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

Authors

I-Ming Chou, 954 National Center, U.S. Geological Survey, Reston, Virginia 20192, U.S.A.Follow
Robert Seal II, U.S. Geological Survey, Reston, Virginia 20192, U.S.A.

Date of this Version

2005

Comments

Published in American Mineralogist, Volume 90, pages 912–917, 2005.

Abstract

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.