An alkali ion-exchange equilibrium curve between molten Na-K bromide and highly-ordered alkali feldspar was determined at 800°C and 1 bar. The miscibility gap in the feldspar crystalline solution occurs between NOr = 0.105 and NOr = 0.50 at a salt composition MKBr = 0.32 (mole fractions). A thermodynamic analysis of combined ion-exchange and two-feldspar data yields the excess Gibbs energy of mixing of the solution with a maximum value of 900 cal mol-1. This is not very different from that for high albite-sanidine feldspars, but is more accentuated in asymmetry towards NaAlSi3O8. Such asymmetry of
is most likely related to the excess entropy of mixing.
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