Dissolution and crystallization rates of silicate minerals as a function of chemical affinity

Abstract

The variation of silicate dissolution and crystallization rates with chemical affinity and solution composition can be quantified by the identification of the rate controlling precursor complex. The nature of this complex depends on the individual mineral structure. Specifically, the destruction of quartz and anorthite frameworks requires the breaking of only one type of structural group. For these minerals, the rate controlling precursor complex has the composition of the mineral itself plus or minus H+, OH−, and/or H2O. As a result, the hydrolysis rates of these minerals do not depend on either the aqueous Al/Si ratio or chemical affinity at far from equilibrium conditions. In contrast, the destruction of the kaolinite, albite, K-feldspar, and kyanite frameworks requires the breaking of more than one structural group. For these minerals the rate controlling precursor complex has a different Al/Si ratio from the mineral and their hydrolysis rates have been found to depend on the aqueous Al/Si ratio. These rates thus appear to depend on chemical affinity at far from equilibrium conditions. Taking into account the identity of these precursor complexes and the framework of transition state theory leads to equations that can accurately describe the hydrolysis rates of each of these minerals as functions of pH, aqueous aluminum and silica concentration, and chemical affinity. © 1995 IUPAC