Jacobsite is a characteristic minor mineral of manganese ores at the Ushkatyn-III deposit in Central Kazakhstan. In the bulk mass of ores, it associates with hausmannite, tephroite, sonolite, friedelite, pennantite, calcite, and some other minerals. A similar assemblage, but without hausmannite, is characteristic of jacobsite from veinlets intersecting ores. The chemical composition of jacobsite varies from Mn1.5Fe1.5O4 to MnFe2O4. The mineral with a high manganese content (>1 atoms per formula unit (apfu)) dominates in the bulk mass of ores, while the veins are dominated by stoichiometric jacobsite (Mn ∼1 apfu). The mineral composition of the ores formed in the process of low-grade metamorphism of the metal-bearing hydrothermal sedimentary deposits. The formation of jacobsite in the metamorphosed metal-bearing deposits occurs under the reducing conditions. With an increase in oxygen fugacity (oxidation potential), jacobsite is replaced by the assemblage of hausmannite with hematite. Jacobsite crystallizes in the veinlets intersecting the hausmannite ores due to the entry of iron-containing hydrothermal solutions into them. It is assumed that jacobsite veinlets serve as mineralogical markers of seepage paths in ore-bearing hydrothermal structures (feeding channels) producing manganese-rich ores.
Brusnitsyn et al. (Mon,) studied this question.