MECHANISMS OF SILICA DISSOLUTION AS INFERRED FROM THE KINETIC ISOTOPE EFFECT

The rate of dissolution of many rock-forming minerals is controlled by the hydrolysis of the bridging silicate bonds at the mineral surface. This hydrolysis can be studied at a detailed level by combining experiments in isotopically distinct solutions with molecular-orbital calculations of the reaction energetics and geometry. These calculations are linked to the macroscopic process of dissolution via the transition-state theory. Rates of silicate leaching and dissolution in D2O and H2O differ for several reasons. First, hydrolysis involves transfer of hydrogen from the solution to a bridging siloxane bond at the mineral surface. The transition state equilibrium describing this reaction varies with the vibrational properties (and, hence, isotopic composition) of the reactant and the transition state. Secondly, the equilibrium acid-base properties of the oxide surface of H2O and D2O are not identical. These differences are important because rates of hydrolysis reactions are enhanced by adsorbed hydrogen and hydroxyl ions. On the basis of the molecular orbital calculations and an assumed mechanism of hydrolysis, quartz dissolution is predicted to be roughly a factor of four slower in D2O than in H2O at pH = pD = 3. The activation energy is predicted to be almost-equal-to 20 kcal/mol, which is in agreement with values measured at hydrothermal conditions. At pH/pD conditions near the isoelectric point of quartz, and in the temperature range 20-70-degrees-C, the measured rate in D2O is only about 15% slower than in H2O, and the activation energy is almost-equal-to 8 kcal/mol. The activation energy is slightly higher at pH/pD = 11, but the kinetic isotope effect remains small. The discrepancy suggests that the hydrogen transfer to bridging oxygen from water at the experimental conditions: more rapid than modeled and may preceed early in the overall reaction.