Dauphine twinning as evidence for an impact origin of preferred orientation in quartzite: An example from Vredefort, South Africa

摘要

Large meteorite impacts cause great changes to geologic features on Earth, ranging from cratering and ejecta blankets to phase transformations in minerals. In this report we describe the effect of a shock wave on the crystallographic orientation of quartz crystals in quartzite from the Vredefort impact site in South Africa. Preferred orientation of bulk quartzite samples was measured by time-of-flight neutron diffraction. With a random distribution of c-axes, a weak but distinct difference between the orientation distribution of positive and negative rhombs was observed, as illustrated with pole figures and inverse pole figures. Results for the natural Vredefort sample are compared with deformation experiments in which the reorientation occurs as a result of mechanical Dauphiné twinning under stress. We conclude that stresses during the impact produced twinning and that the direction of the compressive shock wave can be inferred from the preferred orientation pattern. Specifically, positive rhombs become aligned perpendicular to the direction of compressive stress.