Reaction rate-surface area relationships during the early stages of weathering—I. Initial observations

Abstract

It has commonly been assumed that the weathering rates of feldspars are proportional to their exposed surface areas. However, weathering does not affect the whole surface of a grain uniformily; preferential dissolution occurs at highly localized sites, which are determined by the location of crystalline defects. To examine the importance of reactions at these sites relative to those occurring on bulk surfaces, we compared the rates of dissolution of 5 different size fractions of an alkali feldspar, whose specific surface areas (m 2 /g) spanned a range of a factor of twenty. At any given pH value, the bulk dissolution rates (moles/g/h) for the five fractions varied by less than a factor of two. The results suggest that reactions at crystalline defects dominate weathering processes during the early stages of dissolution. Furthermore, the exposure of these defects is not simply or easily related to total surface area. These findings strongly suggest that care must be taken to distinguish between the surface reaction controlled mechanism and a surface area controlled model which is frequently—and incorrectly—assumed to be equivalent. The data presented here are consistent with the former mechanism but strongly dispute the latter.