Forsterite [Mg 2 SiO 4 )] Carbonation in Wet Supercritical CO 2 : An in Situ High-Pressure X-ray Diffraction Study

Technological advances have been significant in recent years for managing environmentally harmful emissions (mostly CO2) resulting from combustion of fossil fuels. Deep underground geologic formations are emerging as reasonable options for long term storage of CO2 but mechanisms controlling rock and mineral stability in contact with injected supercritical fluids containing water are relatively unknown. In this paper, we discuss mineral transformation reactions occurring with forsterite (Mg2SiO4) exposed to wet supercritical CO2. Forsterite was selected as it is an important olivine group mineral present in igneous and mafic rocks and has been the subject of a large number of aqueous dissolution studies that can be compared with non-aqueous fluid tests in this study. Transformation reactions were examined by in situ high pressure x-ray diffraction in the presence of supercritical carbon dioxide (scCO2) containing dissolved water at conditions relevant to carbon sequestration. Under modest pressures (90 bar) and temperatures (50°C), scCO2 saturated with water was found to convert >70 wt% forsterite to a hydrated magnesium carbonate, nesquehonite (MgCO3 •3H2O) and magnesite (MgCO3), after 72 hours of reaction. However, comparable tests with scCO2 at only partial water saturation (82%) showed a significantly slower carbonation rate with only ~30-39 wt% conversion to nesquehonite and no evidence of the anhydrous form (MgCO3). Further decreases in water content of the scCO2 continued to reduce the extent of carbonation, until a critical moisture threshold (~30%) was crossed where forsterite no longer reacted in the presence of the wet scCO2 to form crystalline carbonates. Increasing the temperature to 75°C produced anhydrous magnesium carbonate, magnesite (MgCO3), preceded by the intermediate phase, hydromagnesite [Mg(CO3)4(OH)2 •4H2O]. Measurements conducted during in situ IR experiments at 50°C and 30% saturation identified the presence of an amorphous carbonate phase as well as the formation of a thin liquid-like water layer on the forsterite surface. The presence of this water film appears to be critical for the mineral carbonation of forsterite exposed to water bearing scCO2. In contrast, our prior studies with the mineral brucite [Mg(OH)2] showed extensive carbonation in the absence of a condensed water layer on the mineral surface. The contrasts in reaction rate and products formed demonstrated by temperature and water-content dependence highlights the im