Reaction rate-surface area relationships during the early stages of weathering. II. Data on eight additional feldspars

Abstract

Dissolution rate data for eight specimens of feldspars have been obtained as functions of mass and exposed surface area in pH 2 and 3 solutions. These measurements suggest that reaction rates are not related to specific surface areas (m 2/g) in any simple fashion, and that the relation expresses itself differently for the alkali feldspars and the plagioclases. The alkali feldspars display rate minima at intermediate size fractions, suggesting that 1) defects associated with exsolution lamella are the primary loci for reaction, and 2) these minerals cleave preferentially along these boundaries, yielding a net reduction in the number of available reaction sites in the finer grain sized fractions. Plagioclase feldspars, as a group, do not display the pronounced rate minima at intermediate size fractions. However, the finer grain-sized fractions do exhibit reduced specific reaction rates, suggesting that the primary sites for the dissolution are not uniformly distributed within the host mineral on the scale of several tens of microns. Two independent approaches are employed to obtain estimates of the precision of individual rate determinations. Replicate rate measurements on related samples have average coefficients of variation of about 8%, with a maximum cv of less than 18%. Variance estimates obtained on the specific rate data from the coarsest size fractions of each mineral yield comparable precision estimates. While there is some uncertainty associated with individual rate determinations, the magnitude of this variance is small compared with discrepancies between field and laboratory-based estimates of absolute rates.