Mechanism of Chemical Reactions between SiO2 and CO2 under Mantle Conditions

Silica (SiO2) is a major component of many minerals on the Earth. Under ambient conditions, silica does not react with carbon dioxide (CO2). However, at high pressure and temperature, the stability of silica may be affected by CO2, which becomes supercritical fluid CO2 under extreme conditions and can percolate into the Earth's mantle to react with silica. Here, we investigated the chemical reactions between zeolite SSZ-56 as a model silicate and CO2 under temperature and pressure conditions close to those orf the mantle transition zone using density functional theory and molecular dynamics calculations. The reactions occurred on the SiO2 surface forming the zeolite's cavities. In the melt, CO2 and SiO2 mixed closely and, upon cooling, formed a solid with disordered Si and C sites similar to a cristobalite SiO2-CO2 solid-solution structure. This structure was thermodynamically stable with respect to alpha-cristobalite and solid CO2 above 9 GPa.