[S-MP23] Physics and Chemistry of Minerals

[SMP23-06] Polymorphism of cristobalite in shock waves

*Sota Takagi¹, Sally Tracy¹, Donghoon Kim², Kento Katagiri³ (1.Carnegie Institution of Washington, 2.Korea University, 3.Stanford University)

Keywords:shock experiments, in situ XRD, cristobalite

SiO₂ is one of the most abundant minerals of Earth’s crust and is also found in a wide-range of meteorites. Observations of high-pressure polymorphs of SiO₂ from terrestrial impact craters and meteorites have been used to indicate past hypervelocity impact events on Earth, Mars, and the Moon. For example, high-pressure phases, including coesite and stishovite, are observed in natural samples. More surprisingly, the seifertite polymorph of SiO₂ (α-PbO₂-type), which is stable above 100 GPa, has also been observed in shocked meteorites. These observations raise questions as it is unlikely these samples have experienced ultrahigh-pressure shock events in their bombardment history. However, recent static high-pressure-temperature experiments indicate it is possible to synthesize seifertite from the high-temperature cristobalite phase of SiO₂ at pressures less than 50 GPa.

Here, we report the results of laser shock experiments for cristobalite. The cristobalite samples used in shock experiments were synthesized from SiO₂ glass in the piston cylinder press at ~1940K, 500 MPa, for a 6h holding time. The laser shock experiments were conducted at Matter in Extreme Conditions (MEC) at Linac Coherent Light Source (LCLS) in SLAC National Accelerator Laboratory. The long pulse laser system was used for the shock driving source. The crystal structure dynamics under the shock conditions were observed using X-ray Free Electron Laser (XFEL), and velocity was observed simultaneously by velocity interferometer system for any reflector (VISAR) system. Data were collected up to ~90 GPa of shock pressure. The data is currently being analyzed, and we will present the data and discuss the formation of seifertite in the presentation.