1Daniel Ozdín#
, 1Gabriela Kučerová,
1Martin Števko, 2Stanislava Milovská
1 Department of Mineralogy and Petrology, Faculty of Natural Sciences, Comenius University,
Mlynská dolina G, SK-842 15 Bratislava, Slovakia, # [email protected]
2 Geological Institute of the Slovak Academy of Sciences, Ďumbierska 1, SK-974 01 Banská
Bystrica, Slovakia
Key words: Mn-oxides, todorokite, hollandite supergroup, limnosilicite, Slovakia
INTRODUCTION
In the Western Carpathians there was a long known the occurrence of Mn-oxides and hydroxides, mainly of Cenozoic limestones and dolomites (e. g. Rojkovič 2002, 2003; Pauliš et al. 2003) and Mn ore mineralization especially in Paleozoic rocks in the Spiš-Gemer Ore Mountains, where the Čučma was the most significant deposit (Rojkovič 2001). Later, Mn- hydroxides were also described from oxidation zone of hydrothermal base and precious-metal veins hosted in Neovolcanites in Banská Štiavnica (e.g. Háber et al. 2002), and from the cave environment (Bónová et al. 2009), and also from travertines (Bónová et al. 2012). Large amount of various black dendrites in different geological environments, diverse rocks of different ages, were not sufficiently identified and these dendrites were mostly determined as psilomelane or wad. Our preliminary study of Mn oxides in the Western Carpathians indicates that different mineral phases occur in genetically variable environment.
METHODS
Manganese minerals were analyzed by a Cameca SX-100 electron microprobe in a wavelength-dispersive mode (State Geological Institute of Dionýz Štúr, Bratislava). Powder X-ray diffraction data were collected with a BRUKER D8 Advance diffractometer and were analyzed by thermal analysis (Derivatograph Q- 1500D) in Faculty of Natural Sciences, Comenius University in Bratislava. Micro Raman spectroscopy was carried out on LabRAM-HR 800 spectrometer (Horiba Jobin-Yvon) using excitation laser at 532 and 633 nm (Geol. Inst. of SAS in B. Bystrica).
RESULTS
Black dendrites in opals generated by residual weathering processes of serpentinite body at the locality Hodkovce are probably formed by romanèchite, which contains up to 0.6 wt. % FeO, 0.5 wt. % V2O3 and mainly up to 4.5 wt. %
NiO and 1.2 wt. % CoO. In limnosilicite from the locality Dúbravica, there are very thin veins composed from radial aggregates of hollandite, whose chemical composition is characteristic by a variable
115 content of BaO (5.98-13.47 wt. %) and K2O (0.16-1.32 wt. %). On the locality
Hriňová-Krivec in andesite tuffs, Mn mineralization is accompanied by a separate opal veins. Manganese minerals are represented by the minerals from cryptomelane-hollandite series. Pyrolusite was identified in the oxidation zone of hydrothermal base metal veins at Banská Štiavnica and Hodruša-Hámre. It contains up to 1.04 wt. % SiO2, 0.46 wt. % CaO,
0.70 wt. % ZnO and up to 0.32 wt. % FeO. We also determined todorokite with variable content of Na2O. Todorokite from
Banská Štiavnica contains up to 9.3 wt. % ZnO. Zn-rich cryptomelane (up to11.7 wt. % ZnO) forms worm-like aggregates in assemblage with crystalline pyrolusite. In this association there was also another Mn oxide observed, which contain up to 19.9 wt. % ZnO and thus is close to chalcophanite or woodruffite. All Mn oxides from Banská Štiavnica and Hodruša-Hámre deposits show increased contents of Zn and Cl (up to 0.8 wt. %). In oxidation zones of siderite deposits Sirk and Krásnohorské Podhradie crystalline pyrolusite was identified. The content of Fe2O3 in hollandite at Krásnohorské
Podhradie deposit is up to 2.52 wt. % and at Sirk deposit cryptomelane contains only 0.6 wt. % Fe2O3. Manganese oxides are
genetically younger in comparison with Fe oxides. They have very small substitution of Fe↔Mn, but goethite has increased content of Mn2O3 (~ 4 wt. %).Black
dendrites in clay limestones (ruin marble) from the Horná Breznica occurrence consist of ranciéite and probably romanèchite or hollandite. Both minerals show variable chemical composition and strong isomorphism of Fe↔Mn. Content of BaO in romanèchite is up to 13.52 wt. %. Chemical composition of both minerals are characteristic by increased content of SiO2 (up to 5.7 wt. %), Al2O3 (up to 4.1 wt.
%), MgO (up to 0.8 wt. %) and K2O (up to
0.9 wt. %). Ranciéite was identified from Kolíňany quarry in association with hydroxylapatite and crandallite in
limestone fissures. It contains (in wt. %): 1.1 Na2O, 1.8, Al2O3, 1.0 MgO, 4.7-8.8
CaO, 1.0 K2O and 4.6 ZnO. In alkaline
environment (limestones) ranciéite is the most common phase, which is in contrast with acidic environment (opals, silicites, quartz veins) were Mn oxides are represented mainly by the minerals of hollandite supergroup.
Acknowledgements: This work was supported by the Slovak Research and Development Agency under the contract No. APVV-0375-12 and GUK/483/2013.
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