X-ray Diffraction Study of Laser-shocked Forsterite (Mg2SiO4) from 20-130 GPa

Forsterite, Mg₂SiO₄, is of fundamental importance for geophysics as the magnesium end-member of the olivine (Mg,Fe)₂SiO₄ solid solution. Olivine is a major phase in the Earth and other terrestrial planets. Phase transitions in olivine are the primary cause of the seismic discontinuities in the upper mantle. Interest in the dynamic behavior of olivine is motivated by understanding the nature of shock-induced phase transition in silicates during hypervelocity collisions. While it is known from gas-gun experiments that olivine transitions to a high-pressure phase under shock compression, there are few constraints on the structure of the high-pressure phase. We have carried out an in situ x-ray diffraction study of laser-shocked polycrystalline and single-crystal forsterite (a-, b-, and c- orientation) from 20 GPa to 130 GPa using the Matter in Extreme Conditions beamline of the Linac Coherent Light Source. Our target package consisted of polycrystalline or single-crystal forsterite sandwiched between a polyimide ablator and LiF window or no window. Consistent with earlier gas-gun experiments (Newman et al., 2018), we observe forsterite III, a metastable structure of Mg₂SiO₄, in the mixed-phase region from 50 to 95 GPa. When compressed above 95 GPa, forsterite III undergoes amorphization. Our results also show a reversion to the ambient forsterite structure during release over nanosecond timescales.