Unit-cell parameters and molar volumes have been determined for seven members of a sanidine-analbite ion-exchange series by least-squares refinements of X-ray powder diffraction data. The quadratic least-squares fit to the molar volume data yields:
where NOr and NAb represent mole fractions of KAlSi3O8 and NaAlSi3O8, respectively. Excess molar volume of K-Na mixing may be expressed as:

The Margules parameter for molar volume, Wv, for this sanidine-analbite ion-exchange series agrees well not only with Wv values determined for several other (hydrothermally synthesized) sanidine-analbite series, but also with those for other alkali feldspar series. Thus, volumes of K-Na mixing in the alkali feldspars are insensitive to Al-Si order-disorder.

Mole fractions of Al in the T1, and T2, tetrahedral sites for all members of the series are 0.271 and 0.229, respectively, based on b and c unit-cell dimensions, making this series somewhat more disordered than those synthesized hydrothermally by others.

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