
Refractive index ~�2.3
Other characteristics weakly magnetic
References [1][2]

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.[1][3]

It occurs as a primary phase or as alteration of other manganese minerals during metamorphism of manganese deposits.[3] Typical associated minerals include hausmannite, galaxite, braunite, pyrolusite, coronadite, hematite and magnetite.[1]

It was first described in 1869 and named for the Jakobsberg Mine, Nordmark, Filipstad, Värmland, Sweden.[2]


Jacobsite, N'Chwaning Mines, Kuruman, Kalahari manganese fields, Northern Cape Province, South Africa. Size 3.8 x 3.5 x 3.2 cm