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LAMELLAR MINERALS OF THE CUPROBISMUTITE SERIES AND RELATED PADRAITE: A NEW OCCURRENCE AND IMPLICATIONS

Nigel J. Cook and Cristiana L. Ciobanu

Geological Survey of Norway, N-7491 Trondheim, Norway

E-mail address: nigel.cook{at}ngu.no

An assemblage consisting of bismuthinite derivatives, mainly in the range gladite – oversubstituted bismuthinite (Cu_0.833Pb_0.833Bi_19.167S₃₀), makovickyite, and intergrowths of cuprobismutite and padraite, 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, Cu_8.07(Ag_0.99Pb_0.21Bi_12.72)_13.92S_24.01, padraite, Cu_7.11(Ag_0.36Pb_1.20)_1.56Bi_11.28S_22.05, and hodrushite, (Cu_7.80Fe_0.23)_8.03(Ag_0.35Pb_0.12Cd_0.02Bi_11.49)_11.98S_21.99. Hodrushite is the least abundant component, containing minor but persistent Fe, up to 0.41 wt.%. An inverse correlation between Ag and Pb is seen between cuprobismutite (Ag_min 2.34 wt.%, Ag_max 2.75 wt.%) and padraite (Pb_min 5.37 wt.%, Pb_max 7.34 wt.%). Compositions 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 padraite. Ag appears to be an essential component of cuprobismutite, our data showing that 1 apfu Ag occupies the Bi position (14 apfu). Hodrushite differs from the type-locality material (Cu_8.12Fe_0.29Bi_11.54S₂₂), 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 padraite, 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 Together with a consideration of other published and unpublished data, this information gives an amended ideal formula for padraite: Cu₇(Ag_0.4Pb_1.2)_1.6Bi_11.4S₂₂. 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 Cu_1.34Ag_0.76Pb_0.14Bi_5.21S_9.64.

Keywords: Bi-sulfosalts, cuprobismutite, hodrushite, padraite, bismuthinite derivatives, makovickyite, compositional data, Ocna de Fier, Romania.

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