In the temperature range 600 ø to 900øC, the activity coefficients of FeS and F1S in pyrrhotite may be expressed in terms of a two-constant Margules solution model, with a linear temperature dependence of the excess partial molar free energy at infinite dilution of the two components. The pyrrhotite solid solution exhibits no measurable excess volume of mixing; for this reason, the activity coefficients are independent of pressure. This fact and the simple form of the activity coefficients facilitate the calcu-lation of equilibria involving pyrrhotite at pressures other than one atmosphere.
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Froese et al. (1976) studied this question.