Because it is ubiquitous in sedimentary basins, the detailed understanding of the kinetics of quartz-water interactions, particularly at the near to equilibrium conditions typical of natural processes, is essential to accurate diagenetic modelling.
Steady-state quartz dissolution and precipitation rates were measured at 200° C in dilute aqueous solutions (pH 3 and pH 5.6) as a function of solution saturation state (or chemical affinity of the reaction). All experiments were conducted in titanium mixed flow reactor systems (Berger et al., 1994). These reactor systems are ideally suited to investigate the rates of water/mineral interactions at both near and far from equilibrium conditions (Gautier et al., 1994). Mixed flow reactors afford numerous advantages over the traditional closed system reactors used for all previous quartz precipitation studies including 1) allowing for direct measurements of steady-state rates, and 2) permitting measurement of rates at specific fluid compositions and chemical affinities by either changing the inlet solution composition or the flow rate, without dismantling the reactor. The synthetic quartz used in both dissolution and precipitation experiments was ground and sieved to the 50-100 m size fraction. Ultra-fine particles were removed by ultrasonic treatment in deionized water. To remove the disturbed surface layer produced by grinding, the sample was pre-leached at 200°C for four days in a mixed flow reactor until the concentration of silica became constant with time. The aqueous silica-rich inlet solutions were obtained by concentrating solutions originally equilibrated with silicic acid at 90°C by evaporation.
Dissolution rates at pH 3 were obtained for steady-state silica concentrations ranging from 2.95x10-5 to 2.17x10-3 m. These rates exhibit a linear dependence on the saturation state consistent with a rate expression derived from Transition State Theory (TST) (Eyring, 1935) and the principle of detailed balancing and given by
r = k+ (1-Q/K) (1)
where r refers to the dissolution rate, k+ designates a rate constant, Q stands for the aqueous silica concentration, and K represents the equilibrium constant for quartz hydrolysis. Regression of the data obtained in the present study at 200°C and pH 3 yields values of 1.84x10-9 mol/m2/s and 4.46x10-3 for k+ and K, respectively.
Quartz precipitation was not observed at pH 3 for inlet aqueous silica solutions having concentrations of up 7.33x10-3 m, corresponding to a chemical affinity of 2.0 kJ/mol for the precipitation reaction. This is in contrast to equation (1) which predicts significant and measurable quartz precipitation at these conditions.
Quartz precipitation was observed, however, at pH 5.6 for inlet aqueous silica solutions having concentrations of up to 8.95x10-3 m. These latter results will be used to determine whether the quartz precipitation is controlled by either a TST linear rate law (1-Q/K) or a dislocation-controlled parabolic rate law (1-Q/K)2 (Nielsen, 1964).
Berger, G., Cadore, E., Schott, J. & Dove, P.M., Geochim. Cosmochim. Acta, 58, 541-551 (1994).
Eyring, H., J. Chem. Phys., 3, 107-120 (1935).
Gautier, J.-M., Oelkers, E.H. & Schott, J., Geochim. Cosmochim. Acta, 58, 4549-4560 (1994).
Nielsen, A.E. Pergammon Press, New York, 148pp (1964).