| Jacobsite | |
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| General | |
| Category | Oxide minerals Spinel group Spinel structural group |
| Formula (repeating unit) | 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] |
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]
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]
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