Solid solutions in the system Cu-Fe-S, part I: The Cu-S and CuFe-S joins

The data of Rau (1967) on the H ₂ S/H ₂ ratios of gas in equilibrium with high digenite solid solutions permit the calculation of the activity of Cu ₂ S as a function of temperature and composition. The activity of Cu ₂ S falls to 0.5 when the mole fraction (in the system Cu ₂ S-S) drops only to 0.87. These calculations also permit the estimation of the activity of S ₂ -temperature curve for the hexagonal chalcocite-high digenite reaction and a revised standard free energy for covellite.New data on the variation of the activity of S ₂ in equilibrium with various metal/sulfur ratios for Cu = Fe in the intermediate solid solution permit the calculation of the activity of CuFeS ₂ and show that it may drop as low as 0.2 at a mole fraction of 0.90 (system CuFeS ₂ -Cu ₂ Fe ₂ S ₂ ). The behavior of the copper-bearing solid solutions closely parallels that of pyrrhotite (Toulmin and Barton, 1964) in which the activity of FeS falls to 0.4 at a mole fraction of 0.9. A phase diagram is given for the CuFe-S join between 400 degrees and 700 degrees C.