Dependence of labradorite dissolution kinetics on CO2(aq), Al(aq), and temperature

Labradorite (Ca0.6Na0.4Al1.6Si2.4O8) dissolution rates were measured using a mixed flow reactor from 30 to 130 degrees C as a function of dissolved CO2 (1.2 x 10(-5) and 0.6 M), and aluminum (10(-6) to 10(-3) M) at pH 3.2. Over these experimental conditions, labradorite dissolution can be described with a single rate expression that accounts for observed increases in dissolution rate with increasing temperature and decreases in dissolution rate with increasing dissolved aluminum: where the apparent dissolution rate constant, k = 10(-5.69) (mol Labradorite cm(-2) s(-1)) and the net activation energy, E-a = 10.06 (kcal mol(-1)). This temperature-dependent rate expression is partly based on the model proposed by Oelkers et al. (1994) [Oelkers, E.H., Schott, J., Devidal, J., 1994. The effect of aluminum, pH, and chemical affinity on the rates of aluminosilicate dissolution reactions. Geochim. Cosmochim. Acta, 58, 2011-2024.] in which the dependence of silicate dissolution rates on dissolved aluminum in acidic solutions is attributed to H+-Al3+ exchange at the mineral surface and formation of silica-rich surface complexes. For this exchange reaction, regression of the experimental data yield a stoichiometric coefficient n = 0.31 and an enthalpy of reaction Delta H=0.54 (kcal mol(-1)). The temperature dependence of the silica-rich surface complex formation constant, K-T, was estimated from the van't Hoff equation and yielded K-T = 4.49 to 5.61 from 30 to 130 degrees C. Elevated CO2(aq) concentrations enhance mineral dissolution indirectly by acidifying solution pH. At temperatures below 100 degrees C, labradorite dissolves incongruently with preferential dissolution of Na, Ca, and Al over Si. (c) 2005 Elsevier B.V. All rights reserved.