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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

Gasc, J; Brunet, F; Brantut, N; Corvisier, J; Findling, N; Verlaguet, A; Lathe, C; (2016) 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. Journal of Petrology , 57 (7) pp. 1389-1408. 10.1093/petrology/egw044. Green open access

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Abstract

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.

Type: Article
Title: 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
Open access status: An open access version is available from UCL Discovery
DOI: 10.1093/petrology/egw044
Publisher version: http://dx.doi.org/10.1093/petrology/egw044
Language: English
Additional information: Copyright © The Author 2016. Published by Oxford University Press. All rights reserved. This is a pre-copyedited, author-produced PDF of an article accepted for publication in the Journal of Petrology following peer review. The version of record [Gasc, J et al (2016) Effect of Water Activity on Reaction Kinetics and Intergranular Transport. Journal of Petrology , 57 (7) pp. 1389-1408. 10.1093/petrology/egw044] is available online at: http://dx.doi.org/10.1093/petrology/egw044
Keywords: metamorphic reaction; kinetics; intergranular transport; water; calcium diffusion
UCL classification: UCL
UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Earth Sciences
URI: https://discovery.ucl.ac.uk/id/eprint/1521315
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