Interfacial Hydrophilicity Controls Mineral Transformation Outcomes for Enstatite and Amorphous MgSiO₃

Subsurface injection of carbon dioxide (CO₂) into mafic-ultramafic rocks for permanent storage via mineralization is being studied to reduce emissions. We investigated the carbonation products of enstatite (MgSiO₃) to assess its efficiency in sequestering CO₂ for safe and permanent storage as carbonate minerals. This was accomplished by conducting variable temperature carbonation reactions with samples of differing crystallinities and surface chemistries. Reaction progress was monitored utilizing in situ X-ray diffraction, and the presence of carbonate products was confirmed using additional techniques, such as thermogravimetric analysis coupled with mass spectrometry and scanning electron microscopy with energy dispersive spectrometry. Our results show that crystalline enstatite produces small amounts of the anhydrous form of MgCO₃ (magnesite), while amorphous MgSiO₃, which was used to simulate mafic glass, more readily converts to the hydrated/hydroxylated hydromagnesite [Mg₅(CO₃)₄(OH)₂·4H₂O]. These results, supplemented with dynamic vapor sorption experiments, suggest that surface properties play a significant role in the pathway and degree of carbonation. These developments concerning the reactivity of CO₂ with reactive mafic phases will help further our understanding of the reactivity of these mafic-ultramafic minerals with implications for permanent carbon storage and other subsurface engineering scenarios involving reactive reservoirs.