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

Forty-four charges with model compositions of Sn- and Ge-substituded tetrahedrite/termantite were weighed out in the systems Cu-Sb (resp. As) - Fe (resp. Zn) - Sn (resp. Ge) - S. They alternatively modelled substitutions Cu-9(Zn,Fe)(3) (Sn,Ge)(2+)(Sb,As)(3)S-3+(13), Cu(Zn,Fe)(4)(Sn,Ge)(2)(2+) (Sb,As)(2)S-3+(13), Cu10Fe23+(Sn,Ge)(2)(2+) (Sb,As)(2)(3+) S-13, Cu-11(Zn,Fe)(0.5)(2+)(Sn,Ge)(0.5)(4+)(Sb,As)(4)S-13, Cu-10(Sn,Ge)(4+)rectangle(Sb,As)(4)S-13,Cu-6 (Sn,Ge)(2)(4+)rectangle(4) (Sb,As)(4)S-13, and Cu-11(Sn,Ge)(2+)(Sn,Ge)(4+)(Sb,As)(3)S-13. 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+) reversible arrow (As,Sb)(3+) + C Cu+ is broadly followed; an alternative (Sn,Ge)(2+) + Fe-3 reversible arrow (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 Cu-0.5 + (Sn,Ge)(0.5)(4+) reversible arrow Fe3+ and, less developed, (Sn,Ge)(4+) + vacancy reversible arrow 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 Angstrom; tennantite with 0.5-0.6 Ge4+ p.f.u. has 10.186 Angstrom. The Sn-substituted tetrahedrite Cu11.4Sn0.84+Sb3.8S13 has a equal to 10.383 Angstrom and such termantite Cu11.7Sn0.54+As3.9S12.9 has a = 10.22 Angstrom. Both in these substitutions and in those involving additional Fe and Zn, the interconvers ions between Cu+ and Cu2+ have decisive influence on the resulting a value.