Dissolution of forsteritic olivine at 65 degrees C and 2 < pH < 5

Dissolution experiments with forsteritic olivine (Fo(91)) were conducted in a batch reactor at a temperature of 65 degrees C between pH 2 and 5. Const ant pH was maintained by using a pH-stat technique. The following dissoluti on rate law for forsteritic olivine at 65 degrees C and acid pH was derived based on the experimental results normalized to the initial surface area: r = 10(-8.51)(a(H+))(0.70) where r is rate in mol forsteritic olivine/cm(2)/s. Our results, combined w ith data from the literature for forsteritic olivine dissolution at 25 degr ees C, show that the pH-dependence of forsteritic olivine dissolution is te mperature-dependent. As temperature increases, the dissolution rate of fors teritic olivine becomes more pH-dependent, which is consistent with a surfa ce protonation model for dissolution at pH < pH(pznpc). The activation ener gy of dissolution, E-a, has been estimated based on our results and literat ure data at 30 +/- 4 kcal/mol. We also observed that at pH between 2 and 4 and at 65 degrees C, the release rates of Mg, Si and Fe were stoichiometric . However, at pH 5, the release rate of Fe was slower than that of Mg and S i, probably due to oxidation of the mineral surface. Results of several dis solution experiments in the presence of Al in solution (pH 3, 0.03-10 ppm A l) show that the dissolution rate of forsteritic olivine under the experime ntal conditions is independent of Al concentration within the experimental error. This result may indicate that Al does not form strong crosslinks to the unpolymerized surface of orthosilicates such as forsteritic olivine, an d therefore does not retard the dissolution of this phase under the experim ental conditions. (C) 2000 Elsevier Science B.V. All rights reserved.