Effect of Water Activity on Reaction Kinetics and Intergranular Transport: Insights from the Ca(OH) 2 + MgCO 3 → CaCO 3 + Mg(OH) 2 Reaction at 1·8 GPa

The kinetics of the irreversible reaction Ca(OH)2 + MgCO3 → CaCO3 + Mg(OH)2 were investigated at high pressures and temperatures relevant to metamorphic petrology, using both in situ synchrotron X-ray diffraction and post-mortem analysis of reaction rim growth on recovered samples. Reaction kinetics are found to strongly depend on water content; comparable bulk-reaction kinetics are obtained under water-saturated (excess water, c. 10 wt %) and under intermediate (0·1–1 wt % water) conditions when temperature is increased by c. 300 K. In contrast, similar reaction kinetics were observed at ∼673 K and 823 K between intermediate and dry experiments, respectively, where dry refers to a set of experiments with water activity below 1·0 (no free water), as buffered by the CaO–Ca(OH)2 assemblage. Given the activation energies at play, this gap—corresponding to the loss of no more than 1 wt % of water by the assemblage—leads to a difference of several orders of magnitude in reaction kinetics at a given temperature. Further analysis, at the microscopic scale, of the intermediate and dry condition samples, shows that intergranular transport of calcium controls the reaction progress. Grain boundary diffusivities could be retrieved from the classic treatment of reaction rim growth rate. In turn, once modeled, this rate was used to fit the bulk kinetic data derived from X-ray powder diffraction, offering an alternative means to derive calcium diffusivity data. Based on a comparison with effective grain boundary data for Ca and Mg from the literature, it is inferred that both dry and intermediate datasets are consistent with a water-saturated intergranular medium with different levels of connectivity. The very high diffusivity of Ca in the CaCO3 + Mg(OH)2 rims, in comparison with that of Mg in enstatite rims found by earlier workers, emphasizes the prominent role of the interactions between diffusing species and mineral surfaces in diffusion kinetics. Furthermore, we show that the addition of water is likely to change the relative diffusivity of Mg and Ca in carbonate aggregates. From a qualitative point of view, we confirm, in a carbonate-bearing system, that small water content variations within the 0–1 wt % range have tremendous effects on both intergranular transport mechanisms and kinetics. We also propose that the water content dependent diffusivity of major species (Mg, Ca) in low-porosity metamorphic rocks is strongly dependent on the interaction between diffusing species and mineral surfaces. This parameter, which will vary from one rock-type to another, needs also to considered when extrapolating (P, T, t, xH2O) laboratory diffusion data to metamorphic processes

Influence of amorphous silica layer formation on the dissolution rate of olivine at 90°C and elevated pCO2

International audienceFor mitigating against rising levels of atmospheric CO2, carbonation of M2+-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 °C 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 (SA/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

NO solubility in water and brine up to 60 MPa and 373 K by combining Raman spectroscopy and molecular simulation

International audienceIn the processes of Carbon Capture and Storage, sulphur and nitrogen oxides (SOx and NOx) would be possibly injected with CO2 depending on the origin of CO2. The thermodynamic properties of these gases in saline aquifer are poorly known. Solubility is one of the key parameter to be implemented in geochemical codes modeling long-term evolution of the aquifer after injection. The solubility of NO is known only at atmospheric pressure. In this study, the solubility of NO in water and NaCl solutions was measured by Raman spectroscopy in the ranges 295-373 K and 2-60 MPa using a High Pressure Optical Cell and after calibration on a few data from molecular simulations. The results show a decrease of solubility when temperature increases and when salinity increases. No modification of NO speciation was observed