REACTION OF SERPENTINE WITH H2O-CO2 FLUIDS TO FORM MAGNESITE: MINERAL SEQUESTRATION OF CO2

Magnesite veins are a common alteration feature of serpentinites. In many places, these veins originated by reaction of serpentine with immiscible H₂O-CO₂ fluids in the shallow crust. These natural reactions suggest a means of addressing the continuing rise of global CO₂ emissions: serpentine deposits could be mined and reacted in an analogous aqueous process with CO₂ derived from a power plant. The resulting Mg-carbonate could be buried at the mine site. Serpentine is a highly suitable candidate for mineral sequestration because of its abundance and the stability of the resulting carbonate (magnesite).

In this study, we report experimental investigations of the reaction of serpentine in the H₂O-CO₂ system at moderate T (<200 °C) and P (<200 bars). At these conditions, CO₂ dissolves into the aqueous phase but also forms an immiscible H₂O-bearing phase with properties similar to supercritical CO₂. Thermodynamic calculations show that the formation of magnesite from serpentine is favorable. The autoclave experiments show that it is possible to rapidly carbonate serpentine in a two-step process: acid dissolution (e.g., HCl) followed by neutralization with a base (e.g., NaOH) and application of CO₂. However, autoclave experiments with weak acids do not result in magnesite. Our measured dissolution rates of serpentine and geochemical modeling indicate that the moderate pH experiments are limited by a combination of slow dissolution rates and nucleation barriers for magnesite.

Fluids associated with natural magnesite veins provide a potential clue to enhanced rates of reactivity: they often contain organic compounds. We have found that dissolution rates can be improved with the use of weak organic acids that complex with Mg²⁺. Autoclave experiments with these ligands have demonstrated greatly enhanced dissolution at moderate pH but none have produced magnesite, apparently because the Mg-ligand is resistant to attack by carbonic acid. The natural systems demonstrate that magnesite formation is possible but methods of rapidly reacting serpentine in the laboratory remain elusive.