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°C as a function of dissolved CO2 (1.2×10−5 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:(A1)Si Rate( mol Labradorite cm −2s−1)=k×10−Ea/2.303⁢R⁢T[(aH+3⁢n/aAl3+n)KT/(1+KT(aH+3⁢n/aAl3+n))]Si Rate where the apparent dissolution rate constant, k=10−5.69 (mol Labradorite cm−2 s−1) and the net activation energy, Ea=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 ΔH=0.54 (kcal mol−1). The temperature dependence of the silica-rich surface complex formation constant, KT, was estimated from the van't Hoff equation and yielded KT=4.49 to 5.61 from 30 to 130 °C. Elevated CO2(aq) concentrations enhance mineral dissolution indirectly by acidifying solution pH. At temperatures below 100 °C, labradorite dissolves incongruently with preferential dissolution of Na, Ca, and Al over Si.