Mn-Fe-Mg ORTHORHOMBIC PYROXENE FROM GONDITE FORMATION OF PRIOLKHONYE (THE WEST BAIKAL REGION)

摘要

Unusual Mn-Fe-Mg orthopyroxene has been found in manganiferous quartzite of the Olkhon metamorphic complex, Lake Baikal, Russia. Main associated minerals are pyroxmangite, Mo-diopside, spessartite and quartz varying proportions. The mineral occurs as small grains (30-40 mkm), sometimes as skeletal crystals (0.3 X 0.5 mm). It is yellowish with an apporoximate hardness of 5-6 and perfect (110) cleavages. Electron microprobe analyses show that the pyroxene consists of three main components: MnSiO_3, FeSiO_3 and MgSiO_3, with manganoan end member prevailing. The averaged chemical formula of the grains studied (mean of 29 analyses) is (Mn_(0.67)Fe_(0.66)Mg_(0.61)Ca_(0.06)) (SIGMA 2.00)Si_(2.00)O_6. The structural formula might be expected as (Mn_(0.67)Fe_(0.27)Ca_(0.06))_(SIGMA 1.00) (Mg_(0.61)Fe_(0.39))_(SIGMA 1.00)Si_(2.00)O_6. X-ray studies indicate that the pyroxene is orthorhombic with space group Pbca and a = 18.47(1), b = 8.97(2), c = 5.243(2) A. It has a cell volume of 868.8(2.7) A~3 with Z = 8. The strongest eight lines of the X-ray powder diffraction pattern are [d in A (I) (hkl)] 3.19 (10) (221), 2.99(8) (321), 2.90(7) (610), 2.57(7) (131), 2.50(10) (231,521), 2.12(9) (322,531), 1.624(8) (10.2.1), 1.499(10) (832). Small size of crystals has not allowed a refinement of structure to be made.