Microspectroscopic and Petrographic Comparison of Experimentally Shocked Albite, Andesine, and Bytownite

摘要

Plagioclase feldspars are common on the surfaces of planetary objects in the solar system such as the Moon and Mars, and in meteorites. Understanding their response to shock deformation is important for interpretations of data from remote sensing, returned samples, and naturally shocked samples from impact craters. We used optical petrography, micro-Raman, and micro-Fourier transform infrared spectroscopy to systematically document vibrational spectral differences related to structural changes in experimentally shocked (0-56 GPa) albite-, andesine-, and bytownite-rich rocks as a function of pressure and composition. Across all techniques, the specific composition of feldspars was confirmed to affect shock deformation, where more Ca-rich feldspars transform to an amorphous phase at lower shock conditions than more Na-rich feldspars. Onset of amorphization occurs at ~50 GPa for albite, between 28 and 30 GPa for andesine, and between 25 and 27 GPa for bytownite. Complete amorphization occurred at pressures greater than ~55 GPa for albite, ~47 GPa for andesine, and ~38 GPa for bytownite. Petrographically, these experimentally shocked samples do not exhibit the planar microstructures common in naturally shocked plagioclase, despite showing the expected trends of internal disordering and deformation as seen in the micro-Raman and infrared spectra. Average spectra of hyperspectral images of these samples mimic macroscale measurements acquired previously. However, we see micro-scale heterogeneity in the shock response, resulting from variations in either composition, crystal orientation, or the inherent heterogeneity of the shock wave topology. This is an important factor to consider when deducing shock pressures in naturally shocked samples.