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PROUDITE: A REDETERMINATION OF ITS CRYSTAL STRUCTURE AND THE PROUDITE–FELBERTALITE HOMOLOGOUS SERIES

William G. Mumme¹, Dan Topa² and Emil Makovicky^3,

¹ CSIRO Minerals, Box 312, Clayton South, Victoria, 3169, Australia
² Fachbereich Materialforschung und Physik, Universität Salzburg, Hellbrunnerstrasse 34, A–5020 Salzburg, Austria
³ Department of Geography and Geology, University of Copenhagen, Østervoldgade 10, DK–1350 Copenhagen, Denmark

E-mail address: emilm{at}geol.ku.dk

The crystal structure of proudite was redetermined with the holotype material from Tennant Creek, Australia, using a Nonius single-crystal diffractometer and MoK radiation. Our determination of the structure, based on 3388 observed reflections, yielded an R₁ value of 8.2% for 306 parameters. Proudite is monoclinic, a 31.814(1), b 4.1002(2), c 36.560(1), β 109.266(1)° and space group C2/m. Results of new electron-microprobe analyses give its composition as Cu_1.9Ag_0.1Pb_15.6Bi_20.4Sb_0.1S_32.4Se_14.5. We report analytical data for associated phases and for Se-free proudite from Felbertal, Austria, as well. The crystal structure contains 18 large cation sites (Pb, Bi, and mixed positions), a single Cu site, and 24 anion sites, nearly all of them being mixed Se,S sites. It consists of regularly sheared double-layers of octahedra that alternate with stepwise sheared (100)_PbS layers, two cation–anion sheets thick. Each straight portion of the former layers is 10 octahedra wide, whereas each straight interval of the PbS-like layers is eight square coordination pyramids (Pb, Bi) wide. Proudite is the highest known member of a homologous series of Pb–Bi– Cu sulfosalts with a mixed character, combining accretion of polyhedra and variable-fit of two layer-type fragments. This series contains proudite, felbertalite, makovickyite and a hypothetical intermediate member not yet known as a pure member, only as a combination with a "makovickyite-like element" in a synthetic selenide. This series is here defined as the proudite–felbertalite homologous series.

Keywords: proudite, crystal structure, proudite–felbertalite homologous series.