Direct Observation of Shock‐Induced Disordering of Enstatite Below the Melting Temperature

Abstract

We report in situ structural measurements of shock‐compressed single crystal orthoenstatite up to 337 ± 55 GPa on the Hugoniot, obtained by coupling ultrafast X‐ray diffraction to laser‐driven shock compression. Shock compression induces a disordering of the crystalline structure evidenced by the appearance of a diffuse X‐ray diffraction signal at nanosecond timescales at 80 ± 13 GPa on the Hugoniot, well below the equilibrium melting pressure (>170 GPa). The formation of bridgmanite and post‐perovskite have been indirectly reported in microsecond‐scale plate‐impact experiments. Therefore, we interpret the high‐pressure disordered state we observed at nanosecond scale as an intermediate structure from which bridgmanite and post‐perovskite crystallize at longer timescales. This evidence of a disordered structure of MgSiO3 on the Hugoniot indicates that the degree of polymerization of silicates is a key parameter to constrain the actual thermodynamics of shocks in natural environments.

Femtosecond X‐Ray Diffraction of Laser‐Shocked Forsterite (Mg2SiO4) to 122 GPa

“…Our results for Mg 2 SiO 4 are similar to those reported recently for [100]‐oriented single crystal enstatite (Mg,Fe)SiO 3 , under nanosecond laser shock compression where a mixture of enstatite and a dense pyroxene‐like phase was observed up to 80 GPa (Hernandez et al., 2020). The dense phase is suggested to be similar to the β‐post‐opx phase identified in 300‐K single‐crystal diamond cell experiments (Finkelstein et al., 2015).…”

“…Pressure‐induced amorphization has been observed in many materials under static and dynamic compression (Hernandez et al., 2020; Sharma & Sikka, 1996; Sikka & Gupta, 1998). Our results provide evidence for shock‐induced amorphization of forsterite above 79 GPa (Figure 6).…”

“…Our results further suggest that Fo may possess Figure 9. Position of the first (FSDP) and second (SSDP) amorphous diffraction peaks for shocked-compressed [010], [001] and polycrystalline forsterite (red circles) compared with laser-shock data for MgSiO 3 (gray squares) (Hernandez et al, 2020;Morard et al, 2020), static-compression data (open symbols) for Mg 2 SiO 4 (Benmore et al, 2011), and MgSiO 3 (yellow: Kono et al, 2018;gray: Morard et al, 2020) glasses. For tabular data, see Table S5. shear-stress anisotropy, leading to orientation-dependent transformation behavior, as well as differences between single crystals and low-porosity polycrystals that are retained up to 100 GPa or more.…”

Melting and density of MgSiO3 determined by shock compression of bridgmanite to 1254GPa

“…The bridgmanite sound velocity was analyzed using both multi-sample and single-sample techniques to determine the overtake time and thickness. Because the single-sample technique requires an assumption of a scaled sound velocity for waves propagating through released material 22 , this technique was used for the shots where velocity could not be determined by the multi-sample method. In both techniques, the wave profile was discretized using linear fits to the constant velocity plateau, longitudinal release (if present), and bulk release (Supplementary Fig.…”

“…Typical dynamic compression experiments use MgSiO 3 enstatite and glass 12,13,[19][20][21][22] as the starting materials, with initial densities <3.2 g/cm 3 . These experiments suffer from the complication of phase transitions associated with large density changes (~28%) during shock compression and limited access to the solid phase region because of the fast-rising of Hugoniot temperature with shock pressure.…”