Contribution to the crystal structures of tennantite and bournonite

摘要

The crystal structures of Zn-free, Fe-rich tennantite (1.75 td%; td = tetrahedrite) from Cornwall, England, of Fe-free, Zn containing As-rich intermediate fahlore (14.75 td%) from Haidbachgraben, Austria, and of bournonite from Felsobanya, Romania, have been refined by single-crystal X-ray diffraction to R-values of 3.1, 2.5 and 2.3 %, respectively. Both fahlores are described in space group 143 m; for tennantite, the lattice parameter a = 10.216(1) A, for the As-rich intermediate fahlore a = 10.269(1) A. Bournonite crystallizes in the space group Pn2_1m, with the lattice parameters a = 7.811(1), b = 8.152(1), and c = 8.704(1) A. In the tennantite sample the threefold coordinated Cu2 site is strongly delocalized perpendicular to the coordination plane defined by three S atoms and splits into two distinct sites, i.e. Cu2a and Cu2b with a predominant occupation of the 'ordinary' Cu2a site. In the As-rich intermediate fahlore sample, the Cu2 site shows only a weak splitting with an equal distribution of the Cu atoms on both sites. In contrast to earlier structure data, the refinement of bournonite revealed almost identical average Sb-S distances for the two crystallographically different Sb atoms forming the SbS_3 pyramids. The coordination sphere of the two different Pb atoms is not strictly defined and can best be described as [6 + 2] and [5 + 2] coordinations, respectively.