The effect of aluminum, pH, and chemical affinity on the rates of aluminosilicate dissolution reactions
Analysis of aluminosilicate steady-state dissolution/precipitation rates indicate that in contrast to what is commonly assumed, the constant pH rates are not independent of chemical affinity at far from equilibrium conditions. Rather, the logarithm of these rates for albite and kaolinite are linear functions of the logarithm of aqueous Al concentration over wide ranges of saturation states. Consideration of both the steady-state rates and the surface chemistry of these minerals following dissolution indicates that these rates are consistent with their control by the decomposition of an Al-deficient, silica-rich surface precursor complex. Taking account of reactions written to form this complex leads to a rate equation for the dissolution/precipitation of these minerals that accurately describes their variation on pH, aqueous Al concentration, and chemical affinity. By analogy, it appears likely that the rates of numerous other aluminosilicate dissolution/crystallization reactions are also consistent with their control by the decomposition of similar precursor complexes. It follows from these observations that (1) the generation of steady state dissolution rate constants from experiments performed in batch type reactors,(2) the interpretation of the pH dependence of aluminosilicate dissolution reactions requires explicit account of the effects of aqueous Al concentration on these rates.