Abstract

The entropy of mixing in high-temperature alkali feldspars has been re-evaluated using recently-published heat-of-solution data in conjunction with the results of earlier equilibrium experiments. We find that the excess entropy of mixing is distinctly positive over most of the K-Na composition range and most strongly so for the more potassic compositions. The excess entropies at the sodic end of the series are much smaller and may even be slightly negative for compositions near the Na end. The asymmetry in the excess entropy appears to be the principal reason for the asymmetry of the alkali feldspar solvus.

Short-range K-Na ordering can only cause a negative excess entropy, hence is either negligible or overwhelmed by other effects of opposite sign. A likely explanation is that there is a positive excess (probably best seen at very low temperatures) in the vibrational heat capacity, and that this effect is greatest for the more K-rich compositions. This asymmetry is what would be expected if it is assumed that a small atom on a large-atom site would have greater vibrational freedom than a large atom on a small-atom site.

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