Sound Velocities and Structural Transitions of Endmember and Fe, Al, Mg, Ti‐Bearing CaSiO ₃ Glasses Up to 23 GPa

Abstract The study of silicate glasses is important to understand the physical and chemical properties of silicate melts under high‐pressure conditions relevant to planetary interiors. We conducted in situ Brillouin spectroscopy measurements on two endmember, low‐impurity CaSiO 3 glasses and one Fe, Al, Mg, Ti‐bearing CaSiO 3 glass up to 23 GPa. We obtained pressure‐dependent acoustic velocities and derived elastic moduli that exhibit discontinuities indicative of structural transitions in all compositions. The endmember CaSiO 3 glasses exhibit velocity softening below 2.6 GPa, consistent with earlier findings in other silicate glasses, while the Fe, Al, Mg, Ti‐bearing CaSiO 3 glass displays a delayed onset of softening and densification. This softening is attributed to intermediate‐range structural rearrangements. Above ∼8 GPa, both glasses show rapid increases in velocities and elastic moduli, reflecting densification associated with structural transitions. Comparison with other silicate glasses demonstrates that Ca acts as a strong network modifier, significantly reducing the stiffness of the glasses, while the addition of Fe, Al, Mg, and Ti collectively has a mixed effect on elasticity. CaSiO 3 glass crosses over in density with its counterpart crystal, wollastonite, at only ∼3 GPa—a pressure lower than any other crossover pressure observed in silicate glasses—suggesting that Ca‐rich melts may become gravitationally stable at much shallower depths in planetary interiors than other silicate melts. These results provide new constraints on the structural evolution and elasticity of Ca‐rich silicate glasses under compression and have implications for modeling the mobility of silicate melts in deep planetary environments.