Experimental determination of the free energy of formation of freibergite fahlore

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Values of at 573 K, and at 673 K, describe the molar free energy of formation of the hypothetical Cu-free endmember Ag 10 Zn 2 Sb 4 S 13 from simple sulfides by the reaction 5 Ag 2 S + 2 Sb 2 S 3 + 2 ZnS = Ag 10 Zn 2 Sb 4 S 13 . (1) At each temperature, virtually identical Ag /( Ag + Cu ) ratios in the coexisting solid solutions (Cu, Ag) 10 Zn 2 Sb 4 S 13 (fahlore), (Ag, Cu) 3 SbS 3 (pyrargyrite), and (Ag, Cu)SbS 2 (miargyrite), with excess ZnS (sphalerite), were obtained by solid state reaction, in evacuated silica tubes, of several bulk compositions composed of a variety of combinations of sulfides and sulfosalts, from 874 to 4503 hours. Results were confirmed by similar reactions at 473 K and by synthesis reactions of finely ground mixtures of Ag 2 S + Cu 2 S + Sb 2 S 3 + ZnS, at 673 K for up to 9400 hours. The resulting silver-copper exchange data are used to derive values for the free energies of formation, from the simple sulfides, of the four (Cu, Ag) 10 (Zn, Fe) 2 Sb 4 S 13 endmembers, using established thermochemical data and the fahlore thermochemical model of S ( 1992). Cell dimensions for high-Ag fahlores are presented, and the stability limits of As-free tetrahedrite-freibergite fahlores across the Fe-Zn join are predicted and compared with experimental data.

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