jacobsite

{{Infobox mineral | name = Jacobsite | category = Oxide minerals Spinel group Spinel structural group | boxwidth = | image = Jacobsite-cktsr-15a.jpg | imagesize = 260px | caption = | formula = iron(II,III) manganese oxide, (Mn,Mg)Fe2O4 | IMAsymbol = Jcb | molweight = | strunz = 4.BB.05 | system = Isometric | class = Hexoctahedral (mm) H-M symbol: (4/m 2/m) | symmetry = Fdm (no. 227) | unit cell = a = 8.457 Å; Z = 8 | color = Black to brownish black | habit = Disseminated to massive, rarely as octahedral crystals | twinning = Spinel law, flattened on {111} or lamellar | cleavage = {111}, probably a parting | fracture = Conchoidal | mohs = 5.5–6.5 | luster = Metallic | refractive = ~2.3 | opticalprop = Isotropic | birefringence = | pleochroism = | streak = reddish black to brown | gravity = 4.76 | density = | melt = | fusibility = | diagnostic = | solubility = | diaphaneity = Opaque | other = 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. thumb|left|Jacobsite, N'Chwaning Mines, Kuruman, Kalahari manganese fields, [[Northern Cape Province, South Africa. Size 3.8 x 3.5 x 3.2 cm]]