Thermodynamic and kinetic constraints on reaction rates among minerals and aqueous solutions. III. Activated complexes and the pH-dependence of the rates of feldspar, pyroxene, wollastonite, and olivine hydrolysis

Abstract

Listen

Translate article

The stoichiometries of activated complexes responsible for controlling the kinetics of mineral dissolution can be inferred from experimental rate data. Constraints are provided by adsorption equilibria, surface ion exchange reactions, and the pH-dependence of the steady-state rates of surface detachment. Adsorption equilibria may lead to accelerated or decelerated rates of hydrolysis due to formation of surface species. They may also result in pH-independent dissolution rates at low pH as a consequence of hydrogen ion surface saturation. Surface exchange reactions of H + or H 3O + for M(2) site cations in pyroxenes, Ca 2+ in wollastonite, and alkali cations in feldspars go essentially to completion for dissolution in solutions that are appreciably undersaturated with respect to the reactant mineral. The rate of surface exchange is proportional to the fraction of exchangeable cations on the reacting surface, which leads to an integrated exponential relation for mass transfer as a function of time. The independence of detachment rates on the degree of surface exchange indicates approximately equivalent formation of activated complexes at both exchanged and unexchanged sites. The degree of hydration or protonation of activated complexes formed from surface species at active sites can be inferred from the dependence of the steady-state hydrolysis rates on pH.