SHOCK AMORPHIZATION OF CRISTOBALITE

Shock amorphization of cristobalite is reported and related to shock metamorphism of quartz, both being silicon dioxide polymorphs. Whereas amorphization of quartz takes place over a broad pressure range and is complete only at 35 to 40 gigapascals (350 to 400 kilobars), amorphization of cristobalite was complete (greater than 99.9 percent) by 28 gigapascals with a relatively sharp phase transformation; lower shock pressures up to 23 gigapascals resulted in no significant amorphization. Also, unlike quartz, there was no sign of lamellar amorphization, which is common in shock compression. Cristobalite amorphization should prove a useful indicator of shock pressure and is the first case of pressure amorphization of isochemical polymorphs. The diaplectic glass that is produced has a refractive index and density essentially identical to those of the diaplectic glass made from quartz, which suggests that both polymorphs collapse during shock to similar disordered phases.