What is the reason for the doubled unit-cell volumes of copper-lead-rich pavonite homologues? the crystal structures of cupromakovickyite and makovickyite

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The crystal Structure of cupromakovickyite, ideally Cu8Pb4Ag2Bi18S36, from the skarn deposit at Baita Bihor, in Romania, has been solved by direct methods and refined to an RI index of 5.36% for 2680 unique reflections measured with MOKU radiation oil it four-circle diffractometer equipped with a CCD detector. Cupromakovickyite has a monoclinic cell with a 13.405(8), b 4.016(3), c 29.949(19) angstrom, beta 99.989(16), V 1587.8(17) angstrom(3), space group C2/m and Z = 1. There are eight metal sites and twelve Sulfur sites in each of the four asymmetric units. The thicker, galena-like layers are composed of parallel chains Bi4-Bi1-Bi1-Bi4 four octahedra long. In the thinner layers, the octahedra of Ag are replaced by two tetrahedrally coordinated Cu sites, whereas one of the paired Square pyramids of Bi is replaced by Pb. The asymmetry of the thin layer is responsible for the doubling of the c parameter for cupromakovickyite with respect to makovickyite. To be able to compare them, the crystal structure of Cu-bearing makovickyite, Cu2.25Ag1.56 (Pb0.27Cd0.13)(Sigma 0.4)Bi10.46S18, from a scheelite deposit at Felbertal, in Austria, has been refined to in R, index of 4.36% for 1320 unique reflections measured with the same equipment. Makovickyite has a monoclinic cell with a 13.239(3), b 4.0547(7), c 14.667(3) angstrom, beta 99.397(4), V 776.7(3) angstrom(3), space group C2/m and Z = 1. There are four metal sites and six sulfur sites in each Of the four asymmetric units. The thicker, galena-like layers are composed of parallel chains Bi2-Bi1-Bi1-Bi2 four octahedra long. In the thinner layers, the octahedra of Ag are partially replaced by two tetrahedrally coordinated Cu sites.
OriginalsprogEngelsk
TidsskriftCanadian Mineralogist
Vol/bind46
Udgave nummer2
Sider (fra-til)515-523
ISSN0008-4476
DOI
StatusUdgivet - 2008

ID: 8805040