Gibbs energies and entropies of K-Na mixing for alkali feldspars from phase equilibrium data; Implications for feldspar solvi and short-range order

Reversed brackets have been achieved at T = 800 °C and P near 1 bar for K-Na partitioning between (K,Na)Br molten salts and members of two topochemically mono-clinic alkali feldspar series that differ substantially in Al-Si distribution. The Gibbs energies of K-Na mixing derived from the phase equilibria for the two series are similar, but significantly less positive than those of a highly ordered topochemically triclinic series. These contrasting relations are consistent with higher critical temperatures for low micro-cline–low albite (Bachinski and Müller, l97l) and adularia-based feldspars, relative to highly disordered series.

Calculated entropies of mixing for highly disordered alkali feldspars, based both on this work and on solvus data, are somewhat uncertain; however, they are asymmetric with respect to composition and lower in magnitude than those from direct low-temperature heat capacity measurements. This, in turn, implies short-range order in such feldspars, consistent with recent observations in a more ordered low microcline–low albite series.

Recommended Margules parameters for internal energy, entropy, and volume for disordered alkali feldspars are W_E.Or = 5450 and W_E.Ab = 4670 cal/mol, W_S.Or = l.5 and W_S.Ab = 2.5 cal/K·mol, W_V.Or = 0.110 and W_V.Ab = 0.078 cal/bar.mol. These parameters reproduce reversed low- and high-pressure solvi based on direct measurements by other investigators and are consistent with enthalpies of mixing from HF solution calorimetry as well as excess Gibbs energies from the present investigation.