Mn-rich norsethite from the Kremikovtsi ore deposit, Bulgaria

摘要

Mn-rich norsethite, Ba(Mg, Mn)(CO_3)_2, is described for the first time in Bulgaria, in the Kremikovtsi ore deposit, Sofia district. The mineral occurs as pale-yellow small clusters of platy crystals and spherulites grown upon fine crusts of spheroid sider-ite crystals in small cavities among granular hydrothermal siderite. Its refraction indices are: n_(omega) = 1.698 and n_(epsilon) = 1.527. Electron microprobe analysis reveals in the mineral (in wt. percent) BaO 52.8-53.3 (53.1 - average content), MgO 10.2-12.1 (11.3), MnO 3.4-5.7 (4.7) and sporadic quantities of FeO that corresponds well to the averaged formula Ba_(1.0)(Mg_(0.8)Mn_(0.2))_(1.0)(CO_3)_2. Powder X-ray diffraction study shows that the studied Mn-rich norsethite is trigonal, space group R3m with slightly larger unit cell parameters than those of the norsethite - a = 5.0318(11) A, c = 16.8705(15) A, V = 369.01(3) A~3, z = 3. The strongest observed reflections in the powder diffraction pattern are (d in A, I observed): 5.629, 38; 3.871, 28; 3.026, 100; 2.109, 25; 1.897, 29; 1.875, 30. The principle absorption bands related to [CO_3]~(2-) groups in the IR spectrum are v_1 - 1114 cm~(-1), v_2 - 880 cm~(-1), v_3 - 1452 cm~(-1), and v_4 -696 cm~(-1). TheDTA-DTG-TG analyses display endothermic effects in 556-776 deg C, 820-940 deg C and 930-1100 deg C temperature regions The observed mineral association and textures suggest interaction of an earlier formed siderite and barite aggregates with hydrothermal solutions and deposition of Mn-rich norsethite and Ca-Mn-Fe bearing norsethite varieties in voids within the hydrothermal siderite.