Experimental determination of the free energy of formation of freibergite fahlore

Details Experimental determination of the free energy of formation of freibergite fahlore Experimental determination of the free energy of formation of freibergite fahlore

Feb 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.1237e&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 58, Issue 4, pp.1237-1242

Computer Science

1

Scientific paper

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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