The use of thermodynamic excess functions in the Nernst distribution law

The Nernst distribution law, which is used by many authors in geology as the theoretical basis of geothermometry and trace-component distribution, is derived using excess functions. It is demonstrated that the free-energy term in the Nernst equation, as it is applied in many of these cases, is an excess function and describes the non-ideal behavior of the phases involved. It is then not necessry to introduce any assumptions concerning activities or activity coefficients, and one may use simple mole fractions instead. Analysis of system data in light of this allows one to make useful predictions concerning the accuracy of the data and the behavior of the system at elevated temperatures. Analysis is made of various data on the diopside-enstatite solvus as an example.