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<報文>東北地方産ケイ酸マンガン鉱の鉱物学的研究(第28報) : 岩手県野川玉川鉱出産含マンガンウィンチ閃石について

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  • <Original Papers>Mineralogical Study of Manganese Silicate Ores in Northeastern Japan. (XXVIII) : Manganese Winchite from Noda-Tamagawa Mine. Iwate Prefecture

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Green-colored amphibole occurs as banded aggregates of short prismatic crystals up to 6×4 mm in size, at the eleventh level of Misago bedded manganese ore deposits of the Noda-Tamagawa mine, in highly metamorphosed Jurassic meta-chert by the intrusion of the Ohtanabu adamellite. Associated minerals are rhodonite, rhodochrosite, plagioclase and quartz. The mineral is dark yellowish-green to dark green in color with vitreous luster. In the thin section, it is weak pleochroic with X=pale greenish-yellow, Y=pale green and Z=pale yellowish-green. The optical properties of the two varieties are α=1.646,1.645,β=1.655,1.654,γ=1.659,1.659 and 2Vα=65°, 65°, respectively. Unit cell dimensions of these two are a_0=9.84,9.83,b_0=18.10,18.09,c_0=5.29,5.30A, β=103.40°, 103.50° and V=916.5,916.4A^3,respectively. On the basis of 24(O, OH, F), the chemical analyses yield the following empirical formulae : (Na_<0.16>K_<0.14>)_<0.30>(Na_<1.18>Ca_<0.22>Mn_<0.60>)_<2.00>(Mg_<2.82>Mn_<1.31>Fe^<2+>_<0.07>Fe^<3+>_<0.52>Al_<0.25>Ti_<0.03>)_<5.00>(Si_<7.96>Al_<0.04>)_<8.00> O_<21.88>(OH)_<2.12> for the dark yellowish-green variety and (Na_<0.27>K_<0.14>)_<0.41>(Na_<1.24>Ca<0.27>Mn<0.49>)_<2.00>(Mg_<2.73>Mn_<1.34>Fe^<2+>_<0.24>Fe^<3+>_<0.40>Al_<0.23>Ti_<0.04>)_<5.00>(Si_<7.96>Al_<0.04>)_<8.00>O_<21.91>(OH_<2.01>F_<0.08>)_<2.09> for the dark green variety. According to the nomenclature approved by the IMA Commission on New Minerals and Mineral Names, winchite is defined as sodic-calcic amphibole in which (Na+K)_A<0.50,(Ca+Na)_B≤1.34,Na_B between 0.67 and 1.34,Si in standard cell between 7.50 and 8.00,and Mg/Mg+Fe^<2+> between 0.50 and 1.00. The composition of two green-colored amphiboles from Noda-Tamagawa mine satisfied the definition of winchite. Refractive indices and unit cell parameters of all six winchites known in the world show an systematic variation with chemical composition. The results of the X-ray study and thermal analysis indicate that the mineral disintegrates into a mixture of monoclinic pyroxene, a jacobsite and roedderite at about 1,075℃.

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