Jacobsite

Jacobsite
General
Category	Oxide minerals Spinel group Spinel structural group
Formula (repeating unit)	iron(II,III) manganese oxide, (Mn,Mg)Fe2O4
IMA symbol	Jcb
Strunz classification	4.BB.05
Crystal system	Isometric
Crystal class	Hexoctahedral (m3m) H-M symbol: (4/m 3 2/m)
Space group	Fd3m (no. 227)
Unit cell	a = 8.457 Å; Z = 8
Identification
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

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 (Mn,Mg)Fe₂O₄ or with oxidation states and substitutions: (Mn²⁺,Fe²⁺,Mg)(Fe³⁺,Mn³⁺)₂O₄.

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

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 is a ferrimagnetic substance, which is weakly attracted by a magnet.

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

References

• National Pollutant Inventory - Manganese and compounds Fact Sheet