Phase relations in the system NaCl-KCl-H₂O. III: Solubilities of halite in vapor-saturated liquids above 445°C and redetermination of phase equilibrium properties in the system NaCl-H₂O to 1000°C and 1500 bars

Halite solubilities along the three-phase curve in the binary system NaCl-H₂O determined by DTA experiment can be represented by the equation Wt.% NaCl (±0.2) = 19.39 − 0.0364t + 3.553 × 10⁻⁴T² − 2.298 × 10⁻⁷T³, where 447≦ T ≦ 800°C. Even though these halite solubilities are up to ~7 wt.% higher than those reported in literature, extrapolated values at temperatures below 447°C merge with the literature values. It is considered that the equation adequately describes halite solubilities between 382 and 800°C. The newly established solubility data are believed to be more reliable because they are compatible with data obtained by using synthetic fluid inclusions and with the observed DTA signals and also because they were measured in a relatively corrosion-free system.

In an earlier publication (GUNTER et al., 1983), we were puzzled greatly by multiple and rather unreproducible DTA peaks appearing during isobaric cooling (heating experiments were nondefinitive) at pressures below about 500 bars. These DTA signals apparently suggested that the “halite liquidus” swung sharply upward in temperature as pressure decreased from about 500 bars to that of the halite-saturated boiling curve. Further analysis of the data and helpful discussions with several individuals have revealed that the behavior is a consequence of the initial (precooling) separation of the fluid into NaCl-poor gas and NaCl-rich liquid that failed to homogenize in the short time encompassed by the DTA experiments. The present analysis is based on extrapolations of the  slopes from pressures above 500 bars.

Through use of these new halite solubility data and the data from synthetic fluid inclusions [formed by healing fractures in inclusion-free Brazilian quartz in the presence of two coexisting, immiscible NaCl-H₂O fluids at various temperatures and pressures (Bodnar et al., 1985)], phase equilibria in the system NaCl-H₂O have been redetermined to 1000°C and 1500 bars.