Abstract
Microprobe analysis of ribbeite from the Kaso mine give SiO2 24.97, MnO 72.01, MgO 0.76, CaO 0.09, total 97.83%, yielding the anhydrous empirical formula: (Mn4.89Mg0.09Ca0.01)Σ4.99Si2O8.98 (basis: Si=2), closer to the theoretical formula than the original magnesian material. The X-ray powder pattern is indexed on an orthorhombic cell with a=10.768Å, b=15.800Å, c=4.832Å. It occurs as light reddish to purplish brown masses included in light grey tephroite layers accompanied by finely bedded rhodonite-quartz ore from Mukaiyama orebody of the Kaso mine. The ore retains the original fine bedded texture despite thermal metamorphism due to nearby or underlying acidic intrusive bodies. The other associates include sonolite, rhodochrosite and hausmannite, the last being apparently absent in Honkô orebody of the mine, where any primary Mn3+-bearing minerals has not been found to date. This is probably due to the difference in the original constituents of ores therefrom.
