<Original Papers>Mineralogical Study of the Manganese Silicate Ores in Northeastern Japan. (X) : Manganiferous Magnesioarfvedsonite from Noda-Tamagawa Mine, Iwate Prefecture

Manganiferous magnesioarfvedsonite occurs as a single crystal up to about 20×10×10mm in size, and lenticular aggregates of these crystals in rhodonite essential mass at the L8-level of Misago bedded manganese ore deposits of Noda-Tamagawa Mine, in the highly metamorphosed Jurassic quartzite by the intrusion of Tanohata granodiorite. The color is dark brown with a red tint and pale yellow in a thin section. The specific gravity is 3.16 and hardness 5. The indices of refraction are α=1.645,β=1.657,γ=1.659 and γ-α=0.014. Optical axial angle (2Vα) is 45°, and extinction angle (c∧X) is 24°. Weak pleochroic with X pale lemon yellow and Y≒Z lemon yellow. X-ray powder data shows it to be isostructual with the alkali amphibole group, i.e., it is monoclinic with α_0=9.97Å, b_0=18.16Å, c_0=5.26Å and β=103.4°. The Chemical analysis gives the formula of (Na_, K_, Ca_)_ [(Mg_, Mn_, Fe^_)_ (Fe^_, Ti_)_]_ (Si_, Al_, Ti_)_O_(OH)_ The high-temperature X-ray study and differential thermal analysis indicate that the mineral breaks down to a mixture of monoclinic pyroxene, Na_2Mg_5Si_O_, jacobsite and glass when heated at about 1,150℃