JACOBSITE

Mn²⁺Fe³⁺₂O₄ 
Cubic, Fd3m, a = 8.4867 Å, Z = 8

Jacobsite, a manganese ferric-iron oxide mineral of the spinel group, has been reported only rarely, but may have been overlooked; it might be more abundant than is presently known. Material of jacobsite composition (Mn²⁺ > Zn or Fe²⁺; Fe³⁺ > Mn³⁺) was first reported from Franklin by Frondel and Klein (1965) and subsequently by McSween (1976). This jacobsite is black, opaque, and has metallic luster. It occurs intimately associated with exsolved hetaerolite in the manganosite and zincite assemblage which occurs as a mutual exsolution at Franklin. Other associated minerals are hausmannite, leucophoenicite, tephroite, and sonolite. Adopting the cation site-assignments given by Shirakashi and Kubo (1979) for franklinite and the crystallochemical constraints of the spinel structure, the material from the this assemblage is properly termed a zincian and manganian jacobsite, containing approximately 60 mole % of the jacobsite member. 

Jacobsite was also noted by the writer as an exsolution along parting planes in zincite. Most of such material, forming thin films, is hetaerolite, but at least one specimen, physically identical to those described under hetaerolite, has jacobsite of composition (Mn_0.50Zn_0.44Mg_0.06)(Fe³⁺_1.89Mn³⁺_0.11)O₄, as proven by microprobe analysis and X-ray diffraction. Jacobsite has not been reported from Sterling Hill, but may occur there.

Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates
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