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One of the major goals of petrology is to retrieve values of the intensive parameters, such as pressure (P), temperature (T) and fluid composition, under which the major mineralogical properties of a rock were established, along with the time scales of evolution of the mineralogical properties. The basic approach in the retrieval of the intensive properties involves comparison of the mineralogical assemblages and the mineral compositions of the rock with the phase equilibrium constraints. The latter are calculated from the internally consistent thermochemical properties of the stoichiometric end members or determined in the laboratory on relatively simple systems, usually involving only the end-member phases, and then corrected for the effects of the compositional departures as observed in a specific natural assemblage. In addition, many mineral pairs (e.g. garnet and biotite) respond to changes of PT conditions, especially temperature, by continuous ion exchange reactions, and thus register the PT condition or PT history of the rock in their compositions. The ion-exchange reactions are also calibrated in the laboratory on relatively simple systems, but require corrections for the effects of additional components that enter into solid solution in the minerals in natural environments. The corrections for the compositional effects rely critically on the thermodynamic mixing properties of the components in the mineral solid solutions and fluid phase which are involved in a specific reaction (e.g. Ganguly & Saxena, 1987). There has, thus, been a sustained effort over the last few decades on the determination of thermodynamic mixing properties of phases in geologically important systems.

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