Acquisition and Evaluation of Thermodynamic Data for Bieberite-Moorhouseite Equilibria at 0.1 MPa

Published estimates for the equilibrium relative humidity (RH) at 25 °C for the reaction: bieberite (CoSO4·7H2O) = moorhouseite (CoSO4·6H2O) + H2O, range from 69.8 to 74.5%. To evaluate these data, the humidity-buffer technique was used to determine equilibrium constants for this reaction between 14 and 43 °C at 0.1 MPa. Reversals along fi ve 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 well with several previously reported values based on vaporpressure measurements. It also agrees well with values calculated from the data derived mostly from calorimetric measurements. Previous studies indicated that the temperature of the invariant point for the assemblage bieberite-moorhouseite-aqueous solution-vapor is near 44.7 °C, and our extrapolated data predict 91.1% RH at this temperature; the predicted position for the invariant point is in excellent agreement with those reported previously. CHOU AND SEAL: BIEBERITE-MOORHOUSEITE EQUILIBRIA 913 (Jambor and Boyle 1965). Moorhouseite forms complete solid solution with nickelhexahydrite at 61 °C, and a miscibility gap exists at lower temperatures (Rohmer 1939). FeO substitution at about 27 mol% in moorhouseite at 50 °C was reported by Balarew and Karaivanova (1976). Other hydrated cobalt sulfate minerals, including aplowite (CoSO4·4H2O) and cobaltkieserite (CoSO4·H2O; approved by IMA 2002), will not be discussed in this study. Dehydration equilibrium The stability of bieberite and moorhouseite can be related by the reaction: CoSO4·7H2O(s) = CoSO4·6H2O(s) + H2O(g) (1) bieberite moorhouseite where (s) and (g) are solid and gas, respectively. Published estimates for the equilibrium relative humidity (RH) at 25 °C range from 69.8 to 74.5% for the reaction. To evaluate these data, the humidity-buffer technique (Polyanskii et al. 1976; Malinin et al. 1977; 1979; Chou et al. 2002) was used in this study to determine equilibrium constants for this reaction between 14 and 43 °C at 0.1 MPa. Reversals were obtained along fi ve humiditybuffer curves. It should be emphasized that Reaction 1 of this study does not involve an aqueous phase. However, as will be presented later, in the presence of an additional aqueous phase at equilibrium at 0.1 MPa, the system becomes invariant with defi ned equilibrium temperature and humidity. The standard Gibbs free energy of reaction, ΔGr°, for Reaction 1 was then derived from the equilibrium constant, K, using the relation: ΔGr° = – RT ln K = – RT ln (aH2O) = – RT ln (fH2O/0.1) = – RT ln [(f*H2O/0.1) · (%RH)/100], (2) where R is the gas constant (8.31451 J/mol·K); T is absolute temperature; aH2O is the activity of H2O; fH2O is the equilibrium H2O fugacity (in MPa); and f*H2O is the fugacity of pure H2O (in MPa). The standard states for minerals and H2O are pure solids and H2O gas, respectively, at 0.1 MPa and temperature. Preliminary results were presented by Chou and Seal (2003c).