Mineral Information and Data:

Jacobsite
Mn2+Fe3+2O4 [CNMNC approved formula]

Named for its original occurrence at Jakobsberg (formerly Jacobsberg), Sweden. First published in 1869. Defined and formalized (IMA, 1980).

IMA status : Approved Species 1980 ()
CLASSIFICATION 
Dana
(8th edition) :
07.02.02.02  
  (07) Multiple Oxides
  (07.02) (A+ B++)2 X4 Spinel group
  (07.02.02) (Iron subgroup)
  
Nickel-Strunz
(10th edition) :
04.BB.05
  (04) Oxides and Hydroxides
  (04.B) Metal:Oxygen = 3:4 and similar
  (04.BB) With only medium-sized cations
  
Crystal system: Cubic System
Point group (H-M): m3m (or 4/m 3 2/m) — hexoctahedral
Space group: Fd3m
Unit cell: a = 8.499Ã…, Z=8
Crystal Habit: Rarely in octahedral crystals, to 4 mm, which may exhibit exsolved hausmannite or galaxite; coarse to fine granular, massive.
Twinning: On {111} as both twin and composition plane, the spinel law, flattened on {111} or lamellar
  
Color: Black to brownish black; grayish white with olive tint in reflected light, with brown internal reflections
Diaphaneity: Opaque, translucent on thin edges
Luster: Metallic, splendent to semimetallic, dull.
Hardness (Mohs): 5.5 - 6.5
Measured Density: 4.76 g/cm3
Cleavage: {111}, probably a parting
Streak: Brown
  
Polymorphism and Series: Dimorphous with iwakiite; forms a series with magnetite.
Geologic Setting: A primary mineral or an alteration product of other manganese-bearing minerals in some metamorphosed manganese deposits.
Mineral Association: Hausmannite, galaxite, braunite, pyrolusite, coronadite, hematite, magnetite
  

 

Jacobsite
Jacobsite on calcite
Origin: N'Chwaning II Mine, Kalahari Manganese Fields, South Africa
Sample size: 2.2 x 2 x 1 cm
Photo: Rob Lavinsky

 

Jacobsite
(zoom-in of above)

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