Different structural behavior of MgSiO₃ and CaSiO₃ glasses at high pressures

Knowledge of the structural behavior of silicate melts and/or glasses at high pressures provides fundamental information for discussing the nature and properties of silicate magmas in the Earth’s interior. The behavior of Si-O structures under high-pressure conditions has been widely studied, while the effect of cation atoms on the high-pressure structural behavior of silicate melts or glasses has not been well investigated. In this study, we investigated the structures of MgSiO₃ and CaSiO₃ glasses up to 5.4 GPa by in situ X-ray pair distribution function measurements to understand the effect of different cations (Mg²⁺ and Ca²⁺) on high-pressure structural behavior of silicate glasses. We found that the structural behavior of MgSiO₃ and CaSiO₃ glasses are different at high pressures. The structure of MgSiO₃ glass changes by shrinking of Si-O-Si angle with increasing pressures, which is consistent with previous studies for SiO₂ and MgSiO₃ glasses. On the other hand, CaSiO₃ glass shows almost no change in Si-Si distance at high pressures, while the intensities of two peaks at ~3.0 and ~3.5 Å change with increasing pressure. The structural change in CaSiO₃ glass at high pressure is interpreted as the change in the fraction of the edge-shared and corner-shared CaO₆-SiO₄ structures. The different high-pressure structural behavior observed in MgSiO₃ and CaSiO₃ glasses may be the origin of differences in properties, such as viscosity between MgSiO₃ and CaSiO₃ melts at high pressures. This signifies the importance of different structural behaviors due to different cations in investigations of the nature and properties of silicate magmas in Earth’s interior.