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The crystal structure and compositional range of mckinstryite

  • U. Kolitsch (a1)

Abstract

The previously unknown crystal structure of mckinstryite, originally described as Ag1.18Cu0.82S 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-Kα radiation, CCD area detector, R1(F) = 3.85%). Mckinstryite has the refined formula Ag4.92Cu3.08S4 or Ag1.23Cu0.77S (idealized Ag5Cu3S4 or Ag1.25Cu0.75S) 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.

A critical evaluation of literature data on the chemical composition and unit-cell parameters of mckinstryite confirms the presence of a small compositional range of mckinstryite which extends approximately from Ag1.18Cu0.82S to Ag1.25Cu0.75S, with the presently studied sample being fairly Agrich. The accurate limits of this range at ambient temperature are still to be determined.

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References

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The crystal structure and compositional range of mckinstryite

  • U. Kolitsch (a1)

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