EXPERIMENTAL-DETERMINATION OF THE FREE-ENERGY OF FORMATION OF FREIBERGITE FAHLORE

Values of DELTA(r)G-degreesBAR = +21.356 +/- 0.267 kJ.mol-1 al 573 K, and DELTA(r)G-degreesBAR = +24.112 +/- 0.380 kJ.mol-1 at 673 K, descri be the molar free energy of formation of the hypothetical Cu-free endm ember Ag10Zn2Sb4S13 from simple sulfides by the reaction 5Ag2S + 2Sb2S 3 + 2ZnS = Ag10Zn2Sb4S13. (1) At each temperature, virtually identical Ag/(Ag + Cu) ratios in the coexisting solid solutions (Cu, Ag)10Zn2Sb 4S13 (fahlore), (Ag, Cu)3SbS3 (pyrargyrite), and (Ag, Cu)SbS2 (miargyr ite), with excess ZnS (sphalerite), were obtained by solid state react ion, in evacuated silica tubes, of several bulk compositions composed of a variety of combinations of sulfides and sulfosalts, from 874 to 4 503 hours. Results were confirmed by similar reactions at 473 K and by synthesis reactions of finely ground mixtures of Ag2S + Cu2S + Sb2S3 + ZnS, at 673 K for up to 9400 hours. The resulting silver-copper exch ange data are used to derive values for the free energies of formation , from the simple sulfides, of the four (Cu, Ag)10(Zn, Fe)2Sb4S13 endm embers, using established thermochemical data and the fahlore thermoch emical model of SACK ( 1992). Cell dimensions for high-Ag fahlores are presented, and the stability limits of As-free tetrahedrite-freibergi te fahlores across the Fe-Zn join are predicted and compared with expe rimental data.