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 [Dunn] 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 (Mn0.50Zn0.44Mg0.06)(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. (Dunn, 1995)

 Location Found: Franklin
 Year Discovered: 1869
 Formula: Mn2+Fe23+O4
 Essential Elements: Iron, Manganese, Oxygen
 All Elements in Formula: Iron, Manganese, Oxygen
 IMA Status: Approved
 To find out more about this mineral at minDat's website, follow this link   Jacobsite

Dunn, Pete J. (1995). Franklin and Sterling Hill New Jersey: the world's most magnificent mineral deposits. Franklin, NJ.: The Franklin-Ogdensburg Mineralogical Society. p.581

The Picking Table References
 PT Issue and PageDescription / Comment
View IssueV. 32, No. 1 - Spring 1991, pg. 15Mineral Notes, Research Reports, Manganese-Rich Ore Assemblages, Jacobsite (small description)
View IssueV. 12, No. 2 - August 1971, pg. 7New Minerals for Franklin-Sterling Validated List - Jacobsite (small article)
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