Equilibrium phase diagrams define the pressure–temperature stability fields of MgSiO₃ polymorphs, yet whether the corresponding phase transformations can proceed under shock compression remains poorly constrained. Here, we present in situ synchrotron X-ray diffraction measurements of pre-synthesized dense MgSiO₃ polymorphs—bridgmanite and akimotoite—subjected to laser-driven shock compression. The experiments were conducted at the Dynamic Compression Sector of the Advanced Photon Source, Argonne National Laboratory. Bridgmanite preserves its perovskite structure up to at least 262 GPa, far beyond the equilibrium perovskite–post-perovskite boundary at ~125 GPa and ~2500 K [1]. Akimotoite retains its ilmenite structure up to 91 GPa, well beyond the akimotoite–bridgmanite boundary [2], and undergoes solid-state amorphization at higher pressures. These observations provide direct evidence that neither the perovskite–post-perovskite transition from bridgmanite nor ilmenite–perovskite transition from akimotoite occurs under the shock conditions investigated. Our results indicate that kinetic limitations dominate MgSiO₃ transformations on nanosecond timescales, such that equilibrium phase relations cannot be directly applied at extreme shock. Moreover, the contrasting responses of bridgmanite and akimotoite show that shock-induced amorphization is not universal, but instead depends on crystal structure, particularly the three-dimensional connectivity of polyhedral networks under kinetic constraints. These findings demonstrate that shock transformation pathways are governed by kinetic accessibility and structural topology, not equilibrium thermodynamics, with direct implications for interpreting shock metamorphism and modeling planetary impact processes.
[1] M. Murakami, K. Hirose, K. Kawamura, N. Sata, Y. Ohishi, Post-Perovskite Phase Transition in MgSiO₃. Science 304, 855-858 (2004).
[2] K. Hirose, T. Komabayashi, M. Murakami, K. Funakoshi, In situ measurements of the majorite-akimotoite-perovskite phase transition boundaries in MgSiO₃. Geophysical Research Letters 28, 4351-4354 (2001).

Dynamic Shock Compression,mineral physics