THE DISSOLUTION OF NATURALLY WEATHERED FELDSPAR AND QUARTZ

Surface area measurements and dissolution experiments were performed o n subsamples from a naturally weathered mineral assemblage (100-1000 m u m) consisting of feldspar and quartz. The subsamples were obtained b y splitting the assemblage into four different ranges of grain density , each of which was sieved to three different size fractions. BET-kryp ton and geometric surface areas, combined with mineralogical data and average grain diameters, showed that (1) surface roughness factors of the subsamples are generally much higher than those of freshly created surfaces by grinding and (2) for individual density ranges (i.e., at constant mineralogical composition), the surface roughness factor decr eases linearly with decreasing grain diameter. Scanning electron micro scopy and X-Ray diffraction showed that contributions to the surface r oughness factors from secondary mineral coatings, macropores (diameter s >50 nm), and etch pits are insignificant. In contrast, krypton adsor ption data indicated that by far most surface roughness is due to the presence of micropores and mesopores (diameters 150 nm). These finding s strongly suggest that, during natural weathering, micropores/mesopor es develop at sites whose density (cm(-2) of geometric surface area) i s approximately proportional to grain diameter. Multivariate linear re gression showed that, at similar grain diameters, the micropore/mesopo re density increases in the order: quartz < microcline < albite < olig oclase/andesine. This sequence is similar to the well-known sequence o f relative weatherability of these minerals, suggesting a relationship between weatherability and micropore/mesopore density. At pH 3 HCl an d ambient temperature, dissolution rates of Na, K, Ca, Al, and Si from the subsamples, normalized to the BET-krypton surface area, were esse ntially independent of the grain diameter. Due to effects from surface roughness, dissolution rates normalized to the geometric surface area were essentially proportional to grain diameter. Comparison with micr opore/mesopore and etch pit densities showed that the dissolution rate s are determined by the pores, rather than by the etch pits. Furthermo re, theoretical arguments indicate that the pore area perpendicular to the mineral surface (i.e., the pore ''walls'') is essentially nonreac tive, and that the dissolution rates are largely determined by the por e area parallel to the mineral surface (the pore ''bottoms''). The por e area parallel to the mineral surface is equivalent to (1) the leache d layer/fresh mineral interface, if the micropores/mesopores develop i n leached layers or (2) the dislocation outcrops where strained minera l material is in contact with the solution, if the micropores/mesopore s develop at crystal defects. Tentative calculations suggest that (1) as in the laboratory, dissolution during the previous natural weatheri ng of the sample occurred from the micropore ''bottoms'' rather than f rom the etch pits, and (2) dissolution from the etch pits becomes more important with increasing exposure time to weathering conditions.