In the temperature range 600 degrees to 900 degrees C, the activity coefficients of FeS and S 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 calculation of equilibria involving pyrrhotite at pressures other than one atmosphere.

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