Ag-Cu exchange equilibria between alpha -miargyrite AgSbS 2 and pyrargyrite Ag 3 SbS 3 and between chalcostibite CuSbS 2 and both pyrargyrite and high-skinnerite Cu 3 SbS 3 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 degrees C. Chalcostibite is found to take very little Ag into its structure. Utilizing 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 solution model for alpha -miargyrite (W AMrgAgcu = 15.0; W BMrgAgCu = 10.0; Delta G (super *) s = 10.0; and Delta G (super *) XCuS = 14.0+ or -5 kJ/gfw) and a one-site Ag-Cu mixing model for chalcostibite (W ChstAgCu = 25.0 kJ/gfw) are formulated. These results are constrained by a miscibility gap between alpha -miargyrite and chalcostibite from 225 to 325 degrees C. The mixing model for alpha -miargyrite is expanded to include As for which alpha -miargyrite has a limited solubility along the Sb-As join with smithite AgAsS 2 .

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