Dissolution kinetics as a function of the Gibbs free energy of reaction: An experimental study based on albite feldspar |
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Authors: | Roland Hellmann Delphine Tisserand |
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Affiliation: | Fluids and Dynamics of the Crust Group, LGIT, CNRS UMR C5559, OSUG, Université Joseph Fourier, 38041 Grenoble Cedex 9, France |
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Abstract: | Here we report on an experimental investigation of the relation between the dissolution rate of albite feldspar and the Gibbs free energy of reaction, ΔGr. The experiments were carried out in a continuously stirred flow-through reactor at 150 °C and pH(150 °C) 9.2. The dissolution rates R are based on steady-state Si and Al concentrations and sample mass loss. The overall relation between ΔGr and R was determined over a free energy range of −150 < ΔGr < −15.6 kJ mol−1. The data define a continuous and highly non-linear, sigmoidal relation between R and ΔGr that is characterized by three distinct free energy regions. The region furthest from equilibrium, delimited by −150 < ΔGr < −70 kJ mol−1, represents an extensive dissolution rate plateau with an average rate . In this free energy range the rates of dissolution are constant and independent of ΔGr, as well as [Si] and [Al]. The free energy range delimited by −70 ? ΔGr ? −25 kJ mol−1, referred to as the ‘transition equilibrium’ region, is characterized by a sharp decrease in dissolution rates with increasing ΔGr, indicating a very strong inverse dependence of the rates on free energy. Dissolution nearest equilibrium, defined by ΔGr > −25 kJ mol−1, represents the ‘near equilibrium’ region where the rates decrease as chemical equilibrium is approached, but with a much weaker dependence on ΔGr. The lowest rate measured in this study, R = 6.2 × 10−11 mol m−2 s−1 at ΔGr = −16.3 kJ mol−1, is more than two orders of magnitude slower than the plateau rate. The data have been fitted to a rate equation (adapted from Burch et al. [Burch, T. E., Nagy, K. L., Lasaga, A. C., 1993. Free energy dependence of albite dissolution kinetics at 80 °C and pH 8.8. Chem. Geol.105, 137-162]) that represents the sum of two parallel reactions R=k1[1-exp(-ngm1)]+k2[1-exp(-g)]m2, |
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