ORIENTATION DEPENDENT INFRARED EMISSION SPECTRA OF SINGLE CRYSTAL QUARTZ AND MINERAL GRAIN ORIENTATION IN SHEARED ROCKS

Thermal infrared emission spectroscopy is a valuable diagnostic tool for studying minerals and rocks. Vibrational modes of bonds in a mineral’s crystal lattice exhibit diagnostic absorption features in their infrared spectra (Michalski et al., 2004, JGR 109). Quartz, SiO₂, the most abundant mineral found in Earth’s continental crust, is an anisotropic mineral with different optical properties in different orientations (Wenrich and Christensen, 1996 JGR, 101). The objective of this study is to investigate the infrared emission spectra of a polished single crystal quartz along different optical axes. The goal is to explore the possibility of using orientation dependent emissivity spectra to evaluate orientation of mineral grains in sheared rocks.

Thermal infrared emission spectra of a clear polished quartz crystal 5 cm in diameter were recorded along the c-axis and a-axis in the 400 cm^-1 to 2000 cm^-1 (25 µm to 5 µm) wavelength range with Nicolet Nexus 670 and Nicolet iS50 spectrometers located in the Mars Space Flight Facility at Arizona State University (Ruff et al., 1997, JGR 102). Infrared emission spectra of 750 µm quartz grains and rocks containing 50 to 80 % quartz collected from the South Mountains and Granite Wash Mountains of Arizona were also recorded. These spectra were analyzed using linear spectral deconvolution and spectral mixture analysis (Ramsey et al., 1998, JGR 103).

Spectral mixture analysis (SMA) of the emission spectrum of quartz grains, with a best-fit match to a custom library of the quartz c-axis and a-axis emission spectra from this study yields a mineral grain orientation of 40% c-axis quartz and 60% a-axis quartz. SMA of chert emission spectra yields mineral grain orientation of 60% c-axis quartz and 40% a-axis quartz. Preliminary results indicate that orientation-dependent infrared emission spectra of single crystals can be used to study the orientation of mineral grains. Work in progress includes analysis of the thermal infrared emission spectra of sheared and unsheared quartz-containing rocks.