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Most rock-forming silicates and many oxides are substitutional solid solutions that are capable of exchanging different cations on crystallographically defined structural sites. The mixing of cations often results in measurable changes in the macroscopic properties such as the thermodynamic functions, G, the Gibbs free energy, H the enthalpy, S the entropy and V the volume. Measurements of these functions, either directly (e.g. calorimetry, X-ray diffraction measurements) or indirectly (e.g. phase diagrams), constitute an important part of mineralogy, petrology and geochemistry. Because of the experimental difficulties often involved in a determination of these quantities and, in addition, considering the large compositional ranges that must be investigated, effort has been made in trying to estimate or predict, with microscopic-based models, the thermodynamic functions. Studies have also been made to try to understand the nature of the microscopic/mesoscopic structural properties (e.g. strain fields, polyhedral distortion, bonding changes, etc.) associated with substitutional solid solutions and, in addition, on how they control the macroscopic properties.

In the field of mineralogy/petrology phase equilibrium experiments have played a central role in determining phase diagrams and in extracting thermodynamic data for rock-forming silicates and oxides. Indeed, Zen (1977) spoke of the phase-equilibrium calorimeter. Presently, there are several internally consistent thermodynamic data bases that contain standard entropies, enthalpies of formation and volumes of end-member oxides and silicates that are to a large degree based on such experiments (e.g. Berman, 1988; Chatterjee et al., 1998). Thermodynamic data on solid solutions, derived from phase-equilibrium studies, are generally fewer and are not as well constrained.

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