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| Titel |
Transport properties of interfacial Si-rich layers formed on silicate minerals during weathering: Implications for environmental concerns |
| VerfasserIn |
Damien Daval, Laurent Remusat, Sylvain Bernard, Bastien Wild, Jean-Sébastien Micha, François Rieutord, Alejandro Fernandez-Martinez |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250107954
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| Publikation (Nr.) |
 EGU/EGU2015-7678.pdf |
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| Zusammenfassung |
| The dissolution of silicate minerals is of primary importance for various processes ranging from chemical weathering to CO2 sequestration. Whether it determines the rates of soil formation, CO2 uptake and its impact on climate change, channeling caused by hydrothermal circulation in reservoirs of geothermal power plants, durability of radioactive waste confinement glasses or geological sequestration of CO2, the same strategy is commonly applied for determining the long term evolution of fluid-rock interactions. This strategy relies on a bottom-up approach, where the kinetic rate laws governing silicate mineral dissolution are determined from laboratory experiments. However, a long-standing problem regarding this approach stems from the observation that laboratory-derived dissolution rates overestimate their field counterparts by orders of magnitude, casting doubt on the accuracy and relevance of predictions based on reactive-transport simulations.
Recently [1], it has been suggested that taking into account the formation of amorphous Si-rich surface layers (ASSL) as a consequence of mineral dissolution may contribute to decrease the large gap existing between laboratory and natural rates. Our ongoing study is aimed at deciphering the extent to which ASSL may represent a protective entity which affects the dissolution rate of the underlying minerals, both physically (passivation) and chemically (by promoting the formation of a local chemical medium which significantly differs from that of the bulk solution). Our strategy relies on the nm-scale measurement of the physicochemical properties (diffusivity, thickness and density) of ASSL formed on cleavages of a model mineral (wollastonite) and their evolution as a function of reaction progress.
Our preliminary results indicate that the diffusivity of nm-thick ASSL formed on wollastonite surface is ~1,000,000 times smaller than that reported for an aqueous medium, as estimated from the monitoring of the progression of a tracer in nm-thick ASSL by means of nanoSIMS analyses. However, estimated diffusion coefficients remain up to 20 orders of magnitude greater than those corresponding to solid-state diffusion, indicating that such ASSL were not formed through a solid-state transformation of wollastonite. In addition, the estimation of diffusion coefficients of the tracer for samples recovered after two different time durations indicates that such a diffusion coefficient may decrease with time. This result may be explained by the spontaneous evolution of the density of ASSL, which increased as the dissolution reaction proceeds, as shown from modeling of time-resolved in situ X-ray reflectivity spectra obtained on the dissolving surfaces. Overall, these results emphasize the need for the recognition of the importance of the interfacial fluid/mineral medium for adequately modeling the dissolution kinetics of silicate minerals relevant for environmental concerns.
[1] Daval, D. et al., 2013. Geochim Cosmochim Acta 107, 121-134. |
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