gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/minmag.1994.58A.1.84 A New Method for Determining Excess Gibbs Energies in Binary Metal-Carbonate Solid Solutions Casey W. H. Department of Land, Air and Water Resources and the Department of Geology, University of California, Davis, CA 95616, USA Rock P. A. Department of Chemistry, University of California, Davis, CA 95616, USA McBeath M. M. Department of Land, Air and Water Resources, University of California, Davis, CA 95616, USA Walling E. M. Department of Chemistry, University of California, Davis, CA 95616, USA Chung J. Department of Land, Air and Water Resources University of California, Davis, CA 95616, USA 8 1994 58A 1 156 157 Copyright © 1994, The Mineralogical Society 1994 Summary

This electrochemical method provides a direct measure of the desired thermodynamic data, as opposed to techniques that obtain similar data from small differences between large numbers. The precision of the measured cell voltages for reversible double cells is in the range of ±0.05 to 0.50 mV, which makes the method sufficiently sensitive to detect small changes in the Gibbs energies. In addition, calculation of the Gibbs energies does not require a model for speciation of solutes in complex mixtures, there is also no correction for a liquid-liquid junction potential, and the reversibility of the cell reaction can be established.