Forsterite [Mg2SiO4)] Carbonation in Wet Supercritical CO2: An in Situ High-Pressure X-ray Diffraction Study

Mechanisms controlling mineral stabilities in contact with injected supercritical fluids containing water are relatively unknown. In this paper, we discuss carbonation reactions occurring with forsterite (Mg2SiO4) exposed to variably wet supercritical CO2 (scCO(2)). Transformation reactions were tracked by in situ high-pressure X-ray diffraction in the presence of scCO(2) containing dissolved water. Under modest pressures (90 bar) and temperatures (50 degrees C), scCO(2) saturated with water converted >70 wt % forsterite to a hydrated magnesium carbonate, nesquehonite (MgCO3 center dot 3H(2)O), and magnesite (MgCO3) after 72 h. However, comparable tests with scCO(2) at only partial water saturation showed a faster carbonation rate but significantly less nesquehonite formation and no evidence of the anhydrous form (MgCO3). The presence and properties of a thin water film, observed by in situ infrared (IR) spectroscopy and with isotopically labeled oxygen (O-18), appears to be critical for this silicate mineral to carbonate in low water environments. The carbonation products formed demonstrated by temperature and water-content dependence highlights the importance of these kinds of studies to enable better predictions of the long-term fate of geologically stored CO2.