Formula: (Mn,Fe,Mg)(Fe,Mn)2O4
Origin of name: After locality; Jakobsberg mine, Varmland, Sweden
Crystal system: Cubic
Crystal class: 4/m -3 2/m
Twinning:
Unit cell:
Colour: Black
Diaphaneity: Opaque
Luster: Metallic to dull
Habit: Crystals octahedral, distorted; massive, coarse or fine granular
Hardness: 5.5 to 6.5
Specific gravity: 4.76
Cleavage: None
Tenacity:
Streak: Brown
Synonyms/varieties:
Comments: Series with magnetite; dimorphous with iwakiite; magnetic
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