The system Ag-Fe-S: Phase equilibria and mineral assemblages

Abstract

Phase relations in the Ag-Fe-S system were determined from 700 to 150 °C by quench experiments with the use of evacuated, sealed, silica tubes as reaction vessels; these data were then used to interpret various aspects of natural occurrences of Ag-Fe-S minerals (e.g. “argentiferous pyrite”). The assemblages Ag2S+Fe1−xS and Ag2S+FeS2 become stable, with decreasing temperature, at 622±2 ° and 607±2 °C, respectively; their establishments involve ternary invariant conditions. The three condensed phases Ag2S+Fe1−xS+FeS2 become stable together at 532±2 °C through a ternary eutectic reaction near Ag2S in composition. An invariant reaction at 248±8 °C results in the formation of the Ag+FeS2 pair from the Ag2S+Fe7S8 assemblage, which is stable at higher temperatures. The associations of native silver and pyrite are found in certain massive sulfide deposits, whereas natural coexistence of argentite and pyrrhotite has not been documented. Experiments demonstrate the feasibility of retrograde reequilibration in ores to produce the silver+pyrite pair from argentite+pyrrhotite. Less than 0.05 and 0.1 at. % Ag are soluble in FeS2 and Fe1−xS, respectively, at 600 °C and less than 0.8 at. % Fe in Ag2S at 500 °C. Silver does not measurably affect the d10.2 values of Fe1−xS or the cell dimension of FeS2 (a25 °C=5.4175±0.0001 A). This study also demonstrates that at low temperatures the binary fugacity data are applicable to ternary assemblages of the Ag-Fe-S system because of these very limited solubilities. The presence of Fe lowers the fcc ⇆ bcc inversion temperature of Ag2S more than 50 °C; the exact amount of lowering is dependent on the associated Ag-Fe-S phases. The bcc ⇆ mono. inversion temperature, however, is not measurably affected. No ternary solid phases were encountered above 150 °C. Heating of sternbergite and argentopyrite (both AgFe2S3) mineral samples shows instability at 152 °C (e.g. partial breakdown of sternbergite in 405 days); rate studies show that a 10 °C temperature increase results in approximately a 5-fold increase in breakdown rate.