Phase equilibrium study in the system Cu2S-PbS-Sb2S3: non-stoichiometry in sulfosalts and isothermal variation in sulfur fugacity

Evacuated silica tube experiments (+halide flux) were conducted in portions of the system Cu₂S-PbS-Sb₂S₃ at 440°C, using two-pyrrhotite indicator method to measure the sulfur fugacity. Product phases were identified by optical and X-ray powder diffraction methods supplemented with microprobe analyses. In addition to the previously reported mineral phases, famatinite (Cu₃SbS₄) appears to be a stable phase in the Sb₂S₃-rich portion of the system. Microprobe data indicate that almost all the sulfosalts depart from stoichiometry. Copper in Pb-Sb sulfosalts and Pb in chalcostibite and skinnerite are indicative of the coupled substitution 2Pb²⁺=Cu⁺+Sb³⁺. Pb-solubility in skinnerite and Cu-solubility in zinkenite are dependent on the initial bulk composition of the charges. The compositions of meneghinite and boulangerite compare well with their natural analogues. The maximum isothermal variation of logf _{s 2} falls in the range of-6.36 (1.06)logf _{s 2}11.12 (0.30). The experimentally derived logf _{s 2} values for some two phase assemblages, compare reasonably well with the respective minimum logf _{s 2} values calculated by the method of Craig and Barton (1973). The stable coexistence of famatinite with zinkenite plus stibnite instead of chalcostibite may be described by the sulfidation reaction: 3CuSbS₂+1/2 S₂=Cu₃SbS₄+Sb₂S₃.