How much copper can the pearceite structure sustain? The case of cupropearceite from Tsumeb, Namibia
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چکیده
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 [Cu6(As1.89Fe0.01) Σ=1.90S7.05][(Ag8.36Cu0.62)Σ=8.98CuS4.06]. Lattice parameters are: a = 7.144(1), c = 11.745(2) Å, V = 519.1(2) Å3. The structure was refined in the space group P3 m1 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 [Cu6As2S7][Cu9CuS4] 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.
منابع مشابه
IN THE Cu - - Fe - - As - - S SYSTEM ( TSUMEB
Haynes, F.M., 1984. A geochemical model for sulfide paragenesis and zoning in the Cu-Fe--As--S system (Tsumeb, South West Africa/Namibia). Chem. Geol., 47: 183--190. The Cu-bearing hypogene mineralization at Tsumeb, South West Africa/Namibia, is interpreted to have resulted from the interaction of hot circulating Cu-rich saline solutions and host dQlomites. Fluids attending formation of the mai...
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