Micro-Raman and Micro-FTIR Spectroscopy of Experimentally Shocked Bytownite

During impact events, feldspars undergo progressive shock metamorphic transformations that change their internal structure. These changes are detectable optically in thin section and with X-ray analyses and vibrational spectroscopy. As part of an ongoing study of the compositional effects on these shock transformations in plagioclase, we report new micro-Raman and micro-FTIR results on experimentally shocked bytownite and compare these to similar studies of andesine and albite. With increasing shock pressure, micro-Raman patterns show a decrease in overall intensity of peaks. At shock pressures below 29 GPa, this manifests as an increase in the 486:507 Dcm-1 peak ratio. Above 29 GPa, bytownite shows micro-Raman patterns indicative of an amorphous material: two broad peaks centered near 500 Dcm-1 and 950 Dcm-1. For comparison, andesine and albite exhibit a broad peak at 490 Dcm-1 and a weak peak (if present) at 950 Dcm-1. Micro-FTIR results also show a pattern of decreasing crystallinity with increased pressure, indicated by loss of the peak at 1050 cm-1 above 29 GPa. Above 38.2 GPa, only one peak near 960 cm-1 is present. In some high pressure samples the peak position varies slightly, which could be due to a crystal orientation effect or minor compositional variations across a given sample. This may suggest that the highly shocked bytownite has retained some small scale remnant feldspar topology despite being amorphous at large scales. Bytownite shares similar patterns to other plagioclases in spectroscopic response to shock. Raman spectra show loss of cation modes first, and an overall decrease in Raman intensity. IR spectra of highly shocked samples show an overall amorphous spectral shape with possible remnant crystal orientation effects. Amorphization pressures for bytownite are slightly lower than andesine and significantly lower than albite. This is consistent with a Ca dependency on shock transformations, as suggested previously. Future work will include Raman studies of samples under static pressures using a diamond anvil cell, and studies using basaltic samples and rocks from Ries and Lonar Craters.