THERMODYNAMIC MIXING MODELS FOR MUSCOVITE–PARAGONITE SOLUTIONS BASED ON SOLUTION CALORIMETRIC AND PHASE EQUILIBRIUM DATA

Abstract

Enthalpies of solution have been measured on a series of muscovite—paragonite micas in 20.1% HF at 50°C under isoperibolic conditions. The molar enthalpy of formation of paragonite at 298 K, for which no calorimetrically measured value is currently available, has been determined to be ?5937.5 (?3) kJ. An inversion of all calorimetric, volumetric and phase equilibrium data has been performed, giving a range of mixing models compatible with most experimental data. The following expressions of the mixing properties of 2M1 micas for enthalpy (Hex) and volume (Vex) at pressures up to 10 kbar, forcing excess entropy (Sex) to be zero and using a subregular mixing model are favoured: Hex(kJ) = [10.6+4.45(1?2Xms)]Xms(1?Xms) Vex(J/bar) = 0.452Xms(1?Xms). However, mixing models of higher order with asymmetric negative Sex are also possible.