Dissolution rates of silicate minerals in water from a subcritical to a supercritical state: effect of solvent properties

Abstract

Hydrothermal reaction experiments of silicate minerals (actinolite, pyroxene, etc.) were carried out using flow-through reactors in the temperature range from 25 to 400 °C at 23 MPa. The dissolution in water of a multi-oxide silicate mineral, for example actinolite or pyroxene may require the breaking of more than one type of metal–oxygen bond. Differences between the rates at which these bonds break are often sufficiently large for dissolution to be non-stoichiometric. Dissolution rates (of Si) for actinolite and pyroxene in water were found to increase with increasing T from 25 to 300 °C, and then decrease with increasing T from 300 to 400 °C. The maximum release rates of Si are reached at 300 °C. The different metals in the minerals often have different release rates at a fixed temperature. At T < 300 °C the release rates of Na, Ca, Mg, Fe, and Al from minerals are usually higher than that of Si. In contrast, release rates of Si are higher than those of the others at T ≥ 300 °C. The hydrolysis of Si–O–Si bonds and metal ion-H+ exchange reactions at T < 300 °C are different from reactions at T ≥ 300 °C, at 23 MPa, because the solvent properties of water (decreasing density and dielectric constant in the region from sub-critical to supercritical state) affect reaction rates. log r (dissolution rates of Si) increases with 1/dielectric constant, as the temperature rises to close to 300 °C (or up to 374 °C) and at the critical pressure.