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

Enthalpies of solution have been measured on a series of musconite-par agonite micas in 20.1% HF at 50 degrees C under isoperibolic condition s. The molar enthalpy of formation of paragonite at 298 K, for which n o calorimetrically measured value is currently available, has been det ermined to be -5937.5 (+/-3) kJ. An inversion of all calorimetric, vol umetric 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 2M(1) micas for enthalpy (H-e x) and volume (V-ex) at pressures up to 10 kbar, forcing excess entrop y (S-ex) to be zero and using a subregular mixing model are favoured: H-ex(kJ) = [10.6+4.45(1-2X(ms))]X(ms)(1-X(ms)) V-ex(J/bar) = 0.452 X(m s),(1-X(ms)). However, mixing models of higher order with asymmetric n egative S-ex are also possible.