An experimental study of forsterite dissolution rates as a function of temperature and aqueous Mg and Si concentrations

Abstract

Steady state forsterite dissolution rates, at far from equilibrium conditions and pH=2±0.04, were measured as a function of aqueous magnesium and silica concentrations, and temperatures from 25°C to 65°C. All rates were measured in mixed flow reactors and exhibited stoichiometric dissolution. Measured rates are found to be independent of both aqueous magnesium and silica concentrations. The temperature dependence of the pH 2 forsterite dissolution rates obtained in this study is consistent with an Arrhenius equation of the form r=A A exp(−E A /RT) where r signifies the overall forsterite steady state dissolution rate, A A refers to a pre-exponential factor equal to 0.190 mol cm −2 s −1, E A designates an activation energy equal to 63.8 kJ/mol, R represents the gas constant, and T denotes absolute temperature. The observed variation of forsterite dissolution rates with aqueous composition is interpreted to originate from its dissolution mechanism. The forsterite structure consists of isolated silica tetrahedra that are branched together by magnesium octahedra chains. MgO bonds apparently break more rapidly than SiO bonds in this structure. The breaking of octahedra chain linking MgO bonds, which is apparently catalyzed by hydrogen ion adsorption at acidic conditions, leads directly to the destruction of the mineral. As the rate-controlling precursor complex for this mineral is formed by a hydrogen adsorption reaction, forsterite dissolution rates are unaffected by aqueous Mg and Si activities.