THE STRUCTURAL ROLE OF EXCESS Cu AND Pb IN GLADITE AND KRUPKAITE BASED ON NEW REFINEMENTS OF THEIR STRUCTURE

Crystal structures of stoichiometric gladite (empirical formula Cu 1.32Pb1.37Bi6.65S12.03) and krupkaite (empirical formula Cu2.00Pb2.03Bi5.99S12.04) from Felbertal, Austria, were refined to R1 = 0.045 and 0.037, respectively, yielding improved positional parameters and interatomic distances. Structures of “oversubstituted” gladite with an excess of Pb + Cu substitution for Bi + tetrahedral vacancy (empirical formula Cu 1.55Pb1.59Bi6.43S12.02, percentage of the aikinite end-member, naik = 39), of “undersubstituted” krupkaite (Cu 1.85Pb1.92Bi6.12S12.06, naik = 47), and of “oversubstituted” krupkaite (Cu 2.32Pb2.40Bi5.64S12.16, naik = 59) from the same locality, were refined to the R 1 values of 0.041, 0.051, and 0.052, respectively. Additional copper (occupancy 0.22) forms a broadly zig-zag pattern in each 1 ½ subcell of gladite. In “undersubstituted” krupkaite (naik = 47), the regular Cu position (Cu 1) was found to be slightly undersaturated. The “oversubstituted” krupkaite (naik = 59) contains additional Cu, located in the Cu 2 sites situated half-way between the fully occupied Cu 1 positions. Fixing their occupancy to 0.18, in agreement with EPMA data, and refining the adjacent large cation sites as two sites, with 0.18 Pb and 0.82 Bi, respectively, yielded the final model refined here. Interatomic distances and other characteristics of the polyhedra are used to evaluate the effects of cation substitution.