A TRIBUTE TO JAMES B. THOMPSON, JR.: HIGHLY PRECISE ENTHALPIES OF K-NA MIXING FOR LOW ALBITE – MICROCLINE CRYSTALLINE SOLUTIONS (Invited Presentation)

One focus of Jim Thompson’s research was the thermodynamic properties of mineral series and the proper formulation of those properties. This is reflected by his article “Thermodynamic Properties of Simple Solutions” (Abelson, Ed., 1967, Researches in Geochemistry), followed by a four-part series of papers on feldspar thermodynamics by Thompson & Waldbaum and Waldbaum & Thompson (1968-69, Amer. Mineral.). Although K-Na mixing properties across various feldspar series were studied by Thompson students (Waldbaum, Hovis), that work was based on data at a limited number of compositions, necessitated at the time by the large sample sizes required for HF solution calorimetric measurements. Thermodynamic mixing properties for mineral series, especially those exhibiting compositionally asymmetric mixing quantities, are best defined when samples at a large number of compositions are utilized. Enabled by the small sample sizes now possible for HF solution calorimetry (Hovis et al., 1998, Amer. Mineral.), we revisit feldspar thermodynamics, having synthesized a low albite - microcline series consisting of samples at 21 compositions. Solution calorimetric experiments on these samples at 50 °C in 20.1 wt% HF under isoperibolic conditions have resulted in highly precise calorimetric data (average standard deviation 0.06% of the heats of solution). Data for the seven samples having compositions at N_Or (mole fraction K) < 0.23 have revealed important new detail at the sodic end of the series, where heats of solution change abruptly with composition. Enthalpies of mixing calculated from the calorimetric data display well-defined asymmetry, showing a maximum value of 8.7 kJ/mol at N_Or = 0.39. The greater mixing magnitudes for this series relative to those for analbite - sanidine solutions (newly investigated via a 20-sample series) correlate well with the comparatively high critical temperature of the low albite - microcline solvus (Bachinski & Müller, 1971, J. Petrology). Entropies of K-Na mixing have been calculated by combining present enthalpy data with Gibbs free energies of mixing derived from phase equilibrium studies (Delbove, 1975, Amer. Mineral.; Bachinski & Müller, ibid; Hovis et al., 1991, Amer. Mineral.); these may be compared with entropy data based on the heat capacity measurements of other investigators.