THERMAL INFRARED EMISSION OF SHOCKED COCONINO SANDSTONE FROM METEOR CRATER, ARIZONA: SPECTROSCOPIC EVIDENCE OF IMPACT POLYMORPHS, COESITE AND STISHOVITE

Coesite and stishovite, the high-pressure polymorphs of quartz, are important markers of shock in terrestrial impact craters. Thermal infrared emission spectroscopy is a valuable diagnostic tool for identifying minerals and rocks in the laboratory and by remote sensing. Minerals have characteristic vibrational modes that give rise to diagnostic absorption features in their thermal infrared spectra (Michalski et al., 2004). In this study we use infrared emission spectroscopy to identify coesite and stishovite in shocked Coconino sandstone samples from Meteor Crater in Arizona and compare the results to those obtained by powder X-ray diffraction.

Thermal infrared emission spectra of three shocked Coconino sandstone samples from Meteor Crater were recorded between 400 cm^-1 to 2000 cm^-1 (25 µm to 5 µm) with the Nicolet iS50 spectrometer located in the Mars Space Flight Facility at Arizona State University (ASU) (Ruff et al., 1997). The infrared spectra were plotted as emissivity vs wavenumber (cm^-1). X-ray diffraction patterns of the powdered sandstone samples will be collected using the Malvern PANalytical Aeris diffractometer in the Eyring Materials Center at ASU.

The infrared emission spectra of the three shocked sandstone samples show distinct spectral features (emissivity minima) that are distinct from those of quartz. These spectral features match several of the infrared absorption features of natural coesite (Lyon et. al., 1962) and synthetic coesite (Koike et. al., 2013). Most notable are the strong absorption bands at 1220 cm^-1 and 1078 cm^-1 which are attributed to the asymmetric Si-O stretching in coesite (Michalski et al., 2003). Spectral features at 960 cm^-1 and 883 cm^-1 match the absorption bands reported for natural stishovite (Lyon et. al., 1962)). Work in progress includes comparison with infrared emission spectra of unshocked Coconino sandstone and powder X-ray diffraction analysis to confirm the presence of coesite and stishovite.