Spectral analysis of synthetic quartzofeldspathic glasses using laboratory thermal infrared spectroscopy

Si-O-Si bond vibrations within silicate materials produce a prominent absorption feature in the 8-12 m region of thermal infrared (TIR) spectra. The wavelength position of this absorption varies with SiO₂ content and may be used to determine rock composition. However, the presence of glass greatly affects the measurement of emitted and reflected TIR energy from the surface, a phenomenon which is not currently well understood. This study examines the TIR spectral characteristics of a suite of synthetic high-SiO ₂ quartzofeldspathic glasses that vary systematically in composition. Glasses were synthesized in the albite-quartz, oligoclase-quartz, and andesine-quartz systems. Microreflectance (∼1000 μm²) and TIR emission spectra (bulk sample) were collected, and the spectral band maxima and minima were determined for each. Reflectance band maximum positions show a strong correlation with emissivity band minimum positions, indicating that the two types of spectra are comparable for this study. Excellent correlations are obtained between weight percent SiO₂ and reflectance maxima position, emission minima position, and spectral shoulder position. Molar Al/[Al+Si] and [Na+Ca]/Si are all well-correlated with reflectance maxima and shoulder positions but slightly less correlated with emission minima. Results of this study will contribute to a better understanding of spectral properties of quartzofeldspathic glasses and will provide a means to more accurately detect and map glassy surfaces (e.g., volcanoes and impact craters) remotely from the ground and from orbit.