The impact of secondary silicate mineral precipitation kinetics on CO₂ mineral storage

The rates of subsurface mineral carbonation are commonly considered to be limited by the dissolution rates of the silicate minerals originally present in the target reservoir rocks. Nevertheless, the rates of secondary silicate precipitation can influence this rate by changing the fluid phase composition during reactive rock-CO₂-water interaction. The degree to which secondary silicate mineral precipitation rates influence the extent and efficiency of mineral carbonation in the subsurface is explored via a suite of geochemical modeling calculations. Calculations were performed using the PHREEQC computer code either by assuming local equilibrium for secondary silicate minerals or calculating their precipitation rates using Transition State Theory-based equations. Calculated carbonation rates of fresh basaltic glass are found to be slower initially when accounting for the sluggish precipitation rates of secondary silicates, including clay minerals and zeolites. These slower rates are due to higher calculated aqueous aluminum concentration, which slows the dissolution rates of the primary basaltic glass. The slower precipitation rates of secondary aluminosilicate minerals, however, may result in less flow path clogging, leading to an overall larger total mineral storage capacity over time. In contrast, the sluggish precipitation rates of secondary silicate minerals accelerate significantly the carbonation rate of altered basalts as a larger percentage of the liberated divalent metals are available for carbonate mineral precipitation. Taken together these results illustrate the importance of considering the rates of secondary silicate precipitation rates to accurately predict the rate and extent of subsurface mineral carbonation efforts.