![]() ![]() We suggest that the non-equilibrium compositions result from retrograde intercrystalline K−Na exchange. Thus temperatures derived from such pairs using any of the proposed two-feldspar geothermometers will necessarily be in error. Coexisting feldspars from such rocks commonly do not plot on a common isotherm. With these caveats in mind we treat the problem of retrograde resetting in high-grade metamorphic rocks that were slowly cooled under essentially dry conditions. Some problems, however, persist: (1) the experimental solvi by Seck (1971a) and Elkins and Grove (1990) differ from each other (2) it is not known to what extent Na−K−Ca exchange equilibrium is approached (3) both solvi are probably metastable with regard to Al, Si order (4) it is difficult to judge how closely high-temperature natural feldspars compare to this situation (5) the thermodynamic treatment neglects phase transformations (6) the temperature dependence of the Margules parameters used to model non-ideal mixing behaviour may not be linear (7) it is not clear which expressions should be used to describe ideal activities. Recent improvements in the experimental and thermodynamic basis of two-feldspar geothermometry allow one to recover temperatures of coexistence more reliably. ![]()
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