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THE CRYSTAL STRUCTURE OF BERRYITE, Cu₃Ag₂Pb₃Bi₇S₁₆

Dan Topa^1,, Emil Makovicky², Hubert Putz³ and W. Gus Mumme⁴

¹ Department of Material Science, Division of Mineralogy, University of Salzburg, Hellbrunnerstrasse 34/III, A–5020, Salzburg, Austria
² Geological Institute, University of Copenhagen, Østervoldgade 10, DK–1350 Copenhagen K, Denmark
³ Department of Geography, Geology and Mineralogy, University of Salzburg, Hellbrunnerstrasse 34/III, A–5020, Salzburg, Austria
⁴ CSIRO Minerals, Bayview Avenue, Clayton 3168, Australia

E-mail address: dan.topa{at}sbg.ac.at

The crystal structure of berryite, ideally Cu₃Ag₂Pb₃Bi₇S₁₆, monoclinic, a 12.703(2), b 4.0305(7), c 28.925(5) Å, ß 102.484(2)°, space group P2₁/m, Z = 2, D_calc = 6.899 g/cm³, has been solved by direct methods and refined to an R₁ index of 6.4% for 2352 unique reflections measured with MoK X-radiation on a three-circle diffractometer equipped with a CCD area-detector. There are fifteen unique Me sites and sixteen S sites in the asymmetric unit. Three Pb sites and two Bi sites are located on the surfaces of PbS-like slabs four layers thick, of two kinds; four Bi sites and two Ag sites are located in the interior of the slabs. Copper atoms in triangular coordination lie in a single S layer, with pseudohexagonal geometry. One Bi site straddles the interspace between the PbS-like slab and the S–Cu layer. Three primitive pseudotetragonal subcells of the PbS-like slab match with two orthohexagonal subcells of the Cu–S layer; this semicommensurate lock-in structure is made possible by the extension of the a parameter of the PbS-like slab by insertion of wide AgS₂₊₄ octahedra (linear S–Ag–S coordinations). The structure determined allowed us to derive model structures of orthorhombic berryite (a polytypic variant) and of watkinsonite, Cu₂PbBi₄(Se,S)₈. Structures of Ca₂Sb₂S₅ and La₄In₅S₁₃ follow the same modular principles.

Keywords: berryite, sulfosalt, crystal structure, electron-microprobe analysis, Grube Clara, Germany.

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