高知県室戸市菜生海岸産コバルト・ニッケル鉱物 <Bull. Natn. Sci. Mus.,>Cobalt and Nickel Minerals from Nabae Coast, Muroto City, Kochi Prefecture, Japan

Cobaltite-gersdorffite solid solutions were found as microscopical aggregates composed of very minute grains in dark grey mudstone exposed at Nabae Coast in the western side of Muroto Peninsula, Muroto City, Kochi Prefecture. It is directly accompanied by bitumen layers involved in mudstone and sandstone of early Miocene Tsuro Formation. They are associated with minor millerite, galena, sphalerite, and a tennantite-tetrahedrite series mineral. Also, tiny smear of molybdenite is found nearby the layers. The constituents of mudstone are quartz, potash feldspar, albite and illite with abundant radiolarian remains. Bitumen and mudstone are cut by veinlets of manganoan ferroan dolomite. The solid solutions have ranging Co : Ni ratios from 7 : 3 to 1 : 9. In analyses with Co>Ni, As is nearly equal to S in mole ratio, whereas those with Ni>Co, As is slightly higher than S. X-ray powder study on the dark grey mudstone indicated that the detected sulphide constituents are the solid solutions, millerite and sphalerite. The diffractions due to the solid solutions are indexed by an apparently cubic cell with α=5.605 A including (100), suggesting the real symmetry to be orthorhombic. The α-dimension indicates Co : Ni to be about 4 : 1,suggesting the extention of the compositional range of orthorhombic cobaltite. Cobalt and nickel sulphides were precipitated around radiolarian remains or siderite chlorite-aggregate probably replacing foraminifera. Closer situation of the dark grey mudstone involving cobaltite-gersdorffite solid solutions and millerite to bitumen suggests the origin of heavy metals to be bitumen, which contained them probably in an absorbed state, and released them during the process of diagenesis.