Thermochemical study of the system Fe-As-S

The results of Toulmin and Barton (1964) for the Fe-S system have been combined with a series of new measurements on As-bearing assemblages in the 500°–850°C temperature range to derive data on the free energies, enthalpies, and entropies of formation for arsenopyrite, loellingite, orpiment, realgar, FeAs, and Fe₂As. The enthalpies and free energies of formation of orpiment and realgar are only approximately one-half as large as indicated in recent compilations of thermochemical data (Wagmanet al., 1965). Data are also presented for the covariation of activity of S₂(g) with temperature and composition of the sulfur-arsenic liquid.