THERMODYNAMICS OF MIXING OF LIQUIDS IN THE SYSTEM CA3(PO4)2-CACL2-CAF2-CA(OH)2

Molten calcium halide and hydroxide salts may be used as flux and reac tants in determining apatite exchange equilibria and solid solution be havior. New experimental data and published phase equilibria are used to determine a thermodynamic model of melts and solids along the binar y joins of the anhydrous apatite system Ca3(PO4)2-CaCl2-CaF2-Ca(OH)2. In this model, melt components are expressed as Ca0.5X, where X is hyd roxide or a halogen, or as Ca1.5PO4. The derived binary interaction pa rameters (W(G)'s) are sufficient to describe deviations from Raoultian behavior for the data. Standard state data are derived for molten Ca( OH)2 and Ca3(PO4)2, and for the intermediate compounds CaClF, CaClOH, and Ca2PO4Cl. Melts in the CaCl2-CaF2 system show ideal mixing behavio r, while the calcium hydroxide-bearing molten salts form asymmetric re gular solutions. The hydroxide-bearing melts show the small positive e nthalpies of mixing typical for mixed-anion salts. Similar behavior is measured in simpler molten salt mixtures. The positive deviations fro m Raoultian behavior are not a mathematical artifact of the model. Dat a are less extensive for the salt-phosphate systems but cover the esse ntial portions of the systems where salt to phosphate ratios are high. The mixtures of the molten salts and phosphates salts show large nega tive enthalpy interaction parameters (W(H)), as is expected in eutecti c systems which mix a high-melting-point crystal with a low-melting-po int flux. Adequate reproduction of the data requires that some entropy interaction parameters be negative, although small. This implies the presence of ordering in the melt, which is manifested in more polymeri zed phosphate liquids and glasses as halogenated and hydroxylated orth ophosphate and pyrophosphate species. Results of the model indicate th at the quaternary system is a good choice for determination of activit y-composition relations for the apatites. Comparison of common sources of standard state thermodynamic data (ROBIE et al., 1979; JANAF, CHAS E et al., 1985; CODATA, GARVIN et al., 1987) shows that the data for t he salts are very similar where they are not identical. At temperature s below 900-degrees-C in the ternary systems, liquid compositions will be on or near the apatite-crystalline salt cotectics, and dissolve le ss than 1 mol% Ca3(PO4)2. This makes CaCl2-CaF2 mixtures excellent can didates for determination of unambiguous apatite activity-compositiona l relations. Only the Ca(OH)2 liquidus is strongly affected by the add ition of phosphate, but this effect is closely described by a regular solution model.