X-ray diffraction study of laser-shocked forsterite (Mg$_{\mathrm{2}}$SiO$_{\mathrm{4}})$ from 20-130 GPa*

Forsterite, Mg$_{\mathrm{2}}$SiO$_{\mathrm{4}}$, is of fundamental importance for geophysics as the magnesium end-member of the olivine (Mg,Fe)$_{\mathrm{2}}$SiO$_{\mathrm{4}}$ solid solution. 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 \textit{in situ} x-ray diffraction study of laser-shocked polycrystalline and single-crystal (a-, b-, and c- orientation) forsterite from 20 GPa to 130 GPa using the Matter in Extreme Conditions beamline of the Linac Coherent Light Source. Consistent with earlier gas-gun experiments (Newman et al., 2018), we observe forsterite III, a metastable structure of Mg$_{\mathrm{2}}$SiO$_{\mathrm{4}}$, from 50 to 110 GPa. When compressed above 110 GPa, forsterite III undergoes amorphization. Our results show a reversion to the ambient forsterite structure during release over nanosecond timescales.