Lamellar minerals of the cuprobismutite series and related paderaite: a new occurrence and implications

Abstract

An assemblage consisting of bismuthinite derivatives, mainly in the range gladite - oversubstituted bismuthinite (Cu0.833Pb0.833Bi19.167S30), makovickyite, and intergrowths of cuprobismutite and paděraite, with minor hodrushite, occurs in the Paulus mine, Ocna de Fier skarn deposit, southwestern Romania. Even though intimately intergrown with one another as sets of lamellae of varying thickness, with back-scattered electron images strongly suggesting that lamellar intergrowths of the two minerals extend below the size of the microprobe beam, homogeneous parts of lamellae or well-developed acicular crystals allow compositional characterization of the components. These are cuprobismutite, Cu8.07(Ag0.99Pb0.21Bi12.72Σ13.92S24.01, paděraite, Cu7.11(Ag0.36Pb1.20Σ1.56Bi11.28S22.05, and hodrushite, (Cu7.80Fe0.23 is seen between cuprobismutite (Agmin 2.34 wt.%, Agmax 2.75 wt.%) and paděraite (Pbmin 5.37 wt.%, Pbmax 7.34 wt.%). Composition for each phase diverge somewhat from previously published data, suggesting that varying degrees of incorporation of Ag and, to a lesser extent, Pb, are features of the cuprobismutite homologous series and of the related paděraite. Ag appears to be an essential component of cuprobismutite, our data showing that 1 apfu Ag occupies the Bi position (Σ14 apfu). Hodrushite differs from the type-locality material (Cu8.12Fe0.29Bi11.54S22), showing that the composition can depart from the ideal compositions within certain limits, with incorporation of Ag, Pb and Fe in the structure. Significantly, the number of Cu atoms is less than 8 apfu in the Ocna de Fier material, with Fe located at the Cu position and minor Ag and Pb at the Bi position. For paděraite, the Ocna de Fier material also differs from type-locality material in that Cu occupancy is 7, instead of 6 apfu, and Ag is less than 1 apfu, based on 42 apfu. Together with a consideration of other published and unpublished data, this information gives an amended ideal formula for paděraite: Cu7(Ag0.4 Pb1.2)Σ1.6Bi11.4S22. Two possible substitutions, 3Ag+ → Bi3+ and Ag+ + Pb2+ → Bi3+ can be written for the cuprobismutite homologues. Furthermore, the substitution Fe2+ → Cu+ + ▷ can be considered for hodrushite with Cu deficiency (<8 apfu), as in our material. In type-locality hodrushite, however, the Bi position is filled by an excess of Cu over 8 apfu, together with Fe, following the substitution Cu+ + Fe2+ → Bi3+. We believe that this association, involving costability and intergrowths among minerals with closely allied structures, constitutes a paragenesis formed in a field of oscillatory chemical gradients of Cu, Pb, Ag and Bi, in which the pavonite series is represented only by makovickyite, with the fixed composition Cu1.34Ag0.76Pb0.14 Bi5.21S9.64.