Jacobsite Locality: Wessels Mine (Wessel's Mine), Hotazel, Kalahari manganese fields, Northern Cape Province , South Africa ( Locality at mindat.org ) Size: miniature, 5 x 4 x 3.25 cm JACOBSITE with crystals to 6mm BRIGHT crystals that are so sharp and lustorus you would think them to be magnetite...for quality, amazing for the species. Additionally, for both isolation and aesthetics, this is a superior miniature display specimen of the species. Taken together, Charlie felt this was the finest example he had seen in his decades in South African mineral dealing. — Photo: Robert M. Lavinsky · CC BY-SA 3.0
Jacobsite
A manganese-iron spinel forming a solid solution with magnetite.
Intro adapted from Wikipedia (CC BY-SA).
Jacobsite is an IMA-recognized mineral species in the oxides & hydroxide class, with the chemical formula Mn²⁺Fe³⁺₂O₄.
It crystallizes in the cubic crystal system (space group Fd3m). It is a member of the Spinel structural group.
Key properties: Mohs hardness 5.5–6.5; specific gravity 4.76; Brown streak; Metallic, splendent to semimetallic, dull luster.
IMA symbol Jcb, IMA status: approved, named in 1869.
A primary mineral or an alteration product of other manganese-bearing minerals in some metamorphosed manganese deposits.
It is found associated with Hausmannite, galaxite, braunite, pyrolusite, coronadite, hematite, magnetite.
The name refers to its original occurrence at Jakobsberg (formerly Jacobsberg), Sweden.