Ag-Cu exchange equilibria between alpha-miargyrite, chalcostibite, pyrargyrite, and high-skinnerite: constraints on Ag-Cu mixing in alpha-miargyrite and chalcostibite

Ag-Cu exchange equilibria between alpha-miargyrite AgSbS2 and pyrargyrite A g3SbS3 and between chalcostibite CuSbS2 and both pyrargyrite and high-skinn erite Cu3SbS3 are reported for the temperature range 150-350 degrees C. All of these features are constrained by Ag-Cu exchange experiments (evacuated silica tubes; variable mass ratio) over the temperature range 120-400 degr ees C. Chalcostibite is found to take very little Ag into its structure. Ut ilizing Ag-Cu data from this study as well as the Ag-Cu solution models for pyrargyrite and high-skinnerite, a two-site Ag-Cu non-convergent ordered s olution model for alpha-miargyrite (W-AgCu(A Mrg) = 15.0; W-AgCu(B Mrg) = 1 0.0; Delta (G) over bar(s)* = 10.0; and Delta (G) over bar(XCus)* = 14.0 +/ - 5 kJ/gfw) and a one-site Ag-Cu mixing model for chalcostibite (W-AgCu(Chs t) = 25.0 kJ/gfw) are formulated. These results are constrained by a miscib ility gap between alpha-miargyrite and chalcostibite from 225 to 325 degree s C. The mixing model for alpha-miargyrite is expanded to include As for wh ich alpha-miargyrite has a limited solubility along the Sb-As join with smi thite AgAsS2.