Abstract
The crystal structure of a Bi-rich izoklakeite, Cu2.6Fe1.4Ag0.9Pb53.8Bi22.6Sb14.8S114, space group Pnnm, Z = 1, a = 34.221(8), b = 37.933(8), c = 4.063(3) Å was refined from X-ray single-crystal data to R = 5.9%, Rw = 7.3%. The specimen was originally included in a quartz crystal and was found near Lake Zervreila, Graubünden, Switzerland. The specimen’s crystal structure is composed of PbS- and SnS-like units and thus is in fair agreement with the structure of Sb-rich izoklakeite from Vena, Sweden (Makovicky and Mumme, 1986). The distribution of Sb-rich atomic positions is determined by site-occupancy refinements. Because Bi3+ and Pb2+ are isoelectronic and cannot be distinguished by their X-ray scattering behavior, a Me-S distance reference diagram for Sb-S, Bi-S, Pb-S, and Tl-S is herein developed on the basis of literature data. Mean Me-S bond lengths are not applicable to sulfosalts owing to the strong distortions of the coordination polyhedra caused by the lone electron pairs characteristic of these ions. A diagram where the mean value of the three shortest Me-S distances is plotted versus the mean value of the next two shortest ones seems suitable to distinguish Sb3+, Bi3+, Pb2+, and T1+. (Pb,Bi)-bearing sulfosalt structures given in the literature are tested with this diagram. The izoklakeite investigated possesses 1 pure Sb site, 10 mixed (Bi,Sb) sites with additional Pb, 2 mixed (Bi,Pb) sites, and 10 Pb sites. Cu and Fe occupy a fourfold-coordinated position. Ag could not be localized, but probably replaces Pb according to the substitution Bi3+ + Ag+ → 2Pb2+.