Quartz to stishovite: Wave propagation in the mixed phase region

Abstract

This work reports on phase transition kinetics and wave propagation in Arkansas novaculite, a fine-grained polycrystalline α quartz rock, when it is subject to high-pressure dynamic loading and relief. The stress region studied is between 150 and 400 kbar, where the polymorphic phase transition from α quartz to stishovite is believed to occur. Particular emphasis was focused on the unloading behavior in the mixed phase region. High-pressure loading is provided by conventional explosive methods. Measurements of the transient flow field are obtained with in-material manganin stress gages or magnetic particle velocity gages. Results showed partial transformation to the high-density phase occurring in the shock front, the degree of transformation depending on peak driving stress. A continuing transformation rate behind the shock front is very small, i.e., at least 3 orders of magnitude slower than the initial transformation rate. Unloading from stress-volume points in the mixed phase region is observed to occur along, or close to, paths of frozen phase concentration down to approximately 80 kbar. Below this stress the data indicate a transition of the high-density phase to a lower-density phase.