Effects of Aluminum on the Precipitation Rate of Silica Minerals

Abstract

The occurrences of silica deposits (quartz veins, silica sinters, silica scales) suggest that precipitation via nucleation and overgrowth on pre-existing mineral surfaces occurs under crustal conditions. We conducted hydrothermal flow-through experiments on silica precipitation using no mineral substrate at 120–430 °C and 31 MPa for understanding
the effects of the aluminum on the mineralogy and for deriving rate of the nucleation-controlled precipitation of silica minerals. The dominant silica mineral changes systematically as amorphous silica → cristobalite → quartz with increasing Al concentration, CAl, from 0.0 to 6.7 ppm. The nucleation-controlled precipitation is expressed by a third-order rate equation, and its rate constant depends on CAl and temperature, with an activation energy of 89.6 kJ/mol (CAl = 0 ppm). The overall rate equation for silica precipitation is written as the summation of rates of surface-controlled and nucleation controlled precipitation. This rate predicts that the dominant precipitation mechanism changes from surface-controlled to nucleation-controlled precipitation with increasing temperature, degree of supersaturation ratio and fracture aperture. This result provides useful constraints on interpretations of the formation of scales in pipelines at geothermal power plants.