Characterization of the dissolution of hornblende with application to natural waters

Dissolution rates of hornblende and tremolite were studied in constant pH batch experiments over the pH range 4-6, in order to understand the acid neutralizing role of hornblende in watersheds with low alkalinity. Hornblende and tremolite exhibit linear dissolution kinetics within one or two days after the onset of weathering. During the first 80-100 hours of weathering, base cations are released preferentially to silica in both minerals. During this period a leached surface layer similar in structure to the original material, but altered in composition is believed to be formed. Release rates of Si, Ca, Na, and Mg from hornblende exhibit weak fractional dependence on pH: d[Mg] /dt = k₁[H⁺] 0.13, d[Ca] /dt = k₁[H⁺] 0.065, d[Na] /dt = k₁[H⁺] 0.17, and d[Si] /dt = k₁[H⁺] 0.045. As a result, it is believed that, for large and transient influxes of acidified water, hornblende is not an important pH buffer. Because of rapid dissolution rates, however, hornblende could be an important source of acid neutralizing capacity