The dependence of dissolution rates on grain size for some fresh and weathered feldspars

Abstract

Ground samples from naturally weathered mineral fragments can still contain substantial proportions of weathered BET surface area. If the freshly ground and naturally weathered surfaces in such samples have different dissolution rates, data must be interpreted by a kinetic model in terms of both types of surface. Such a kinetic model is presented. The model was used to reexamine literature data for ground size fractions from nine different naturally weathered feldspars. Ratios of dissolution-reactive BET to measured BET surface area were analyzed as a function of the grain size, both for the fresh and weathered- surfaces in the samples. For the naturally weathered surfaces, the model strongly suggests that the ratio of reactive BET to measured BET surface area is either independent of the grain size or, alternatively, increases with decreasing grain size. If so, grinding does not destroy, and possibly creates, weathered reactive surface area (relative to weathered surface area). For the freshly ground surfaces, the ratio of reactive BET to measured BET surface area was approximately directly proportional to the grain size. Thus, grinding destroys fresh reactive surface area (relative to fresh surface area) at all grain sizes. Additional dissolution experiments on samples essentially containing fresh BET surfaces only have confirmed this finding. In the literature, ground samples from large naturally weathered fragments are frequently implied to contain fresh BET surface only. Observed dissolution rates can then be explained if dissolution rates per unit of fresh BET surface area are approximately independent of the grain size. In fact, however, two (partly) counteracting processes appear to be active during grinding of naturally weathered mineral fragments. These are 1. (1) increase of the ratio of freshly ground BET to measured (freshly ground plus naturally weathered) BET surface area and 2. (2) decrease of the dissolution rate per unit of fresh BET surface area. These observations demonstrate that rate constants and dissolution rates, when normalized to fresh BET surface, are valid at one grain size only. Tentative calculations further suggest that dissolution rates, normalized to dissolution-reactive BET surface, are approximately two orders of magnitude higher for fresh feldspar surfaces than for naturally weathered feldspar surfaces.