Dissolution of plagioclase (andesine) into acidic, neutral and alkaline aqueous solutions at 20°C and 150°C was studied using batch reactors. Dissolution under neutral conditions are congruent, whereas, that under acidic conditions would be incongruent with selective leaching of Al, Ca and Na, which was estimated from compositions and surface observation. Dissolution rates (log K [mol feldspar /m2s]) at 25°C estimated from Si release were -10.93 in H2O (pH 5.8), -9.37 in 0.1N HCl (pH 1.1), -10.25 in 0.1N HCOOH (pH 2.2), and -9.71 in 0.01N NaOH (pH 12.2) solutions respectively. Those at 150°C were -7.76 in H2O, -7.49 in 0.003N HCI, -6.42 in 0.1N HCl, and -7.76 in 0.01N NaOH. Activation energies were 63.0kJ/mol in 0.1N HCI, 61.2kJ/mol in H2O, and 37.7kJ/mol in 0.01N NaOH. The small activation energy under alkaline conditions suggests that the rate controlling process is different from that under neutral and acidic conditions.
