ACQUISITION AND EVALUATION OF THERMODYNAMIC DATA FOR BIEBERITE-MOORHOUSEITE EQUILIBRIA AT 0.1 MPA

Metal sulfates are common constituents in mine drainage settings, both as dissolved species and as solids. Thermodynamic data for end-member salts are essential for understanding more complex natural systems and for modeling speciation in high ionic strength solutions.  Thermodynamic data for these salts are either not available or in poor agreement.  This study extends the database to the Co end-member system.

Published estimates for the equilibrium relative humidity (RH) for the reaction: Bieberite (CoSO₄•7H₂O) = Moorhouseite (CoSO₄•6H₂O) + H₂O, at 25 ¢XC range from 69.8 to 74.5%.  To evaluate these data, the humidity-buffer technique (Chou et al., 2002, Amer. Mineral., 87, 108-114) was used to determine equilibrium constants for this reaction between 14 and 43 ¢XC at 0.1 MPa.  Reversals along five humidity-buffer curves yield ln K=18.03- 6509.43/T, where K is the equilibrium constant, and T is temperature in K. The derived standard Gibbs free energy of reaction is 9.43 kJ/mol, which agrees very well with values based on the vapor pressure measurements of Schumb (1923, J. Am. Chem. Soc., 45, 342-345), Carpenter and Jette (1923, J. Am. Chem. Soc., 45, 578-590), and Boers and Van Welie (1965, Rec. Trav. Chim., 84, 789-798). It also agrees well with values calculated from the data compiled by Wagman et al. (1982, J. Phys. Chem. Ref. Data, v. 11, Sup. No. 2) and DeKock (1982, Bur. Mines Inf. Cir., 9810).  Previous studies indicate that the invariant point for the assemblage bieberite-moorhouseite-aqueous solution-vapor is located at 45.1 ¢XC and 91.4 %RH (Carpenter and Jette, ibid.), and 44.7 ¢XC and 89.9 %RH (Boers and Van Welie, ibid.). Our extrapolated data predict 91.6 %RH at 45.1 ¢XC and 91.2 %RH at 44.7 ¢XC for this invariant point; the predicted RH values are in excellent agreement with those reported previously.