Jacobsite from Manganese Deposits of the Kiuragi Mine, Saga Prefecture

Abstract

Jacobsite, with chemical composition MnFe₂O₄, belonging to magnetite series of spinel group, has been found in the manganese deposits from the Kiuragi Mine, about 15 kilometers west of Saga-City, Kyushu, Japan. This mineral is fine-grained and in tephroite ores associated with rhodonite, spessartine, rhodochrosite and a small amount of penwithite. It shows the following characters;
Colour is magnetite-like iron black with metallic luster. Streak, blackish brown. Strongly magnetic. Hardness, 6± In thin section this mineral is opaque, usually associated with galaxite which shows close similarity in crystal structure. In the polished section it is greyish white with olive tint. Standard etch reactions are negative except that of HCl(conc.)+SnCL₂.
Its X-ray powder pattern is in very close agreement with those of jacobsite from Jacobsberg and artificial MnFe₂O₄. Analysed specimens contain a small amount of galaxite (5.46% wt.). Analytical results are as follows; SiO₂ tr., Fe₂O₃ 56.19, MnO 29.34, MgO 0.10, CaO 0.13, TiO₂ 3.37, AL₂O₃ 11.62, BaO none, total 100.75. After deducing MnAL₂O₄ for galaxite as impurities, the chemical formula can be written as (Mn, Fe^II, Ca, Mg)_0.990 (Fe^III, Al, Ti)_1.950O_4.000. A precise determination of the lattice dimension by the extrapolation-method gave 8.452±0.004A.