Structure measurements of MgSiO3 and CaSiO3 glasses at high pressure

The structure of silicate melt at high pressure is a key to understanding its properties such as density, viscosity, and melt-crystals partitioning in Earth's interior. Glasses have been studied as analogs of silicate melts. Previous measurements have been performed only on SiO₂ and MgSiO₃ glasses under lower mantle conditions [e.g., Sato and Funamori 2010, PRB; Kono et al. 2018, PNAS], and other components such as CaSiO₃ have not been examined yet.

Here we investigated the pressure-induced structural evolution of MgSiO₃ and CaSiO₃ glasses up to 131 GPa based on synchrotron X-ray diffraction measurements in a diamond-anvil cell. The obtained intensity data were normalized and converted into the structure factor S(Q). The radial distribution function g(r) was calculated by the Fourier transform of S(Q). Our results show that the first sharp diffraction peak (FSDP) positions in S(Q) of CaSiO₃ glass are approximately 10% smaller than those of MgSiO₃ glass at pressures above 15 GPa. The calculated g(r) indicates that the Ca-O distances are 1.5 times larger than the Si-O distances in CaSiO₃ glass through the experimental pressure range, while the Mg-O distances in MgSiO₃ glass seem to be almost similar with the Si-O distances. For CaSiO₃ glass, the first and second peaks on g(r) are considered to represent Si-O and Ca-O contributions, respectively. The calculated Si-O coordination number in CaSiO₃ glass gradually increases with increasing pressure, whereas the Ca-O coordination number rapidly increases below 20 GPa and is almost constant at higher pressures. The change of the FSDP position of CaSiO₃ with increasing pressure is slightly smaller than that of MgSiO₃, implying that CaSiO₃ glass is less compressible than MgSiO₃ glass as predicted by recent theoretical calculations [e.g., Bajgain et al. 2015, PCM; Ghosh et al. 2014, AmMin].