| Abstract |
Feldspar minerals are part of the most abundant minerals in the earth crust. Predicting their long term evolution dissolution represents a critical issue for several geological and engineering concerns. For example, in the context of the soultz-sous-forêts enhanced geothermal system (Alsace, france), the pumping of hot water and exploitation of heat at surface may provoke a partial re-equilibration of the aqueous fluid composition, and re-injection of cooled water at depth may favor the dissolution of the main rock-forming minerals of the reservoir (Such as k-feldspar) While promoting the precipitation of secondary phases (Fritz et al., 2010). The relative intensities of primary mineral leaching and secondary phase formation significantly affect porosity and permeability of the reservoir, thereby influencing its hydraulic performance and the efficiency of the geothermal site. To determine the long term evolution of fluid/Rock interactions in natural environments, the common strategy consists in investigating experimentally, the influence of different parameters (Ph, temperature, concentration of cations, deviation from equilibrium….) On dissolution rates in conditions in which dissolution is relatively fast (High temperature, far-from equilibrium conditions…) To be measured. Then, resulting kinetic rate laws are implemented in geochemical codes which are ultimately used to make an extrapolation to field conditions. With this method, five orders of magnitude are reported between the laboratory-based silicate weathering rates and those measured in field (White and brantley, 2003). In actual reactive transport codes, the evolution of surface area is related to the amount of dissolving mineral through a shrinking core model. The dissolution is considered as to be isotropic. However, recent studies suggest that some faces could dissolve up to 1,000 times faster than other (Daval et al., 2013). Besides, etch pitting is responsible for the development of microfacets less energetic (I.E. Less reactive) During long-term dissolution (See fig. 1 and e.G. Smith et al., 2013). These results cast doubt of the relevance of a surface model based on shrinking spheres. The aim of this study is to quantify the impact of k-feldspar anisotropy on dissolution rates in order to develop a more comprehensive model of the evolution of the reactive surface of silicate minerals. Our global objective is to propose alternate kinetic rate laws which could in turn be implemented into reactive transport codes and ultimately improve the predictive ability of geochemical simulations. |