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| Titel |
Revisiting classical silicate dissolution rate laws under hydrothermal conditions |
| VerfasserIn |
Marion Pollet-Villard, Damien Daval, Giuseppe Saldi, Kevin Knauss, Bastien Wild, Bertrand Fritz |
| 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 |
250105623
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| Publikation (Nr.) |
 EGU/EGU2015-5155.pdf |
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| Zusammenfassung |
| In the context of geothermal energy, the relative intensities of primary mineral leaching and
secondary mineral precipitation can affect porosity and permeability of the reservoir, thereby
influencing its hydraulic performance and the efficiency of the geothermal power station.
That is why the prediction of reaction kinetics of fluid/rock interactions represents a critical
issue in this context.
Moreover, in several geothermal systems such as the one of Soultz-sous-Forêts (Alsace,
France), the circulation of aqueous fluids induces only modest modifications of their chemical
composition. Therefore, fluid-rock interactions take place at close-to-equilibrium
conditions, where the rate-affinity relations are poorly known and intensively debated
[1].
To describe more precisely the dissolution processes, our strategy consists in investigating
the dissolution of the main cleavages of K-spar minerals (one of the prevalent primary
minerals in the reservoir of Soultz-sous-Forêts geothermal system) over a wide
range of Gibbs free energy (δG) conditions. The aims are to decipher the impact
of crystallographic orientation and microstructural surface modifications on the
dissolution kinetics and to propose a relation between K-spar dissolution rate and
δG.
Our experimental work relies on a coupled approach which combines classical
experiments of K-spar dissolution monitored by aqueous chemical analyses (ICP-AES) and
innovative techniques of nm- to μm-scale characterization of solid surface (SEM, AFM, VSI)
[2].
Our results confirm that K-spar dissolution is an anisotropic process: we measure a
tenfold factor between the slowest and the fastest-dissolving surfaces. Moreover, the
formation of etch pits on surfaces during their alteration has been evidenced on all of the
different faces that have been studied. This complex evolution of the surface topography
casts doubt of the relevance of a surface model based on shrinking particles and
represents a possible cause of an apparent modification of silicate dissolution rate over
time.
In addition, we evidenced that the relation between K-spar dissolution rate and δG
depends on the crystallographic orientation of the altered surface, and differs from the
transition state theory currently implemented into geochemical codes. Importantly, this
theoretical curve overestimates the dissolution rates measured in close-to-equilibrium
conditions.
Taken together, the new findings show promise as a means for improving the accuracy of
geochemical simulations.
[1] Schott, J., Pokrovsky, O. S., and Oelkers, E. H., 2009. The Link Between Mineral
Dissolution/Precipitation Kinetics and Solution Chemistry. Rev Mineral Geochem 70,
207-258.
[2] Daval, D., Hellmann, R., Saldi, G. D., Wirth, R., and Knauss, K. G., 2013. Linking
nm-scale measurements of the anisotropy of silicate surface reactivity to macroscopic
dissolution rate laws: New insights based on diopside. Geochim Cosmochim Acta 107,
121-134. |
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