DETERMINATION OF EPSOMITE-HEXAHYDRITE EQUILIBRIA BY THE HUMIDITY-BUFFER TECHNIQUE AT 0.1 MEGAPASCAL

Epsomite (MgSO₄•7H₂O) and hexahydrite (MgSO₄•6H₂O) are common minerals found in marine evaporite deposits and as precipitates in saline lakes. They also occur in weathering zones of coal and metallic deposits as well as in some soils and their efflorescences (Jambor et al., 2000, Rev. Mineral. Geochem., v. 40, p. 303). These minerals have been proposed to have formed as evaporite deposits on the surface of Europa, a satellite of Jupiter (McCord et al. 1998, Science, v. 280, p. 1242). Hexahydrite can also form as concretions in humans (Gibson, 1974, Amer. Mineral., v. 59, p. 1177).

Thermodynamic properties of these two minerals reported in the literature are in poor agreement. For example, published estimates for the equilibrium relative humidity (RH) for the reaction: Epsomite=Hexahydrite + H₂O, at 25 °C range from 36 to 53%. These discrepancies hinder our capability for estimating their stability in various physical conditions. To resolve these discrepancies, the humidity-buffer technique (Chou et al., 1998, Eos, v. 79, p. S364) was used to determine equilibrium constants for this reaction between 25 and 45 °C at 0.1 MPa. Reversals along four humidity-buffer curves yield ln K=20.00 + 7182.07/T, where K is the equilibrium constant, and T is temperature in K. The derived standard Gibbs free energy of reaction is 10.1 kJ/mol, which is essentially the same value as that calculated from the vapor pressure measurements of Carpenter and Jette (1923, J. Am. Chem. Soc., v. 45, p. 578). However, this value is 1.0 and 0.8 kJ/mol lower than those calculated from the data compiled by Wagman et al. (1982, J. Phys. Chem. Ref. Data, v. 11, Sup. No. 2) and DeKock (1986, Bur. Mines Inf. Cir., 9081), respectively. Many previous observations indicate that the temperature of the invariant point for the assemblage epsomite-hexahydrite-aqueous solution-vapor is near 48 °C (Link, 1965, Solubilities, vol. 2). When extrapolated to this temperature, our data predict RH=84.3% at this invariant point, which agrees well with RH=84.7% based on the vapor-pressure measurements of hexahydrite-saturated solutions reported by Carpenter and Jette (1923), and also agrees reasonably well with RH=82.8% predicted by Pitzer’s model for epsomite-saturated solutions. This correspondence with previous experimental and predicted data strongly supports our results.