Influence of amorphous silica layer formation on the dissolution rate of olivine at 90 °C and elevated pCO 2

Abstract

For mitigating against rising levels of atmospheric CO 2, carbonation of M 2+-bearing silicates has been proposed as a possible option for sequestering CO 2 over long time spans. Due to its rapid far-from-equilibrium dissolution rate and its widespread occurrence in mafic and ultramafic rocks, olivine has been suggested as a potentially good candidate for achieving this goal, although the efficacy of the carbonation reaction still needs to be assessed. With this as a goal, the present study aims at measuring the carbonation rate of San Carlos olivine in batch experiments at 90 °C and pCO 2 of 20 and 25 MPa. When the reaction was initiated in pure water, the kinetics of olivine dissolution was controlled by the degree of saturation of the bulk solution with respect to amorphous silica. This yet unrecognized effect for olivine was responsible for a decrease of the dissolution rate by over two orders of magnitude. In long-term (45 days) carbonation experiments with a high surface area to solution volume ratio ( SA/ V = 24,600 m −1), the final composition of the solution was close to equilibrium with respect to SiO 2(am), independent of the initial concentration of dissolved salts (NaCl and NaClO 4, ranging between 0 and 1 m), and with an aqueous Mg/Si ratio close to that of olivine. No secondary phase other than a ubiquitous thin (≤ 40 nm), Si-rich amorphous layer was observed. These results are at odds with classic kinetic modeling of the process. Due to experimental uncertainties, it was not possible to determine precisely the dissolution rate of olivine after 45 days, but the long term alteration of olivine was indirectly estimated to be at least 4 orders of magnitude slower than predicted. Taken together, these results suggest that the formation of amorphous silica layers plays an important role in controlling the rate of olivine dissolution by passivating the surface of olivine, an effect which has yet to be quantified and incorporated into standard reactive-transport codes.