<Original Papers>Mineralogical Study of the Manganese Silicate Ores in Northeastern Japan. (I) : Knebelite from Hakozaki Mine, Iwate Prefecture

The bedded manganese ore deposits at Hakozaki mine, Iwate Prefecture, occur in the clay-slate of Kamaishi formation of Permian age. The manganese ore bodies and the rocks of Kamaishi formation are thermally metamorphosed, by the Miyako granitic mass. Main metamorphic rock is cordierite-biotite-hornfels which contains cordierite, biotite, green hornblende, plagioclase and quartz, and the ore minerals have been altered to the manganese silicates such as rhodonite, tephroite and knebelite. In this report, knebelite was examined by optical micrograph. X-ray diffraction, chemical and thermal analyses. Its physical and optical properties are : greenish black in colour, specific gravity 4.04 (obs.) and 4.30 (calc.), Vickers microhardness 514,optical axial angle 2Vα=50°, and the indices of refraction are α=1.796,β=1.837,γ=1.846,γ-α=0.050. It is orthorhombic, with a_0=4.842,b_0=10.561,C_0=6.127 Å and cell volume 311.80 Å^3. From chemical analysis showed in table 3,the formula is given as (Fe^_, Mn_, Fe^_, Mg_, Ca_, Ti_)_ (Si_, Al_)_ O_, and the molecular percentages of endmembers are calculated as Fe_2SiO_4 67.9,Mn_2SiO_4 30.5,Mg_2SiO_4 1.5,Ca_2SiO_4 0.1 mol.%. The differential thermal analysis curve shows distinguished exothermic reaction at 880℃. The result of thermal transformation experiment is as follows : the pattern of mineral heated at 770℃ was mainly that of knebelite, and the lines of knebelite were even weaker and some of them disappeared. At 1,050℃, all of diffraction lines of knebelite disappeared, and the diffraction lines corresponding to braunite, jacobsite and hematite were observed