Determination of the Gibbs Free Energy of Formation of Magnesite by Solubility Methods1
- J. A. Kittrick and
- F. J. Peryea2
The current values reported for the Gibbs free energy of formation for magnesite [MgCO3(s)] are bimodally grouped, centered at −1012 kJ mol−1 and in the range −1026 to −1030 kJ mol−1. The solubilities of three natural magnesites in aqueous solution were determined in order to verify the appropriate values. The samples were equilibrated at high solid-to-liquid ratios with solutions that were initially undersaturated or supersaturated with respect to magnesite. The chemical activities of pertinent system components at equilibrium were measured analytically or calculated using an ion-association computer model. The results of 50 solubility determinations yielded a mean Gibbs free energy of formation of −1026.0 ± 2.1 kJ mol−1. Identical values were obtained from undersaturation, using an immiscible displacement technique, and from supersaturated initial conditions. These data suggest that the more negative values reported for the Gibbs free energy of formation are correct; hence, magnesite will be the only thermodynamically-stable Mg carbonate phase in surface or near-surface geochemical environments. The attainment of equilibrium in initially undersaturated systems containing either magnesite only or a mixture of magnesite, kaolinite, and montmorillonite was not impeded by the presence of soluble silicon (Si). This observation indirectly discounts the hypothesis that dissolved Si may form magnesium silicate coatings on magnesite surfaces, thus limiting the rate of dissolution and preventing the establishment of chemical equilibrium.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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