%0 Journal Article %1 hellmann2006 %A Hellmann, Roland %A Tisserand, Delphine %D 2006 %J Geochimica et Cosmochimica Acta %K 2005 Dissolution-kinetics Gibbs-free-energy albite-feldspar experimental-study function %N 2 %P 364-383 %R DOI: 10.1016/j.gca.2005.10.007 %T Dissolution kinetics as a function of the Gibbs free energy of reaction: An experimental study based on albite feldspar %U http://www.sciencedirect.com/science/article/B6V66-4HTBM4M-1/2/f235c682d95cab2422cf0d6c588403bf %V 70 %X Here we report on an experimental investigation of the relation between the dissolution rate of albite feldspar and the Gibbs free energy of reaction, DeltaGr. 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 DeltaGr and R was determined over a free energy range of -150 < DeltaGr < -15.6 kJ mol-1. The data define a continuous and highly non-linear, sigmoidal relation between R and DeltaGr that is characterized by three distinct free energy regions. The region furthest from equilibrium, delimited by -150 < DeltaGr < -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 DeltaGr, as well as Si and Al. The free energy range delimited by -70 less-than-or-equals, slant DeltaGr less-than-or-equals, slant -25 kJ mol-1, referred to as the `transition equilibrium' region, is characterized by a sharp decrease in dissolution rates with increasing DeltaGr, indicating a very strong inverse dependence of the rates on free energy. Dissolution nearest equilibrium, defined by DeltaGr > -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 DeltaGr. The lowest rate measured in this study, R = 6.2 × 10-11 mol m-2 s-1 at DeltaGr = -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 reactionsR=k11-exp(-ngm1)+k21-exp(-g)m2,where k1 and k2 are rate constants that have been determined by regression, with values 1.02 × 10-8 and 1.80 × 10-10 mol m-2 s-1, g reverse not equivalent DeltaGr/RT is a dimensionless number, and n, m1, and m2 are adjustable fitted parameters (n = 7.98 × 10-5, m1 = 3.81 and m2 = 1.17). Based on measurements of the temporal evolution of RSi and RAl for each experiment, steady-state dissolution rates appear to be congruent at all DeltaGr. In contrast, non-steady-state dissolution is incongruent, and is related to DeltaGr. Scanning electron microscopy (SEM) images of post-reaction grain surfaces indicate that dissolution close to equilibrium (DeltaGr > -25 kJ mol-1) resulted in the precipitation of a secondary crystalline phase, but there are no indications that this altered the measured R-DeltaGr relation.

@article{hellmann2006, 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, [Delta]Gr. 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 [Delta]Gr and R was determined over a free energy range of -150 < [Delta]Gr < -15.6 kJ mol-1. The data define a continuous and highly non-linear, sigmoidal relation between R and [Delta]Gr that is characterized by three distinct free energy regions. The region furthest from equilibrium, delimited by -150 < [Delta]Gr < -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 [Delta]Gr, as well as [Si] and [Al]. The free energy range delimited by -70 [less-than-or-equals, slant] [Delta]Gr [less-than-or-equals, slant] -25 kJ mol-1, referred to as the [`]transition equilibrium' region, is characterized by a sharp decrease in dissolution rates with increasing [Delta]Gr, indicating a very strong inverse dependence of the rates on free energy. Dissolution nearest equilibrium, defined by [Delta]Gr > -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 [Delta]Gr. The lowest rate measured in this study, R = 6.2 × 10-11 mol m-2 s-1 at [Delta]Gr = -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 reactionsR=k1[1-exp(-ngm1)]+k2[1-exp(-g)]m2,where k1 and k2 are rate constants that have been determined by regression, with values 1.02 × 10-8 and 1.80 × 10-10 mol m-2 s-1, g [reverse not equivalent] [Delta]Gr/RT is a dimensionless number, and n, m1, and m2 are adjustable fitted parameters (n = 7.98 × 10-5, m1 = 3.81 and m2 = 1.17). Based on measurements of the temporal evolution of RSi and RAl for each experiment, steady-state dissolution rates appear to be congruent at all [Delta]Gr. In contrast, non-steady-state dissolution is incongruent, and is related to [Delta]Gr. Scanning electron microscopy (SEM) images of post-reaction grain surfaces indicate that dissolution close to equilibrium ([Delta]Gr > -25 kJ mol-1) resulted in the precipitation of a secondary crystalline phase, but there are no indications that this altered the measured R-[Delta]Gr relation.}, added-at = {2009-10-12T10:13:37.000+0200}, author = {Hellmann, Roland and Tisserand, Delphine}, biburl = {https://www.bibsonomy.org/bibtex/2e3953b61d11b4ca4d47b94301c20b67f/aglinda}, description = {ScienceDirect - Geochimica et Cosmochimica Acta : Dissolution kinetics as a function of the Gibbs free energy of reaction: An experimental study based on albite feldspar}, doi = {DOI: 10.1016/j.gca.2005.10.007}, interhash = {10279e06a9dbb828a197e3e121fdf26a}, intrahash = {e3953b61d11b4ca4d47b94301c20b67f}, issn = {0016-7037}, journal = {Geochimica et Cosmochimica Acta}, keywords = {2005 Dissolution-kinetics Gibbs-free-energy albite-feldspar experimental-study function}, number = 2, pages = {364-383}, timestamp = {2009-10-12T10:13:37.000+0200}, title = {Dissolution kinetics as a function of the Gibbs free energy of reaction: An experimental study based on albite feldspar}, url = {http://www.sciencedirect.com/science/article/B6V66-4HTBM4M-1/2/f235c682d95cab2422cf0d6c588403bf}, volume = 70, year = 2006 }