Thin Water Films Enable Low-Temperature Magnesite Growth Under Conditions Relevant to Geologic Carbon Sequestration.

Injecting supercritical CO (scCO) into basalt formations for long-term storage is a promising strategy for mitigating CO emissions. Mineral carbonation can result in permanent entrapment of CO; however, carbonation kinetics in thin HO films in humidified scCO is not well understood. We investigated forsterite (MgSiO) carbonation to magnesite (MgCO) via amorphous magnesium carbonate (AMC; MgCO·HO, 0.5 < < 1), with the goal to establish the fundamental controls on magnesite growth rates at low HO activity and temperature. Experiments were conducted at 25, 40, and 50 °C in 90 bar CO with a HO film thickness on forsterite that averaged 1.78 ± 0.05 monolayers. In situ infrared spectroscopy was used to monitor forsterite dissolution and the growth of AMC, magnesite, and amorphous SiO as a function of time. Geochemical kinetic modeling showed that magnesite was supersaturated by 2 to 3 orders of magnitude and grew according to a zero-order rate law. The results indicate that the main drivers for magnesite growth are sustained high supersaturation coupled with low HO activity, a combination of thermodynamic conditions not attainable in bulk aqueous solution. This improved understanding of reaction kinetics can inform subsurface reactive transport models for better predictions of CO fate and transport.