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 sampl es in pH 4.0, 0.01 M HOAc-LiOAc buffer at 298 K. To minimize the effec ts 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 r ates were higher for the smaller particle size fractions. The rates of release of Al, Fe, and Mg were first order with respect to surface ar ea, but the order of release for Si was 0.39. These results suggest th at the direct relationship between the quantity of exposed crystalline defects and dissolution rates suggested by previous investigators doe s not hold true for horn-blende. 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 elec tron micrographs and N2 adsorption-desorption isotherms (77 K) indicat ed that the formation of etch pits and hollow dissolution cores contri buted 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 for mation of 0.5-13 nm pores. Most of the increased adsorption of N2 in t he surface area determinations, however, was due to the formation of l arger etch features.