Observations on the inversion of Stishovite to silica glass

Stishovite, a form of SiO2 with Si in sixfold coordination, inverts under certain conditions to silica glass with Si in fourfold coordination. The inversion rate determined at ten temperatures between 300° and 800°C for Meteor Crater Stishovite can be extrapolated to the time for total inversion of Stishovite to silica glass at each temperature. The resulting time-temperature curve constitutes a limit for thermal distribution models of Meteor Crater immediately after passage of the shock wave generated by meteoritic impact and initial cooling by adiabatic expansion. The data indicate the virtual impossibility that Stishovite can be formed and preserved at the surface of the earth by any mechanism other than meteoritic impact. The transformation proceeds by nucleation of glass centers along grain boundaries and dislocation surfaces in the Stishovite grains, then by growth from these centers into the host material. The growth rate of the already formed glass centers is the principal mechanism controlling the observed transformation rate in the temperature range of investigation. The transformation appears to proceed by the formation of a dense sixfold-coordinated glass from the Stishovite, then by a breakdown of the dense glass to the observed fourfold-coordinated silica glass.