Heats of solution have been measured for seven members of a sanidine-analbite ion-exchange series in 20.1 percent HF at 49.7°C under isoperibolic conditions. A cubic least-squares fit to the heat-of-solution data from twenty-eight experiments yields:
−H¯soln(kcal/mol)=154.411−6.233 NOr+7.474 NAbNOr2+4.117 NOrNAb2
where NOr and NAb are the mole fractions of KAlSi3O8 and NaAlSi3O8, respectively. Excess molar enthalpy of K-Na mixing for this series may be expressed as
H¯ex(kcal/mol)=WH,Ab NAbNOr2 + WH,OrNOrNAb2

where WH,Ab and WH,Or are the Margules parameters for the enthalpy of K-Na mixing and are equivalent Lo 7.474 and 4.ll7 kcal/mol, respectively.

This series of feldspars has a topochemically monoclinic, relatively disordered Al-Si distribution. Comparison of the enthalpy of K-Na mixing for these feldspars with that of a microcline-low albite series of feldspars (Waldbaum and Robie, l97l) having a topochemically triclinic, relatively ordered Al-Si distribution shows that WH,Ab > WH,Or for both; however WH,Ab and WH,Or are 1.8 and 1.0 kcal/mol less, respectively, for the (monoclinic) disordered series than for the (triclinic) ordered feldspars, probably due to a reduction in the strain associated with K-Na substitution in the less dense, disordered feldspar series

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