Abstract
The intriguing recovery of an extremely Cu-rich cupropearceite crystal (Cu = 7.62 a.p.f.u.) from Tsumeb, Namibia, allowed us to study the effect of the entry of Cu in the positions usually occupied by Ag in the B module layer in the structure of the minerals belonging to the pearceite-polybasite group. Electron microprobe analysis yields the formula [Cu 6 (As 1.89 Fe 0.01 ) Σ=1.90 S 7.05 ][(Ag 8.36 Cu 0.62 ) Σ=8.98 CuS 4.06 ]. Lattice parameters are: a = 7.144(1), c = 11.745(2) A, V = 519.1(2) A 3 . The structure was refined in the space group P m 1 up to R = 0.0278 using 489 observed reflections [2σ( I ) level]. The entry of these elevated amounts of Cu in the structure does not induce a structural change and shows that pearceite is able to accommodate higher contents of Cu than previously thought. This allows hypothesis of the existence of an ideal [Cu 6 As 2 S 7 ][Cu 9 CuS 4 ] end member in nature. Such a proposed Ag-free mineral would show the highest structural disorder so far observed for a pearceite structure and, consequently, the highest ionic conductivity and could find applications in silver photography as sensitizer or in optics and microelectronics as rewritable storage medium.
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