High-pressure behavior of shock-compressed novaculite using in-situ X-ray diffraction

Quartz is an important phase in the mantle and serves as an archetype for silicate minerals of the deep Earth. As the most abundant mineral in Earth's crust, the dynamic response of quartz is also important for understanding natural impact events. High-pressure phase transitions in a -quartz have been extensively studied using both static and dynamic methods. Despite numerous shockwave studies on quartz, the sequence of phase transitions and the structure of the high-pressure phase(s) under shock loading remain topics of continued debate. Questions remain concerning the role of kinetics and the strain-rate dependence of high-pressure phase transformations. Based on continuum Hugoniot data, it has long been assumed that the high-pressure phase on the quartz Hugoniot corresponds to the stishovite phase observed in static experiments. Recent X-ray diffraction (XRD) results under gas-gun shock compression challenge this interpretation, revealing a -quartz transforms into a metastable defective niccolite structure between 39-65 GPa (Tracy et al., 2020). This result indicates a significant kinetic barrier hindering a transformation to stishovite on gas-gun timescales (100s of ns).

To clarify the role of compression timescale and provide new details of the high-pressure structure, we have carried out in situ XRD study on polycrystalline quartz (novaculite) under laser-based shock-compression at the Linac Coherent Light Source. The intense ultrafast X-ray pulses at LCLS allow for the characterization of the time dependent evolution under shock loading. Two high-powered lasers were used to generate ~5-nanosecond duration compression waves in quartz samples achieving Hugoniot pressures of 35 GPa and 48 GPa. Femtosecond XRD data were collected both on loadingand up to 25 ns after release to study the high-pressure crystal structure and any structural modifications on release. Our results demonstrate a crystallographic phase transformation to a metastable structure on the Hugoniot followed by amorphization combined with a -quartz recrystallization on release. The diffraction peaks on loading are not consistent with a -quartz or stishovite but can be interpreted as a disordered metastable phase. This result is consistent with recent gas-gun experiments despite largely different time scales.