Effects of Aluminum on the Precipitation Rate of Silica Minerals

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 nucleationcontrolled 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.