Thermal emission spectroscopy of microcrystalline sedimentary phases: Effects of natural surface roughness on spectral feature shape. Issue 3 (31st March 2016)

• Record Type: Journal Article
• Authors: Hardgrove, C. J.; Rogers, A. D.; Glotch, T. D.; Arnold, J. A.
• Is Part Of: Journal of geophysical research. Volume 121:Issue 3(2016:Mar.)
• Page Start: 542
• Page End: 555
• Publication Date: 2016-03-31
• Subjects: thermal emission spectroscopy -- microcrystalline spectroscopy -- surface roughness thermal emission spectra -- surface roughness -- chert -- micrite; Planets -- Periodicals; Geophysics -- Periodicals; 559.9
• DOI: 10.1002/2015JE004919
• Languages: English
• ISSNs: 2169-9097

Abstract

Abstract: Distinguishing between microcrystalline and macrocrystalline mineral phases can help constrain the conditions under which those minerals formed or the degree of postdepositional alteration. This study demonstrates the effects of crystal size and surface roughness on thermal infrared emission spectra of micro and macrocrystalline phases of the two most common minerals on Earth, quartz and calcite. Given the characteristic depositional and environmental conditions under which microcrystalline minerals form, and the recent observations of high‐silica deposits on Mars, it is important to understand how these unique materials can be identified using remote infrared spectroscopy techniques. We find that (a) microcrystalline minerals exhibit naturally rough surfaces compared to their macrocrystalline counterparts at the 10 µm scale; and that (b) this roughness causes distinct spectral differences within the Reststrahlen bands of each mineral. These spectral differences occur for surfaces that are rough on the wavelength scale, where the absorption coefficient ( k ) is large. Specifically, the wavelength positions of the Reststrahlen features for microcrystalline phases are narrowed and shifted compared to macrocrystalline counterparts. 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. Petrographic and topographic analyses of microcrystalline samples suggest a relationship between crystal size and surface roughness. Together, these observations suggest it may be possible to make general inferences about microcrystallinity from the thermal infrared spectral character of samples, which could aid in reconstructions of sedimentary rock diagenesis where corresponding petrographic or microimaging is not available. Key Points: Surface roughness and crystal size can influence infrared spectral shape Changes in spectral shape due to multiple surface scattering at wavelength scale Infrared spectra can be used to make general inferences about microcrystallinity