IngentaConnect Mn-rich norsethite from the Kremikovtsi ore deposit, Bulgaria

Authors: Zidarov, Nikola; Petrov, Ognyan; Tarassov, Mihail; Damyanov, Zhelyazko; Tarassova, Eugenia; Petkova, Vilma; Kalvachev, Yuri; Zlatev, Zlati

Source: Neues Jahrbuch für Mineralogie - Abhandlungen, Volume 186, Number 3, October 2009 , pp. 321-331(11)

Publisher: E. Schweizerbart'sche Verlagsbuchhandlung

Abstract:

Mn-rich norsethite, Ba(Mg, Mn)(CO₃)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 siderite crystals in small cavities among granular hydrothermal siderite. Its refraction indices are: n_ω = 1.698 and n_ε = 1.527. Electron microprobe analysis reveals in the mineral (in wt. %) 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_0.8Mn_0.2)_1.0(CO₃)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) Å, c = 16.8705(15) Å, V = 369.01(3) Å³, z = 3. The strongest observed reflections in the powder diffraction pattern are (d in Å, 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 [CO₃]^2- groups in the IR spectrum are ν₁ - 1114 cm^-1, ν₂ - 880 cm^-1, ν₃ - 1452 cm^-1, and ν₄ - 696 cm^-1. The DTA-DTG-TG analyses display endothermic effects in 556-776 °C, 820-940 °C and 930-1100 °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.

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