<Original Papers>Mineralogical Study of Manganese Silicate Ores in Northeastern Japan. (IV) : Boron-Manganiferous Eckermannite from Noda-Tamagawa Mine, Iwate Prefecture
Boron-manganiferous eckermannite occurs as prismatic crystals up to 1.5x 1.2x 1.0cm in size in manganiferous aegirine-rhodonite-alkali feldspar quartz bearing pegmatitic veinlets, cutting through the Misago bedded manganese silicate ore bodies of Noda-Tamagawa mine of older Cretaceous age. The color is brownish dark green in hand-specimen, the specific gravity is 3.03 and the hardness is 4(1/2). In a thin section the pleochroism weak : X=pale yellow, Y=Z=yellow. The indices of refraction are α=1.640 β=1.653 γ=1.658 γ-α=0.018,extinction angle is c∧Z=39° and optic axial angle is 2Vα=70°, X-ray powder data showed it to be isostructual with the alkali amphibole group, i.e., it is monoclinic with a_0=9.88,b_0=18.05,c_0=5.32Å, β=104.0° and Z=2. The chemical analysis gives the formula of (Na_, K_, Ca_)_ (Mg_, Mn_, Fe^_, Fe^_, Al_, Ti_)_ (Si_, B_, Al_)_ O_ (OH_, F_)_, and the Si^ atoms are replaced partly byB^ atoms. The high-temperature X-ray diffraction and DTA indicate that the mineral breaks down to a mixture of clinopyroxene, jacobsite and glass when heated at about 1,100℃
