Compositional dependence of structural transition pressures in amorphous phases with mantle-related compositions

Properties of silicate melts are key to understanding the evolution of the mantles of the Earth and terrestrial planets. Although remarkable progress has been made in first-principle calculations for melts in recent years, structural measurements of silicate melts at in situ high P-T remain one of the most challenging tasks. The study of glasses, kinetically frozen melts, at high pressure can provide valuable insights into related melts in the mantle. We report Raman scattering of MgSiO₃ glass revealing a structural transition at 19-38 GPa, which is associated with increases in the Si{single bond}O coordination number, and another transition at 65-70 GPa. However, in CaSiO₃ and Mg₂SiO₄ glasses, the former transition occurs at higher pressures by 5-10 GPa and the latter transition is not observed to our maximum pressure (80 GPa), indicating that a less polymerized Si{single bond}O network increases the transition pressures. Our results suggest that the pressure for the structural transitions in these glasses is influenced strongly by the concentration of network former cations and the ionic size of the network modifiers. This observation may have important implications for compositional differentiation in the early magma ocean and the present-day mantle.