This study provides new insight into the transformation of forsterite under nanosecond-duration shock loading. The first two sharp diffraction peaks of amorphous Mg 2SiO 4 show a similar trend with compression as those observed for MgSiO 3 in both recent static- and laser-driven shock experiments. For - and -oriented crystals, a mixture of crystalline and amorphous material is observed to 108 GPa, whereas the -oriented forsterite adopts an unknown phase at 122 GPa. Polycrystalline samples undergo amorphization above 79 GPa. At higher stress, the response is sample-dependent. Densities inferred from X-ray diffraction are consistent with earlier gas-gun shock data. Instead, we observe forsterite and forsterite III, a metastable polymorph of Mg 2SiO 4, coexisting in a mixed-phase region from 33 to 75 more » GPa for both polycrystalline and single-crystal samples. Under laser-based shock loading, forsterite does not transform to the high-pressure equilibrium assemblage of MgSiO 3 bridgmanite and MgO periclase, as has been suggested previously. Here, we have carried out an in situ X-ray diffraction study of laser-shocked polycrystalline and single-crystal forsterite from 19 to 122 GPa using the Matter in Extreme Conditions end-station of the Linac Coherent Light Source. The response of forsterite, Mg 2SiO 4, under dynamic compression is of fundamental importance for understanding its phase transformations and high-pressure behavior. for Shock Physics Sponsoring Org.: USDOE National Nuclear Security Administration (NNSA), Office of Defense Programs (DP) OSTI Identifier: 1493746 Grant/Contract Number: NA0002442 Resource Type: Accepted Manuscript Journal Name: Geophysical Research Letters Additional Journal Information: Journal Volume: 45 Journal Issue: 16 Journal ID: ISSN 0094-8276 Publisher: American Geophysical Union Country of Publication: United States Language: English Subject: 58 GEOSCIENCES forsterite shock compression x‐ray = , Publication Date: Wed Jul 11 00:00: Research Org.: Washington State Univ., Pullman, WA (United States). Washington State Univ., Pullman, WA (United States).Cornell High Energy Synchrotron Source, Ithaca, NY (United States).of Minnesota, Minneapolis, MN (United States) of Technology (CalTech), Pasadena, CA (United States) In contrast to previous interpretations of the forsterite shock Hugoniot, we find that forsterite does not decompose but instead reaches the forsterite III structure, which is a metastable structure of Mg 2SiO 4 with orthorhombic space group Cmc2 1. In situ X–ray diffraction measurements were used to probe the crystal structure(s) in the shock state and to investigate potential decomposition into periclase and bridgmanite. Here we present the results of plate impact experiments on polycrystalline forsterite conducted at the Dynamic Compression Sector of the Advanced Photon Source. Previous shock recovery experiments on forsterite, which has orthorhombic space group Pbnm, show discrepant results as to whether forsterite undergoes segregation into its equilibrium phase assemblage of compositionally distinct structures upon shock compression. Furthermore, direct observations of the phase or phases making up the measured states along the forsterite Hugoniot are essential to assess whether kinetic factors inhibit the achievement of the expected equilibrium, phase–separated assemblage. Shockwave data on mineral–forming compounds such as Mg 2SiO 4 are essential for understanding the interiors of Earth and other planets, but correct interpretation of these data depends on knowing the phase assemblage being probed at high pressure.
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