A refined view of the thermodynamic mixing quantities for alkali feldspars and the quandary of excess configurational entropy

Abstract

Solution calorimetric measurements have been made at 50 °C in 20.1 weight percent HF for three alkali-feldspar ion-exchange series consisting of a total of fifty-five samples. Such a large number of specimens has produced well-defined enthalpies of K-Na mixing (H_ex) for all three series in terms of both H_ex magnitude and H_ex variation with composition. The new results plus earlier-reported data for two additional K-Na exchange series give excellent definition to the decline in maximum H_ex with Al-Si disorder, changing from 7.7 kJ/mol for low albite - microcline to 4.8 kJ/mol for analbite - sanidine. Although maximum H_ex for the four most ordered series occurs at mole fractions potassium of 0.40 ± 0.02, the most disordered series displays enthalpies of mixing that are nearly symmetric with respect to K:Na ratio. It is proposed that both the magnitude and compositional asymmetry of H_ex are related to volume expansion associated with Al-Si disorder, which makes substitution of the relatively larger potassium ion into a sodic feldspar energetically “easier” in disordered than ordered structures.

The combination of our H_ex values with Gibbs free energies of K-Na mixing (G_ex) derived from phase equilibrium data for several different alkali feldspar series has produced data that show variation in entropy of K-Na mixing (S_ex) with Al-Si order. Although total S_ex consists of both vibrational and configurational components, heat capacity data from other laboratories have allowed the computation of excess configurational entropy alone. The latter is found to play an important role in alkali feldspar thermodynamic behavior, varying from strongly negative values for Al-Si disordered alkali feldspars to strongly positive quantities for ordered ones. While short-range K-Na order can explain the former, there is no clear explanation for the latter.