Abstract
Equilibrium phase diagrams define the pressure-temperature stability fields of MgSiO
3
polymorphs, yet whether these phase transformations can proceed under shock compression remains poorly constrained. Here, we present in situ synchrotron x-ray diffraction (XRD) measurements of presynthesized dense MgSiO
3
polymorphs—bridgmanite and akimotoite—subjected to laser-driven shock compression. Bridgmanite preserves its perovskite structure up to at least 274 gigapascals, far beyond the equilibrium perovskite-postperovskite boundary, whereas akimotoite retains its ilmenite structure up to 87 to 116 gigapascals and loses crystallinity at 344 to 401 gigapascals. These observations provide direct evidence that neither the perovskite-postperovskite transition from bridgmanite nor the ilmenite-perovskite transition from akimotoite was observed under the shock conditions investigated. Our results indicate that kinetic limitations dominate MgSiO
3
transformations on nanosecond timescales, such that equilibrium phase relations cannot be directly applied at extreme shock. Placed in the context of recent in situ XRD studies across a range of silicate minerals, these results suggest an important role for kinetic accessibility relating to crystal topology—particularly the three-dimensional connectivity and stiffness of the polyhedral network—rather than thermodynamic stability alone, in the structural response to shock, with implications for mineral physics and modeling planetary impact processes.