Using Thermal Infrared Emission Spectroscopy to Detect High-pressure Silica Polymorphs in Shocked Coconino Sandstone from Meteor Crater

Coesite and stishovite, the high-pressure polymorphs of quartz, are important markers of shock in terrestrial impact craters. Thermal infrared emission spectroscopy (TIES) is a valuable tool for identifying minerals in rocks in the laboratory and by remote sensing. In this study we combine TIES with powder X-ray diffraction (XRD) to test the detectability of coesite and stishovite in shocked Coconino Sandstone from Meteor Crater, Arizona. We studied twenty shocked sandstone samples including five samples with up to 30% coesite, and unshocked Coconino sandstone. Thermal emission spectra were recorded with a Nicolet iS50 spectrometer and plotted as emissivity vs wavenumber (cm-1). Powder X-ray diffraction patterns were recorded using a Malvern Panalytical Aeris diffractometer. Infrared spectra of shocked sandstones show spectral features (emissivity minima) that are distinct from those of pure quartz and unshocked sandstone. Some of the emissivity minima, notably those around 1220 cm-1 and 1090 cm-1 match the infrared absorption features of natural and synthetic coesite. Shallow features around 960 cm-1 and 880 cm-1 are attributed to surface volume effects of shock-pulverized sandstones. Additional features in the regions 725-750 cm-1 and 655-665 cm-1 are not present in the spectra of quartz. Powder X-ray diffraction patterns of the shocked sandstone samples exhibit diagnostic coesite peaks at 28.9 and 26.0 (2) confirming the presence of coesite in some of the samples. Modeling with mixed XRD patterns shows coesite contents from 0 to 20% and stishovite up to ~ 0.5%. Spectral mixtures of silica polymorphs and glass do not yield a good fit to the emission spectra of shocked sandstones. This is likely a result of spectral features caused by mechanical shock effects. Infrared emission spectra of shocked sandstones contain evidence for the presence of coesite as well evidence of mechanical shock effects. However, the coesite content cannot be quantitatively determined because of nonlinear mixing effects and complex mechanical shock effects. Our results show that thermal infrared emission spectroscopy can be useful for remotely identifying shocked samples associated with impact craters.