The crystal structure of paarite, the newly discovered 56 Å derivative of the bismuthinite-aikinite solid-solution series

We have determined the crystal structure of paarite, a newly discovered 56 Angstrom derivative of the bismuthinite-aikinite series from the metamorphosed scheelite deposit of Felbertal, Austria. Its electron-microprobe-established composition is Cu1,7Pb1.7Bi6.3S12, and the structure-derived composition is Cu1.6Pb1.6Bi6.4S12, with a 4.007(1), b 55.998(8), c 11.512(2) Angstrom, space group Pmcn, Z = 5. The structure has been solved by direct methods and difference-Fourier syntheses, and refined to a residual value R-1 of similar to7.0%. The unit cell of this "fivefold derivative" contains two "bismuthinite-like" Bi4S6 ribbons and eight "krupkaite-like" CuPbBi3S6 ribbons. The fully occupied Cu sites occur in zigzag [001] rows at y = 0.15, 0.30, 0.70 and 0.85, comprising gladite-like and krupkaite-like intervals in the structure in regular 1: 1 alternation. Coordination polyhedra have been analyzed using the polyhedron-distort ion parameters devised recently by Balic-Zunic & Makovicky, and comparisons to the structures of salzburgite and lindstromite are given. This new derivative occurs at Felbertal both as independent grains and as a component of exsolution pairs with krupkaite.