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

D. E. Harlov
Projektbereich 4.1, GeoForschungsZentrum, Telegrafenberg, D-14473 Potsdam, Germany

Abstract: Ag-Cu exchange equilibria between α-miargyrite AgSbS2 and pyrargyrite Ag3SbS3 and between chalcostibite CuSbS2 and both pyrargyrite and high-skinnerite Cu3SbS3 are reported for the temperature range 150–350°C. All of these features are constrained by Ag-Cu exchange experiments (evacuated silica tubes; variable mass ratio) over the temperature range 120–400°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 α-miargyrite (WAgCuA Mrg=15.0;WAgCuA Mrg=10.0;ΔG¯s*=10.0; and Δ G ¯ X C u S * = 14.0 ± 5 kJ/gfw) and a one-site Ag-Cu mixing model for chalcostibite (WAgCuChst=25.0 kJ/gfw) are formulated. These results are constrained by a miscibility gap between α-miargyrite and chalcostibite from 225 to 325°C. The mixing model for α-miargyrite is expanded to include As for which α-miargyrite has a limited solubility along the Sb-As join with smithite AgAsS2.

Keywords: Ag-Cu exchange experiments • miargyrite • chalcostibite • pyrargyrite • skinnerite • Ag-Cu solution model • As-Sb solution model

Mineralogical Magazine; June 1999 v. 63; no. 3; p. 399-411; DOI: 10.1180/002646199548493
© 1999, The Mineralogical Society
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