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 WE.Or = 5450 and WE.Ab = 4670 cal/mol, WS.Or = l.5 and WS.Ab = 2.5 cal/K·mol, WV.Or = 0.110 and WV.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.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.