Polymorphism, superheating, and amorphization of silica upon shock wave loading and release

We present a detailed and quantitative examination of the thermodynamics and phase change mechanisms (including amorphization) that occur upon shock wave loading and unloading of silica. We apply DebyeGrüneisen theory to calculate both the Hugoniot of quartz and isentropic release paths. Quartz converts to stishovite between 15 and 46 GPa without undergoing pressure-induced amorphization and persistence of stishovite above its liquidus (i.e., superheating) is confirmed between 77 and 110 GPa. Calculations compare favorably to measurements of shock and post-shock temperatures. For silica, the method of measuring post-shock temperature is insensitive to predicting whether phase transitions actually occur during release. Measurements of release states in pressure-particle velocity space in comparison with frozen-phase release paths, suggest transformation of stishovite to lower density phases including quartz, liquid, or dense amorphous glass. Such transformations occur on release paths from peak pressure of 26 GPa and above. The isentropic release assumption appears to be approximately valid. A pressure-temperature scale for impact events involving quartz is proposed. Although recovery of coesite or stishovite upon shock loading of non-porous quartz is not predicted, trace stishovite from shock loading between 15 and 26 GPa is expected.