Scanning electron microscopy, cathodoluminescence, and Raman spectroscopy of experimentally shock-metamorphosed quartzite

Abstract

We studied unshocked and experimentally (at 12, 25, and 28 GPa, with 25, 100, 450, and 750 deg degrees C pre-shock temperatures) shock-metamorphosed Hospital Hill quartzite from South Africa using cathodoluminescence (CL) images and spectroscopy and Raman spectroscopy to document systematic pressure or temperature-related effects that could be used in shock barometry. In general, CL images of all samples show CL-bright luminescent patchy areas and bands in otherwise non- luminescent quartz, as well as CL-dark irregular fractures. Fluid inclusions appear dominant in CL images of the 25 GPa sample shocked at 750 degrees C and of the 28 GPa sample shocked at 450 degrees C. Only the optical image of our 28 GPa sample shocked at 25 degrees C exhibits distinct planar deformation features (PDFs). Cathodoluminescence spectra of unshocked and experimentally shocked samples show broad bands in the near-ultraviolet range and the visible light range at all shock stages, indicating the presence of defect centers on, e.g., SiO4 groups. No systematic change in the appearance of the CL images was obvious, but the CL spectra do show changes between the shock stages. The Raman spectra are characteristic for quartz in the unshocked and 12 GPa samples. In the 25 and 28 GPa samples, broad bands indicate the presence of glassy SiO2, while high-pressure polymorphs are not detected. Apparently, some of the CL and Raman spectral properties can be used in shock barometry.