Most experiments are conducted in relatively simple chemical systems in which only very limited formation of solid solutions takes place. Sometimes, however, natural phases are used as starting materials. In such cases the activities for any deviation from pure end-member composition were calculated assuming ideal mixing.
In some experimentally investigated mineral systems, two phases of a solid solution series coexist, forming a miscibility gap. In such a case appropriate activities had to be calculated.
For the system calcite-dolomite activities were calculated using the mixing model formulated by Gottschalk & Metz (1992).
In the system diopside-enstatite activities were calculated using the mixing model by Lindsley et al. (1981).
Tschermaks substitution (MgSiT <==> AlAlT) was considered only for the pyroxenes, since only very few experimental results exist for other phases such as amphiboles and phyllosilicates. The thermodynamics of the tschermaks substitution in these phases is also poorly understood, and tschermaks substitutions are not only a function of pressure and temperature but also of the phase assemblage. Activities due to tschermaks substitution in pyroxenes were calculated following the approach by Gasparik (1986).
For the binary cordierite - H2O-cordierite a new mixing model was derived (Gottschalk, 1997).