高知市鴻ノ森産 desautelsite

概要

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Desautelsite from Konomori, Kochi City, occurs as the last fissure mineral in a fractured serpentinite, forming somewhat flocculent crust generating from the underlying manganese dioxide film. The wet chemical analysis gave MgO 36.0,NiO 0.3,Mn_2O_3 21.0,Fe_2O_3 0.1,CO_2 8.5,H_2O 33.6,total 99.5%, corresponding to (Mg_<6.14>Ni_<0.03>)_<Σ6.17>(Mn^<3+>_<1.83>Fe^<3+>_<0.01>)_<Σ1.84>(CO_3)_<1.32>_(OH)_<15.22>_(H_2O)_<1.46>・3.75H_2O on the basis of total cation=8 and total anionic group (including (H_2O))=18,reasonably close to the ideal formula Mg_6Mn^<3+>_2CO_3(OH)_<16>・4H_2O, with a slight excess of CO^<2->_3,uncommonly known in the minerals of pyroaurite series. It is trigonal, R3m of R3^^-m, a 3.110(1), c 23.488(9)A, Z=3/8. It is bright corange in fresh colour with the same streak and vitreous to earthy luster. Cleavage basal, perfect. Optically it is uniaxial and negative with nE=1.544(2) and nO=1.568(2). Dichroism is strong with axial colours E=orange yellow and O=deep orange. The mineral was formed after a reaction of magnesium and carbonate-bearing meteoritic water with a filmy amorphous manganese dioxide covering fissure walls in a fractured serpentinite. The acid treatment on some materials labelled as pyroaurite disclosd the additional occurrence of desautelsite at five localities in Japan, indicating its commoner occurrence.
国立科学博物館の論文
1984-09-22