Shocked quartz: A 29Si magic-angle-spinning nuclear magnetic resonance study

Quantitative 29Si NMR spectra of single-crystal a-quartz, shock compressed to 12–38 GPa and recovered, provide new information about the complex response of quartz to shock loading. Spectra from samples recovered from shock pressures of 12–20 GPa show a broadening of the 29Si NMR peak and the development of asymmetry toward lower NMR frequency (indicating an increase in the mean Si-O-Si intertetrahedral bond angle). NMR spectra of samples shock compressed above ;25 GPa show increasing amounts of a separate amorphous phase of SiO2 with a mean Si-O-Si bond angle roughly 58 narrower, and 10–15% denser, than fused SiO2. Small amounts of crystalline material remain with a mean Si-O-Si bond angle up to 38 larger than unshocked a-quartz. The recovery of dense glass indicates that post-shock temperatures were sufficiently low to also preserve stishovite, had any been created in our experiments. The paucity of stishovite or [6]Si in an amorphous phase in our recovered samples suggests that the formation of stable, highcoordinated Si is kinetically hindered in shock compression experiments up to about 35– 40 GPa, except in regions of high temperature, such as planar deformation features (PDFs), microfaults (pseudotachylites), or voids.