The U.S. Department of Energy has identified mineral sequestration as a promising CO2 sequestration technology option, converting anthropogenic CO2 and magnesium silicates into permanent carbonate minerals. Advantages of a mineral CO2 mitigation scheme include (1) enormous deposits of ultramafic rock such as serpentine [Mg3Si2O5(OH)(4)] and olivine [(Mg,Fe)(2)SiO4], exist in nature, providing a large potential capacity for CO2 sequestration, (2) resulting magnesite (MgCO3) product is thermodynamically stable and environmentally benign, and (3) overall process is exothermic with potential to be implemented with acceptable economics. High carbonation efficiencies were attained at elevated temperatures and supercritical CO2 pressures employing magnesium silicate ores. However, unlike olivine, serpentine required an additional high temperature pre-treatment step to drive off hydroxyl groups prior to carbonation. X-ray diffraction analysis (XRD) identified magnesite as the primary reaction product. Mechanistic aspects of mineral dissolution/magnesium carbonate precipitation were investigated with the aid of scanning electron microscopy-energy-dispersive spectroscopy (SEM-EDS) of polished cross sections of solid process grains. Alterations to both thermally pre-treated serpentine and natural olivine grains revealed outer surfaces to be noticeably depleted in magnesium in comparison to its inner core. Formation of silica rims on the reaction surfaces may slow further dissolution of magnesium from silicate grains.