THE pH DEPENDENCE OF HORNBLENDE DISSOLUTION

Hornblende minerals are important in proton neutralization and base cation release processes in many soils. This study was conducted to determine the pH dependence of hornblende dissolution rates and the rate of proton consumption during weathering under laboratory conditions. Hornblende was crushed mechanically and purified by magnetic and density separation. The samples were initially weathered (22 days) in pH 4.0, 0.01mol/L HOAc-LiOAc buffer to minimize the effect of grinding on dissolution rates. The pH range investigated was from 3.4 to 4.6 at approximately 0.2 pH unit intervals. The hornblende dissolution experiment was conducted in a 0.01 mol/L HOAc-LiOAc buffer in batch-type reactors for 30 days. The rate of proton consumption was determined for both the initially weathered samples and for samples not initially weathered in 0.01 mol/L LiCl at pH near 4.0. The dissolution was nonstoichiometric during the study period, with preferential release of Al, Fe, and Mg relative to Si. Nearly linear kinetics were observed for the release of Si, but the release rates of Al, Fe, and Mg decreased with time. Hornblende dissolution rates in the pH range studied decreased as pH increased. The overall apparent linear reaction orders, with respect to solution pH, were found to be 0.71, 0.39, 0.42, and 0.79 for Al, Fe, Mg, and Si release, respectively. The reaction orders at pH values between 3.4 and 4.2 were lower than those between pH 4.2 and 4.6. The quantity of proton consumed was equivalent to the total charges of the cations released. The proton consumption rate of samples not initially weathered was five times higher than that of the initially weathered samples. Dissolution rates obtained in acetate buffer were 3 to 4 times higher than those obtained in unbuffered LiCl solution at comparable pH and ionic strengths. The rate of hornblende dissolution is a function of solution pH and concentration of organic ligand.