Crystalline forsterite to 160 GPa: the striking metastability of one of Universe's most abundant minerals

TL;DR

This study demonstrates that crystalline forsterite remains metastable and retains long-range order up to 160 GPa, challenging previous beliefs about pressure-induced amorphization and revealing its resilience under extreme conditions relevant to planetary processes.

Contribution

The paper provides the first evidence of forsterite's metastability up to 160 GPa and details the long-range ordering persistence, offering new insights into mineral behavior under extreme pressures.

Findings

Forsterite retains long-range order up to 160 GPa.

Metastable forsterite III persists from 58 GPa to 13 GPa during decompression.

Discrepancy observed between static compression data and shock wave discontinuities.

Abstract

Among Universe's most consequential events are large impacts generating rapidly-evolving extreme pressures and temperatures. Crystalline and amorphous forms of (Mg, Fe)2SiO4 are abundant and widespread, within planets and in space. The behavior of these minerals is expected to deviate form thermodynamic equilibrium in many of the processes that are critical to the formation and evolution of planets, particularly shock events. To further the understanding of the behavior of the silicate under extreme conditions, we statically compressed a crystal of forsterite up to 160.5 GPa, far beyond the compound's stability field, and probed its long-range ordering with synchrotron microdiffraction. We found that forsterite retains long-range ordering up to the highest pressure reached. Forsterite III, emerging at about 58 GPa, persists in compression to 160.5 GPa and in decompression down to about 13 GPa, for a rare combined occurrence of a metastable phase of nearly 150 GPa. These observations dispute earlier reports of pressure-induced amorphization and are a unique testimony of the resilience of the crystalline state in quasi hydrostatic compression. We confirm that highly disordered forsterite can be obtained from the decompression of forsterite III as suggested from the substantial loss of long-range ordering observed at 7 GPa after further decompression. Such kinetic pathway may explain how synthetic olivine glass have been obtained in shock experiments and could be a mechanism of generation of amorphous forsterite in cosmic dust. The 120 GPa Hugoniot discontinuity finds no correspondence in our data, marking a departure from the parallelism between static "cold compression" and dynamic compression.