The kinetics of silica-water reactions

Abstract

A differential rate equation for silica-water reactions from 0–300°C has been derived based on stoichiometry and activities of the reactants in the reaction SiO 2(s) + 2H 2O(l) = H 4SiO 4(aq) ( ∂a H 4SiO 4 ∂t ) P.T.M. = ( A M )(γ H 4SiO 4 )(k+a SiO 2 a 2 H 2O − k_a H 4SiO 4 ) where ( A M ) = (the relative interfacial area between the solid and aqueous phases/the relative mass of water in the system), and k + and k − are the rate constants for, respectively, dissolution and precipitation. The rate constant for precipitation of all silica phases is log k − = − 0.707 − 2598 T (T, K) and E act for this reaction is 49.8 kJ mol −1. Corresponding equilibrium constants for this reaction with quartz, cristobalite, or amorphous silica were expressed as log K = a + bT + c T . Using K = k + k − , k was expressed as log k + = a + bT + c T and a corresponding activation energy calculated: a b c E act(kJ mol -1) Quarts 1.174 -2.028 x 10 3 -4158 67.4–76.6 α-Cristobalite -0.739 0 -3586 68.7 β-Cristobalite -0.936 0 -3392 65.0 Amorphous silica -0.369 -7.890 x 10 -4 3438 60.9–64.9 Upon cooling a silica-saturated solution below the equilibrium temperature, the decreasing solubility of silica causes increasing super saturation, which tends to raise the precipitation rate, but the rate constants rapidly decrease, which tends to lower the precipitation rate. These competing effects cause a maximum rate of precipitation 25–50°C below the saturation temperature. At temperatures below that of the maximum rate, silica is often quenched into solution by very slow reaction rates. Consequently, the quartz geothermometer will give the most accurate results if samples are taken from the hottest, highest flow rate, thermal springs which occur above highly fractured areas.