The major differences between the evolutionary histories of ternary feldspars undergoing equilibrium or fractional crystallization at high or low temperatures and under H 2O-unbuffered or H 2O-buffered conditions provide important criteria for the interpretation of the histories of natural igneous rocks. Nonetheless, when combined with models for silicate melts in the feldspar system, most of the models for the solid solutions yield the same general trends in compositional evolution of feldspars during crystallization or melting (although the specific temperatures and pressures at which various changes in behavior are manifested vary with the model chosen). However, because of the paucity of data at high temperatures and in Ab-poor regions, there are significant variations in the predicted high-temperature characteristics of the solvus. Several thermodynamic models describing the mixing behavior of components in ternary feldspars have been developed for use in geothermometry. The shape of the solvus in T-X space indicates that this solubility increases significantly with temperature but only slightly with pressure, thereby laying the foundation for ternary feldspar geothermometry. The presence of a solvus in the ternary feldspar system indicates that there is limited solubility of the three components at magmatically reasonable temperatures and pressures. The compositions of ternary feldspars coexisting in equilibrium with melts are constrained by the subsolidus characteristics in the system Ab-An-Or.
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