Modelling silicate mineral interfaces for carbon dioxide sequestration: structure and stability of orthoenstatite surfaces

The atomic structure and stability of the surfaces of orthoenstatite (Mg₂Si₂O₆), a silicate often present in natural environments where carbon dioxide is captured and stored into minerals or reduced, have been investigated for the first time using density functional theory and a classical rigid-ion force field model. Only non-polar stoichiometric surfaces have been considered, with a specific focus on the {210}, {010}, {100}, {110} and {120} surfaces that have been most frequently reported in the literature. It turns out that there are only three main surfaces ({210}, {010}, {100}), all the others being microfacets (<i>i.e.</i> surfaces composed of multiple crystalline facets) exhibiting combinations of the three possible surfaces. The {100} surface, which is the main one reported in the literature, is the most unstable and undergoes surface reconstruction due to major rearrangement of magnesium atoms and their coordination shell. Results suggest that this surface would either be hydroxylated or undergo other forms of reconstruction altering the Si-O bonding pattern (<i>i.e.</i> forming a phyllosilicate layer) depending on the chemical environment. This work provides the background knowledge and model that can be used to investigate CO₂ reduction at the enstatite-gas interfaces and to develop future models able to describe the enstatite-liquid interface in the context of CO₂ capture.