Change in surface area and dissolution rates during hornblende dissolution at pH 4.0

Four particle-size fractions (0.045-0.075, 0.075-0.11, 0.11-0.25, and 0.50-1.00 mm sieve sizes) were used to study the relationships of bulk dissolution rates (mol g^-1s^-1) to particle size and measured specific surface area. All reactions were performed on sonically cleaned samples in pH 4.0, 0.01 M HOAc-LiOAc buffer at 298 K. To minimize the effects of grinding, dissolution rates were not determined until 30 days of weathering had occurred; subsequent bulk dissolution rates were still nonlinear with time and the reaction was incongruent. Bulk reaction rates were higher for the smaller particle size fractions. The rates of release of Al, Fe, and Mg were first order with respect to surface area, but the order of release for Si was 0.39. These results suggest that the direct relationship between the quantity of exposed crystalline defects and dissolution rates suggested by previous investigators does not hold true for hornblende. The surface area of hornblende grains increased 99% on average during the first 30 days of weathering, and an additional 12% during the next 29 days of weathering. Scanning electron micrographs and N₂ adsorption-desorption isotherms (77 K) indicated that the formation of etch pits and hollow dissolution cores contributed to the increased surface area. High resolution TEM observations of surface materials removed by ultrasonic treatment and N₂ adsorption hysteresis data suggest weathering along cleavage planes with the formation of 0.5-13 nm pores. Most of the increased adsorption of N₂ in the surface area determinations, however, was due to the formation of larger etch features.