%0 Journal Article %1 Dixit:2007:Geochem-Trans:17386113 %A Dixit, S. %A Carroll, S. A. %D 2007 %J Geochemical Transactions %K 2007 diopside dissolution experiments saturation-state %N 1 %R 10.1186/1467-4866-8-3 %T Effect of solution saturation state and temperature on diopside dissolution %U http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1852097 %V 8 %X ABSTRACT: Steady-state dissolution rates of diopside are measured as a function of solution saturation state using a titanium flow-through reactor at pH 7.5 and temperature ranging from 125 to 175C. Diopside dissolved stoichiometrically under all experimental conditions and rates were not dependent on sample history. At each temperature, rates continuously decreased by two orders of magnitude as equilibrium was approached and did not exhibit a dissolution plateau of constant rates at high degrees of undersaturation. The variation of diopside dissolution rates with solution saturation can be described equally well with a ion exchange model based on transition state theory or pit nucleation model based on crystal growth/dissolution theory from 125 to 175C. At 175C, both models overpredict dissolution rates by two orders of magnitude indicating that a secondary phase precipitated in the experiments. The ion exchange model assumes the formation of a Si-rich, Mg-deficient precursor complex. Lack of dependence of rates on steady-state aqueous calcium concentration supports the formation of such a complex, which is formed by exchange of protons for magnesium ions at the surface. Fit to the experimental data yields an Mg-H exchange coefficient = 1.39, apparent activation energy = 332 kJ/mol, and apparent rate constant = 10;41.2 mol diopside/cm;2/s. Fits to the data with the pit nucleation model suggest that diopside dissolution proceeds through retreat of steps developed by nucleation of pits created homogeneously at the mineral surface or at defect sites, where homogeneous nucleation occurs at lower degrees of saturation than defect-assisted nucleation. Step edge energy for homogeneously nucleated pits were higher (275 to 65 mJ/m;2) than the pits nucleated at defects (39 to 65 mJ/m;2) and the activation energy associated with the temperature dependence of site density and the kinetic coefficient for homogeneously nucleated pits (2.59 x 10;-16 mJ K-1) were lower than the pits nucleated at defects (8.44 x 10;-16 mJ K-1).

@article{Dixit:2007:Geochem-Trans:17386113, abstract = {ABSTRACT: Steady-state dissolution rates of diopside are measured as a function of solution saturation state using a titanium flow-through reactor at pH 7.5 and temperature ranging from 125 to 175C. Diopside dissolved stoichiometrically under all experimental conditions and rates were not dependent on sample history. At each temperature, rates continuously decreased by two orders of magnitude as equilibrium was approached and did not exhibit a dissolution plateau of constant rates at high degrees of undersaturation. The variation of diopside dissolution rates with solution saturation can be described equally well with a ion exchange model based on transition state theory or pit nucleation model based on crystal growth/dissolution theory from 125 to 175C. At 175C, both models overpredict dissolution rates by two orders of magnitude indicating that a secondary phase precipitated in the experiments. The ion exchange model assumes the formation of a Si-rich, Mg-deficient precursor complex. Lack of dependence of rates on steady-state aqueous calcium concentration supports the formation of such a complex, which is formed by exchange of protons for magnesium ions at the surface. Fit to the experimental data yields an Mg-H exchange coefficient = 1.39, apparent activation energy = 332 kJ/mol, and apparent rate constant = 10;41.2 mol diopside/cm;2/s. Fits to the data with the pit nucleation model suggest that diopside dissolution proceeds through retreat of steps developed by nucleation of pits created homogeneously at the mineral surface or at defect sites, where homogeneous nucleation occurs at lower degrees of saturation than defect-assisted nucleation. Step edge energy for homogeneously nucleated pits were higher (275 to 65 mJ/m;2) than the pits nucleated at defects (39 to 65 mJ/m;2) and the activation energy associated with the temperature dependence of site density and the kinetic coefficient for homogeneously nucleated pits (2.59 x 10;-16 mJ K-1) were lower than the pits nucleated at defects (8.44 x 10;-16 mJ K-1).}, added-at = {2009-09-30T16:13:16.000+0200}, author = {Dixit, S. and Carroll, S. A.}, biburl = {https://www.bibsonomy.org/bibtex/2e626022154e7d631ccdfb1b160fa316d/schepers}, description = {Effect of solution saturation state and temperature on diopside dissolution}, doi = {10.1186/1467-4866-8-3}, interhash = {3aa88cb93d6b662fda49285b40004ec6}, intrahash = {e626022154e7d631ccdfb1b160fa316d}, journal = {Geochemical Transactions}, keywords = {2007 diopside dissolution experiments saturation-state}, number = 1, pmid = {17386113}, timestamp = {2009-09-30T16:13:16.000+0200}, title = {Effect of solution saturation state and temperature on diopside dissolution}, url = {http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1852097}, volume = 8, year = 2007 }