Silica Apparent Solubilities and Rates of Dissolution and Precipitation

Abstract

A method for the determination of the solubilities of ca. 20 common silica-containing minerals is proposed, wherein the release rate of dissolved silica is monitored in terms of mol cm-2 sec-1 from the mineral surface, using a series of extractions with pH 7.5-8.5 seawater and having the dissolved silica concentration adjusted to 5, 50, 100 and 200 x 10-6mol 1-1 (the range of dissolved silica concentrations commonly found in the deep oceans). These extractions reveal both long-term dissolution rates and precipitation magnitudes for a given mineral at a given dissolved silica concentration. Preliminary results suggest the following solubility ranges with respect to dissolved silica in seawater at 1-2°C for the following minerals: albite, 55-110 µM; kaolinite, 10-300 µM; hornblende, 50-120 µM; nepheline, 35-75 µM, chlorite, 40-80 µM; biotite, 60-120 µM; illite, 40-70 µM; montmorillonite, 80-250 µM; hyperstene, 200-275 µM; bentonite, 115-340 µM; epidote, 150-200 µM; tremolite, 200-275 µM; olivine, 300-700 µM; quartz, variable but between 50 and 800 M. The above approach may allow us to predict a stability sequence or weathering series for these minerals in seawater and deep-sea sediments having a variety of dissolved silica concentrations.