Jacobsite

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.^[2][4]

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

Jacobsite
General
Category	Oxide minerals Spinel group Spinel structural group
Formula	iron(II,III) manganese oxide, (Mn,Mg)Fe2O4
IMA symbol	Jcb[1]
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	[2][3]

It occurs as a primary phase or as alteration of other manganese minerals during metamorphism of manganese deposits.^[4] Typical associated minerals include hausmannite, galaxite, braunite, pyrolusite, coronadite, hematite and magnetite.^[2] 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.^[3]