Original paper

Contribution to the crystal structures of tennantite and bournonite

Kharbish, Sherif; Giester, Gerald; Beran, Anton

Abstract

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 Felsöbanya, 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 I43m; for tennantite, the lattice parameter a = 10.216(1) Å, for the As-rich intermediate fahlore a = 10.269(1) Å. Bournonite crystallizes in the space group Pn21m, with the lattice parameters a = 7.811(1), b = 8.152(1), and c = 8.704(1) Å. 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 SbS3 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.

Keywords

tennantitefahlorebournonitesite delocalizationcrystal structure refinement