Gageite, one of main constituent minerals of "chocolate ore" in the bedded manganese deposits in Japan

Abstract

Dosulite which was named after the Dosu mine by YOSHIMURA (1967) is considered to be a major consti-tuent mineral of "chocolate ore" in the bedded manganese deposits in Japan. However, it is identified as a mixture of gageite and the associated hausmannite, rhodochrosite and Mn-humites based on electron microprobe and X-ray powder diffraction analyses in this study. The Dosu gageite has a composition in good agreement with the ideal chemical formula proposed by FERRARIS et al. (1987) and shows a similar X-ray diffraction pattern to that of MOORE (1968).The gageite is also associated with cymrite and is assumed to be more stable phase compared with Mn-humites such as alleghanyite and sonolite under high PH2O/T ratio conditions. Seven occurrences of gageite have been found in Japan including another two from Nagatani and Manako mines in this study and four by MATSUBARA and KATO (1979) and almost all these are located in the northern Chichibu Belt where the sediments containing the manganese deposits were affected by low temperature metamorphism of Mesozoic age. Hence, it is assumed that primary gageite which ac-cepted more high temperature metamorphic event was decomposed to common assemblages containing Mn-humites and so on.