Formula: Mn2+Fe3+2O4 multiple oxide, spinel group
Jacobsite forms a series with magnetite
Specific gravity: 4.76
Hardness: 5½ to 6½
Streak: Reddish black or brownish black
Colour: Black, gray in reflected light
Epitaxy: Jacobsite forms oriented intergrowths with hausmannite.
Jacobsite is a primary mineral or a
secondary alteration product of other manganese-bearing minerals in some
metamorphosed manganese deposits, associated with hausmannite,
hematite or magnetite
In Sweden jacobite occurs in crystalline limestone with native copper at the Jakobsberg mine, and with tephroite and calcite at Långban (Dana). The Hutter Mine locality, Pittsylvania County, Virginia, is a metamorphosed magnetite deposit, with subsidiary manganoan marble, that occurs in schist. Manganese oxides and spinel group minerals at the Hutter Mine include hausmannite, magnetite, galaxite and jacobsite. It seems that the maximum temperature developed here was 550 to 600oC (AM87.690).
Common impurities: Zn
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