AG-CU EXCHANGE EQUILIBRIA BETWEEN PYRARGYRITE, HIGH-SKINNERITE, AND POLYBASITE SOLUTIONS

A model is developed for the thermodynamic properties of pyrargyrite ( Ag,Cu)3SbS3 and high-skinnerite (Cu, Ag)3SbS3 solid solutions over the temperature range 150-350-degrees-C. It makes explicit provision for Ag-Cu exchange equilibria between pyrargyrite, high-skinnerite, and po lybasite and for the presence of a miscibility gap between pyrargyrite and high-skinnerite. These features are constrained by 150-350-degree s-C Ag-Cu exchange experiments (evacuated silica tubes; variable mass ratio) between pyrargyrite and poly-basite and high-skinnerite and pol ybasite. Ag-Cu exchange experiments between polybasite and equimolar m ixtures of pyrargyrite and skinnerite are used to define the limbs of a miscibility gap between pyrargyrite and high-skinnerite. Explicit pr ovision for site distortion resulting from the Cu+ for Ag+ substitutio n (or Ag+ for Cu+ substitution) is made by assuming that the standard state properties of the Ag-pyrargyrite and Cu-pyrargyrite endmembers a re linearly dependent on the Cu/(Cu + Ag) ratio. Copper-substitution i n pyrargyrite is shown to be asymmetric and substantially nonideal (W( CuAg)Pyr = 21.0; W(AgCu)Pyr = 11.0 +/- 0.5 kJ/gfw). In contrast, subst itution of Ag in high-skinnerite is strongly ordered (DELTAG(s) = 10 +/- 0.5 kJ/gfw) on two sites (2:1) (W(CuAg)A Skn = 12.0; W(CuAg)B Skn = 9.0 +/- 0.5 kJ/gfw). Finally, these equilibria are consistent with e stimates of Gibbs energies of formation (from the simple sulfides) of a hypothetical Cu16Sb2S11 polybasite endmember of 15.00 kJ/gfw (400-de grees-C).