Quantitative 29 Si NMR spectra of single-crystal alpha -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 29 Si 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 approximately 25 GPa show increasing amounts of a separate amorphous phase of SiO 2 with a mean Si-O-Si bond angle roughly 5 degrees narrower, and 10-15% denser, than fused SiO 2 . Small amounts of crystalline material remain with a mean Si-O-Si bond angle up to 3 degrees larger than unshocked alpha -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 (super ) Si in an amorphous phase in our recovered samples suggests that the formation of stable, high-coordinated 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.