Original paper

Exploratory studies on substitutions in tetrahedrite-tennantite solid solution. Part IV. Substitution of germanium and tin

Klünder Hansen, Maiken; Makovicky, Emil; Karup-Møller, Sven

Abstract

Forty-four charges with model compositions of Sn- and Ge-substituded tetrahedrite/tennantite were weighed out in the systems Cu–Sb (resp. As) – Fe (resp. Zn) – Sn (resp. Ge) – S. They alternatively modelled substitutions Cu9(Zn,Fe)3 (Sn,Ge)2+(Sb,As)33+S13, Cu(Zn,Fe)4(Sn,Ge)222+(Sb,As)23+S13, Cu10Fe23+(Sn,Ge)22+ (Sb,As)23+S13, Cu113(Zn,Fe)0.52+(Sn,Ge)0.54+(Sb,As)4S13, Cu10(Sn,Ge)4+□(Sb,As)4S13,Cu6 (Sn,Ge)24+□4 (Sb,As)4S13, and Cu11(Sn,Ge)2+(Sn,Ge)4+(Sb,As)3S13. The resulting phases were investigated by microprobe analyses and X-ray powder diffraction. In no case the substitutions reached the planned end-members, producing a number of Sn- or Ge-containing phases that were detected and analysed in detail. For Sn & Ge substituting for semimetals, the substitution scheme (Sn,Ge)2+ + (Fe,Zn)2+ (As,Sb)3+ + Cu+ is broadly followed; an alternative (Sn,Ge)2+ + Fe3 (As,Sb)3+ + Fe2+. The highest solubility observed is about 1.6 (Sn,Ge)2+ p.f.u. In the case of tetravalent Sn and Ge, the trends Cu0.5 + (Sn,Ge)0.54+ Fe3+ and, less developed, (Sn,Ge)4+ + vacancy Cu+ + Fe3+ are observed. In Zn, Fe-free charges a double role of Sn, Ge was observed as well. The Ge-substituted tetrahedrite Cu11.6Ge0.64+Sb3.9S12.9 has a equal to 10.345 Å; tennantite with 0.5–0.6 Ge4+ p.f.u. has 10.186 Å. The Sn-substituted tetrahedrite Cu11.4Sn0.84+Sb3.8S13 has a equal to 10.383 Å and such tennantite Cu11.7Sn0.54+ As3.9 S12.9 has a = 10.22Å. Both in these substitutions and in those involving additional Fe and Zn, the interconversions between Cu+ and Cu2+ have decisive influence on the resulting a value.