The crystal structure and compositional range of mckinstryite

摘要

The previously unknown crystal structure of mckinstryite, originally described as Ag_1.18Cu_0.82S or (Ag,Cu)₂S, was solved and refined using single-crystal X-ray diffractometer data collected from a sample from the Clara mine, Black Forest (Mo-K radiation, CCD area detector, R1(F) = 3.85%). Mckinstryite has the refined formula Ag_4.92Cu_3.08S₄ or Ag_1.23Cu_0.77S (idealized Ag₅Cu₃S₄ or Ag_1.25Cu_0.75S) and crystallizes in space group Pnma (no. 62), with a = 14.047(3) Å, b = 7.805(2) Å, c = 15.691(3) Å, V = 1720.3(7) Å3, Z = 8. The structure contains five Ag, six Cu and eight S sites in the asymmetric unit. One of the Ag sites shows minor Cu-for-Ag substitution. The topology is based on flat, interrupted (010) layers of Cu and S atoms (all atoms on y = 0.25), in which the Cu atoms show triangular or two-coordination to S (interrupted {6,3} tiling). These layers alternate with uneven layers consisting of Ag atoms showing irregular three- to two-coordination to S. Some fairly short Ag-Cu contact distances (2.781-2.884 Å) strongly indicate that metal-metal interaction plays an important role in mckinstryite. The topology is related to that of stromeyerite (AgCuS) which contains complete flat layers of Cu atoms triangularly coordinated to S atoms, alternating with layers of loosely packed Ag atoms.