EMISSION SPECTROSCOPY OF SILICA MINERALS AND IMPLICATIONS FOR REMOTE SENSING OF MARS AND EARTH

Silica is one of the most important and abundant geological materials on Earth and may be locally important on Mars. On Earth, there are a variety of silica minerals, each of which constrains the possible geologic origin of the siliceous material. Due to its long crustal residence time, silica is also one of the most important materials for studies of early life, and potentially for astrobiology. In order to understand the spectral variations due to crystal structure, texture, and crystallinity, and the implications for thermal infrared remote sensing of silica bearing materials, laboratory emission spectroscopy of silica minerals was performed. All amorphous forms, including silica glass, hyalite (opal-AN), and opal (A), exhibit very similar emission spectra. Partially crystalline silica, such as opal-CT is distinguishable from amorphous forms by the depth and placement of the major restrahlen feature. Analysis of the minerals tridymite, cristobalite, coesite, quartz, and rocks bearing these minerals is under way. In addition to structural effects on emission spectra, variations due to texture are being examined. Preliminary results show that microcrystalline quartz has a different spectral shape from coarsely crystalline quartz, but similar to that of packed, fine particles, due to minor volume scattering at microcrystalline interfaces. Microcrystalline varieties have a positive spectral (emissivity) slope between 1220 and 1185 cm^-1 and coarsely crystalline varieties have a negative slope in this region. It is possible that this spectral slope can be used to estimate the texture of chert or the grain size of particulate quartz. All coarsely crystalline varieties have similar spectra in the region 1400 - 400 cm^-1. Microcrystalline samples are generally similar to each other, but the spectral shape is drastically changed when larger amounts of non-silica material are present. Chert with both microcrystalline calcite and quartz exhibits a drastically different spectral shape due to increased volume scattering, which dominates the shape of the rock spectrum. These results demonstrate that emission spectroscopy provides information about the structure and texture of silica and suggest that hyperspectral thermal emission remote sensing may be used to characterize silica forms on planetary surfaces.