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Bendadaite from Krásno near Horní Slavkov (Czech Republic), description and Raman spectroscopy

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SEJKORA Jiří TVRDÝ Jaromír ČEJKA Jiří VRTIŠKA Luboš DOLNÍČEK Zdeněk

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Časopis / Zdroj Bulletin Mineralogie Petrologie
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www http://bullmineral.cz/paper/download/197/fulltext
Klíčová slova bendadaite; arthurite group; powder X-ray diffraction data; unit-cell parameters; chemical composition; Raman spectroscopy; Krásno near Horní Slavkov; Czech Republic
Popis A rare Fe2+-Fe3+ dominant arsenate of the arthurite group, bendadaite, was determined at two samples from an abandoned Huber open pit in the Krásno ore district near Horní Slavkov, Slavkovský les area (Czech Republic). Bendadaite occurs there as brownish to olive green crystalline aggregates up to 2 - 6 mm in size in cavities of quartz gangue. The aggregates are composed by elongate prismatic crystals up to 100 - 200 um in length, partly in radial arrangement. It is opaque to semi-translucent (aggregates) to translucent (thin fragments). It has vitreous to subadamantine (crystals) or greasy to dull (aggregates) lustre. Bendadaite is monoclinic, space group P21/c, with the unit-cell parameters refined from X-ray powder diffraction data: a 10.183(2), b 9.672(2), c 5.536(1) A, beta 94.15(2)°, V543.8(1) A3 (sample NM) and a 10.175(2), b 9.682(2), c 5.532(1) A, beta 94.13(2)°, V 543.6(1) A3 (sample JT). The chemical composition of bendadaite agrees with general stoichiometry of the arthurite group minerals and corresponds to the following empirical formulae: (Fe0.52Zn0.25Cu0.02Mg0.02X0.19)1.00(Fe3+1.80Al0.20)2.00 [(AsO4)1.66(PO4)0.34]2.00(OH)2·4H2O (sample NM) and (Fe0.63Zn0.26X0.11)1.00(Fe3+1.87Al0.13)2.00[(AsO4)1.62(PO4)0.38]2.00(OH)2·4H2O (sample JT). The Raman spectra of both studied bendadaite sam-ples as well as tentative assignment of observed bands are given in this paper. Origin of bendadaite from Krásno is connected to in-situ supergene weathering of primary arsenopyrite, sphalerite and phosphates and high activity of arsenate and Fe2+, Fe3+ ions in acidic supergene fluids.