Identification of microcrystalline rocks using thermal emission spectroscopy

Abstract:

High-silica deposits on Mars have been discovered from orbit (Holden Crater, Mawrth Vallis) and from landed surface missions to both Gusev Crater (Spirit) and Gale Crater (Curiosity). The character of these silica deposits can be used to understand both the depositional environment (i.e. fumarole vs. sinter) and/or diagenetic process. Initial work has shown that, in the case of opaline silica, there are differences in spectral shape that may be related to surface textural features imparted during formation or post-depositional alteration. Due to the increasing importance of understanding microcrystalline deposits on Mars, here, we study the effects of crystal size and surface roughness on thermal infrared emission spectra of micro- and macro-crystalline quartz. The spectra of chert and macro-crystalline quartz have significant differences in both spectral contrast, and in the rounded doublet between ~1000-1250 cm^-1, which can shift and appear less rounded in microcrystalline samples. We find that microcrystalline minerals exhibit naturally rough surfaces compared to their macrocrystalline counterparts at the 10 micron scale; and that this roughness causes distinct spectral differences within the Reststrahlen bands. We find that surface roughness, if rough on the scale of the wavelengths where the wavelength-dependent absorption coefficient (k) is large, can cause not only decreased spectral contrast, but also substantial changes in spectral shape. The spectral shape differences are small enough that the composition of the material is still recognizable, but large enough such that a roughness effect could be detected. We find that my studying the thermal infrared spectral character of the sample, it may be possible to make general inferences about microcrystallinity, and thus aid in the potential reconstruction of sedimentary rock diagenesis.