Journal of
GEOsciences
  (Formerly Journal of the Czech Geological Society)

Original Paper

Cu-rich members of the beudantite-segnitite series from the Krupka ore district, the Krušné hory Mountains, Czech Republic

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Abstract:
Copper-rich members of the beudantite-segnitite series (belonging to the alunite supergroup) were found at the Krupka deposit, KruÅ¡né hory Mountains, Czech Republic. They form yellow-green irregular to botryoidal aggregates up to 5 mm in size. Well-formed trigonal crystals up to 15 μm in length are rare. Chemical analyses revealed elevated Cu contents up to 0.90 apfu. Comparably high Cu contents were known until now only in the plumbojarosite-beaverite series. The Cu²⁺ ion enters the B³⁺ position in the structure of the alunite supergroup minerals via the heterovalent substitution Fe³⁺Cu²⁺_-1 (AsO₄)^3-(SO₄)^2-_-1 . The unit-cell parameters (space group R-3m) a = 7.3265(7), c = 17.097(2) Å, V = 794.8(1) ų were determined for compositionally relatively homogeneous beudantite (0.35 - 0.60 apfu Cu) with the following average empirical formula:
Pb_1.00(Fe_2.46Cu_0.42Al_0.13Zn_0.01)_Ε3.02 [(SO₄)_0.89(AsO₃OH)_0.72(AsO₄)_0.34(PO₄)_0.05]_Ε2.00 [(OH)_6.19F_0.04]_Ε6.23 .
Interpretation of thermogravimetric and infrared vibrational data is also presented. The Cu-rich members of the beudantite-segnitite series are accompanied by mimetite, scorodite, pharmacosiderite, cesàrolite and carminite. These minerals are characterised by refined unit-cell parameters and by quantitative chemical analyses. The assemblage of supergene minerals corresponds to the following crystallization sequence: mimetite → beudantite-segnitite → pharmacosiderite, scorodite, cesàrolite, and carminite.