The crystal structure and compositional range of mckinstryite
The crystal structure and compositional range of mckinstryite
Mineralogical Magazine (February 2010) 74 (1): 73-84
The previously unknown crystal structure of mckinstryite, originally described as Ag (sub 1.18) Cu (sub 0.82) S or (Ag,Cu) (sub 2) S, was solved and refined using single-crystal X-ray diffractometer data collected from a sample from the Clara mine, Black Forest (Mo-Kalpha radiation, CCD area detector, R1(F)=3.85%). Mckinstryite has the refined formula Ag (sub 4.92) Cu (sub 3.08) S (sub 4) or Ag (sub 1.23) Cu (sub 0.77) S (idealized Ag (sub 5) Cu (sub 3) S (sub 4) or Ag (sub 1.25) Cu (sub 0.75) S) and crystallizes in space group Pnma (no. 62), with a=14.047(3) Aa, b=7.805(2) Aa, c=15.691(3) Aa, V=1720.3(7) Aa (super 3) , 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 Aa) strongly indicate that metal-metal interaction plays an important role in mckinstryite. The topology is related to that of stromeyerite ( approximately 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 Ag (sub 1.18) Cu (sub 0.82) StoAg (sub 1.25) Cu (sub 0.75) S, with the presently studied sample being fairly Ag-rich. The accurate limits of this range at ambient temperature are still to be determined.