『Abstract
　For mitigating against rising levels of atmospheric CO2, carbonation of Mg²⁺-bearing silicates has been proposed as a possible option for sequestering CO2 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℃ and pCO2 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 (SAC/V = 24,600 m^-1), the final composition of the solution was close to equilibrium with respect to SiO2 (am), independent of the initial concentration of dissolved salts (NaCl and NaClO4, 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.

Keywords: Olivine; Carbonation; CO2 sequestration; Passivation; Kinetic modeling』

1. Introduction
2. Materials and methods
　2.1. Starting materials
　2.2. Experimental equipment and protocols
　　2.2.1. High SAC/V experiments
　　2.2.2. Low SAC/V experiment
　2.3. Characterization and analytical procedures
　　2.3.1. Fluid analyses
　　2.3.2. Bulk solid analyses (ENS experiments)
　　2.3.3. Microscopic observations
　2.4. Thermodynamic calculations and kinetic modeling
3. Results
　3.1. High SA/V experiments
　　3.1.1. Estimation of magnesite content and microscopic observations
　　3.1.2. Solution chemistry at the end of each run
　3.2. Low SA/V experiment
4. Discussion
　4.1. Dissolution of olivine: measured and predicted release of Mg and Si　
　4.2. Rate-controlling step of olivine carbonation at 90℃ in closed systems
　4.3. Implications for long-term olivine-water-CO2 interactions at 90℃
　4.4. Understanding the physical and chemical properties of silica layers
5. Conclusions
Acknowledgments
Appendix A
　Calculation of the upper bound of Mg release rate, based on the STEM-EDXS profile of Mg
References