Effects of CO₂ on the structure of silicate melts considering the degree of polymerization under pressure

Abstract

Carbon dioxide (CO₂) is a prevalent volatile in Earth’s interior, but its effects on the structural properties of magmas or silicate melts remain insufficiently understood. Previous studies have indicated that the addition of CO₂ can decrease the viscosity of silicate melts, but only if they are fully polymerized. In this study, we explored the effects of CO₂ considering the degree of polymerization on the structure of silicate melts at high pressures of up to ∼ 5 GPa using in situ synchrotron X-ray diffraction (XRD) measurements and classical molecular dynamics (MD) simulations. The first sharp diffraction peak (FSDP) position of the X-ray structural factor S(Q), which shows the periodicity of an intermediate-range structure, was not affected by the addition of CO₂ for partially depolymerized sodium silicate melt (Na₂Si₃O₇). On the other hand, the height of the FSDP for fully polymerized silica melt (SiO₂) slightly decreased, indicating that the Si-O network structure was disordered by the addition of CO₂. This difference in the behavior of the FSDP may be attributed to the type of carbon species.