Ab initio thermodynamics of magnesium carbonates and hydrates in water-saturated supercritical CO2 and CO2-rich regions

Abstract ab initio Thermodynamics is used to determine how free energies of magnesium carbonates and hydrates in a CO 2 -rich environment change with water concentration across a range temperature and pressure relevant to geochemistry and carbon sequestration (275 K to 375 K and pCO 2 of 1 to 210 bar). The methodology is based on first principles density-functional theory (DFT) calculations of the total energies and vibrational entropy of periclase, magnesite, brucite, nesquehonite, and hydromagnesite coupled to the experimental chemical potentials of CO 2 and H 2 O. The impact of water in supercritical CO 2 (scCO 2 ), even though less than 1% at saturation, is found to have a significant impact on the stability of hydrated carbonate minerals. Hydromagnesite and nesquehonite are found to be more thermodynamically stable than periclase and brucite in water-saturated scCO 2 and hence may be expected to result kinetically from carbonation of these minerals during CO 2 sequestration. Under dehydrating conditions nesquehonite destabilizes rapidly, whereas hydromagnesite is much more likely to persist in a CO 2 -rich environment, consistent with the observations that hydromagnesite is widespread in nature and nesquehonite is relatively rare.