The activity coefficients of FeS and ZnS in sphalerite in the temperature range 580 to 850°C can be expressed in terms of a two-constant Margules solution model. The standard state of FeS in sphalerite is chosen as pure FeS with the sphalerite structure, rather than FeS with the structure of pyrrhotite. The fit parameters of an ordinary least-squares regression indicate that the activity of FeS and ZnS in sphalerite can be adequately represented by a solution model in which the activity is independent of temperature. The excess partial molar free energy at infinite dilution is given by.

The solution model can be extrapolated to lower temperatures and will accurately predict the composition of sphalerite in sphalerite-pyrrhotite experiments. If aFeSpo is allowed to be constant below 525°C the matrix compositions of sphalerite from published sphalerite-pyrrhotite-pyrite experiments can be reproduced. If aFeSpo is allowed to vary with temperature below 525°C the solution model predicts sphalerite compositions between the composition of the matrix and iron-rich patches in published experiments.

The P-T-X relations of sphalerite-pyrrhotite-pyrite assemblages can be calculated using published data for the pyrite-pyrrhotite equilibrium and molar volumes and activities from the sphalerite solution model. Calculated isobars are in agreement with published experiments at elevated pressures.

Application of the calculated isobars to sphalerite compositions from the Stall Lake Mine, near Snow Lake, Manitoba, indicates the total pressure of metamorphism was 7 ± 1.0 kbar.

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