Magnesite dissolution and precipitation rates at hydrothermal conditions

Magnesite (MgCO3) is the stable anhydrous member of a series of Mg-carbonates with different degrees of hydration. Despite its relative scarcity in the natural environments, it constitutes an important mineral phase for the permanent sequestration of CO2 as carbonate minerals. Experimental determination of magnesite precipitation and dissolution rates at conditions representative of the storage sites is therefore fundamental for the assessment of magnesite sequestration potential in basaltic and ultramafic rocks and the optimization of the techniques of CO2 storage. Magnesite precipitation rates have been measured using mixed-flow and batch reactors as a function of temperature (100 ≤ T ≤ 200 °C), solution composition and CO2 partial pressure (up to 30 bar). Rates were found to be independent of aqueous solution ionic strength at 0.1 M < I < 1.1 M but decrease significantly with increasing aqueous CO32- activity at pH > 8. All rates obtained from mixed flow reactor experiments were found to be consistent with the model of Pokrovsky et al. (1999) where magnesite precipitation rates are proportional to the concentration of the >MgOH2+ surface species. The study of magnesite crystallization using hydrothermal atomic force microscopy (HAFM) demonstrated the consistency of the rates derived from microscopic measurements with those obtained from bulk experiments and showed that these rates are also consistent with a spiral growth mechanism. According to AFM observations this mechanism controls magnesite growth over a wide range of temperatures and saturation states (15≤ Ω ≤ 200 for 80 ≤ T < 120 °C). Precipitation rates dependence on solution composition recommends the use of relatively high pCO2 to accelerate the rate of the overall carbonation process, avoiding the inhibiting effect of carbonate ions on magnesite precipitation and increasing the rates of Mg-silicate dissolution via acidification of reacting solutions. Determination of magnesite dissolution rates by mixed flow reactor at 150 and 200 °C and at neutral to alkaline conditions allowed us to improve and extend to high temperatures the surface complexation model originally developed at 25 °C. The decrease of dissolution rates observed from 150 to 200 °C can be explained by the increasing carbonation and hydrolysis of the rate controlling >MgOH2+ sites. As a result of the decreasing rates of dissolution, the achievement of alkaline conditions and temperatures higher than 100 °C by CO2-rich fluids represents a favorable condition for CO2 sequestration as dissolved alkalinity in deep aquifers where carbonate minerals are major constituting phases. The use of a hydrogen electrode concentration cell (HECC) corroborates the kinetic data obtained at close to equilibrium conditions by the precise determination of magnesite solubility product as a function of temperature (50-200°C). These measurements allowed generating the thermodynamic properties of this phase and comparing them with those obtained from calorimetric measurements and phase equilibria experiments. The results of this study significantly improve our understanding of the kinetic behaviour of carbonate minerals in hydrothermal systems and provide an essential database for the future study of dissolution/precipitation reactions of carbonate minerals in complex systems. This work also provides important kinetic constraints for the geochemical modeling of CO2 sequestration processes and will help the evaluation of impact and risks connected to a long-term storage.