Forsterite dissolution and magnesite precipitation at conditions relevant for deep saline aquifer storage and sequestration of carbon dioxide

The products of forsterite dissolution and the conditions favorable for magnesite precipitation have been investigated in experiments conducted at temperature and pressure conditions relevant to geologic carbon sequestration in deep saline aquifers. Although forsterite is not a common mineral in deep saline aquifers, the experiments offer insights into the effects of relevant temperatures and PCO2 levels on silicate mineral dissolution and subsequent carbonate precipitation. Mineral suspensions and aqueous solutions were reacted at 30 °C and 95 °C in batch reactors, and at each temperature experiments were conducted with headspaces containing fixed PCO2 values of 1 and 100 bar. Reaction products and progress were determined by elemental analysis of the dissolved phase, geochemical modeling, and analysis of the solid phase using scanning electron microscopy, infrared spectroscopy, and X-ray diffraction. The extent of forsterite dissolution increased with both increasing temperature and PCO2. The release of Mg and Si from forsterite was stoichiometric, but the Si concentration was ultimately controlled by the solubility of amorphous silica. During forsterite dissolution initiated in deionized water, the aqueous solution reached supersaturated conditions with respect to magnesite; however, magnesite precipitation was not observed for reaction times of nearly four weeks. Magnesite precipitation was observed in a series of experiments with initial solution compositions that simulated extensive forsterite dissolution. The precipitation of magnesite appears to be limited by the process of nucleation, and nucleation requires a critical saturation index between 0.25 and 1.14 at 95 °C and 100 bar PCO2. Magnesite precipitation is fastest in the presence of an initial magnesite seed. Although magnesite precipitates do form on the surfaces of forsterite particles, the presence of the forsterite surface does not significantly accelerate magnesite precipitation relative to solid-free systems.