Analysis of Synthetic Quartzofeldspathic Glasses Using Electron Microprobe and Thermal Infrared Spectroscopic Methods

Thermal infrared (TIR) remote sensing has been used to analyze the physical properties of silicic volcanic products that may otherwise be hazardous to study at a ground-based level. Si-O bond vibrations within silicates produce a prominent absorption feature in the 8-12 micron region. The depth and wavelength position of this absorption varies with SiO₂ content, and may be used to determine silicate material 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. We are developing laboratory and field-based TIR methods for analysis of these glasses and eventually melts. This study examines the TIR spectral characteristics of a suite of synthetic quartzofeldspathic glasses that vary systematically in composition. Mineral or oxide mixtures were synthesized at high temperatures at one atmosphere in air, and rapidly quenched to form glasses. Samples were synthesized in the following systems: Albite-Quartz (Ab-Q, ten glasses), Oligoclase-Q (Olig-Q, four glasses), Andesine-Q (An-Q, four glasses), Kspar-Q (Or-Q, five glasses), and Or-Ab-Q (three glasses). The compositions were verified using electron microprobe analysis. In addition, four micro- reflectance TIR spectra (~1000 μm²) were averaged, and the reflectance maximum for each spectrum was derived. TIR emissivity data were also collected and the spectral minimum was determined using a bulk sample of each glass. Reflectance maxima correlate well with SiO₂, Al₂O₃, and molar Si/O and Al/O (R²>0.8). Excellent correlations are obtained between reflectance maxima and normative quartz (with Ab/Anorthite or Ab/Or constant) and normative albite (with Q/Anorthite or Q/Or constant). 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 terrestrial glassy surfaces remotely from the air, space, and using hand-held devices.