Ag-Cu Exchange Equilibria Between α-Miargyrite, Chalcostibite, Pyrargyrite, and High-Skinnerite: Constraints on Ag-Cu Mixing in α-Miargyrite and Chalcostibite

Abstract

Ag-Cu exchange equilibria between a-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 150À3508C. All of these features are constrained by Ag-Cu exchange experiments (evacuated silica tubes; variable mass ratio) over the temperature range 120À4008C. 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 a-miargyrite (W AgCu A Mrg = 15.0; W AgCu B Mrg = 10.0; DG Å s * = 10.0; and DG Å * X Cu s = 14.0+5 kJ/gfw) and a onesite Ag-Cu mixing model for chalcostibite (W AgCu Chst = 25.0 kJ/gfw) are formulated. These results are constrained by a miscibility gap between a-miargyrite and chalcostibite from 225 to 3258C. The mixing model for a-miargyrite is expanded to include As for which a-miargyrite has a limited solubility along the Sb-As join with smithite AgAsS 2 .