WELL-CONSTRAINED ENTHALPIES OF K-NA MIXING IN AL-SI ORDERED AND DISORDERED ALKALI FELDSPAR SERIES FROM NEW HF SOLUTION CALORIMETRIC MEASUREMENTS ON FORTY-ONE SAMPLES

Enthalpies of K-Na mixing for microcline – low albite and sanidine – analbite ion-exchange series determined more than a quarter century ago (Hovis, 1988, Journal of Petrology) were based on just 7 samples for each series, owing to the large sample size (one gram per experiment) required for HF solution calorimetric measurements at the time. Results from those studies revealed subtle compositional asymmetry in the heats of mixing for microcline – low albite, but symmetric relationships for sanidine – analbite. With the advent of smaller calorimetric sample sizes for such experiments (Hovis, Roux, and Pascal, 1998, American Mineralogist), it is now possible to study a greater number of samples in order to better characterize heats of mixing. Thus, we have synthesized two new K-Na ion-exchange series consisting of 21 Al-Si disordered and 20 Al-Si ordered feldspar samples. Heats of solution have been measured for all samples in 20.1 weight % HF at 50 °C under isoperibolic conditions (temperature of the calorimeter "surroundings" held constant). The new data more precisely define both the magnitude and asymmetry of the mixing quantities for both series. Maximum enthalpies of K-Na mixing for the sanidine – analbite series are, in fact, skewed toward sodic compositions, consistent with asymmetry of the corresponding alkali feldspar solvus (Smith and Parsons, 1974, Mineralogical Magazine). Maximum enthalpies of mixing for microcline – low albite are skewed in the same direction, but greater in magnitude than the latter, correlating well with the higher critical temperature of the solvus for Al/Si-ordered feldspars (Bachinski and Müller, 1971, Journal of Petrology). Interestingly, though, the interrelationships among (1) Gibbs free energies of mixing calculated from phase equilibrium studies (e.g., Delbove, 1975, American Mineralogist; Hovis, Delbove, and Bose, 1991, American Mineralogist), (2) enthalpies of mixing from the current work and (3) entropies of mixing from heat capacity measurements (Haselton, Hovis, Hemingway, and Robie, 1983, American Mineralogist; Benisek, Dachs and Kroll, 2014, American Mineralogist) remain an enigma. One way of bringing all data into agreement is to invoke the presence of short-range order in these minerals, as suggested in our earlier work and also by Benisek et al. (ibid).