Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p43c1684h&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P43C-1684
Physics
[3934] Mineral Physics / Optical, Infrared, And Raman Spectroscopy, [5415] Planetary Sciences: Solid Surface Planets / Erosion And Weathering, [5464] Planetary Sciences: Solid Surface Planets / Remote Sensing, [5470] Planetary Sciences: Solid Surface Planets / Surface Materials And Properties
Scientific paper
Sedimentary rocks have been identified on Mars in increasing numbers and at scales ranging from cobbles to regional outcrops. For this reason, it is important to assess the potential of using thermal infrared (TIR) spectra to obtain quantitative mineralogical information of sedimentary samples. A single sedimentary sample can be a complex mixture of clasts and chemical precipitates of varying crystal size; thus the assumption that the spectral contribution from each component combines linearly in the bulk rock spectrum may not hold true. The spectral properties of some microcrystalline (<20 um) phases also differ slightly from their macrocrystalline counterparts; within the microcrystalline silica suite ("cherts"), wide spectral variability is observed between samples [1]. Thus our first step is to understand the spectral variability observed within microcrystalline chemical precipitates that are found in terrestrial sedimentary rocks. Here we describe several causes of thermal infrared spectral variability in terrestrial "chert" samples, which were identified using emission, micro-FTIR, micro-Raman spectroscopy and SEM analyses. In this work, "chert" refers to microcrystalline silica with fibrous (a.k.a. chalcedony) or non-fibrous fabric, and may consist of pure alpha-quartz or a mixture of alpha-quartz with the low-temperature silica polymorph, moganite (which typically occurs as intergrowths within chert). Moganite is so prevalent within terrestrial cherts that it has been suggested that its absence indicates high water to rock ratios in the formation environment. Using microspectroscopy, we have isolated the first TIR reflectance spectrum for moganite. Increasing proportions of moganite within chert samples primarily has the effect of decreasing the 8.47-um reflectance peak within the quartz restrahlen ~9 um "doublet", thus accounting for some of the spectral variability observed between chert samples. A second, widely observed, spectral characteristic of some natural chert surfaces are narrow and pointed quartz doublet reflectance peaks (or emission minima) compared to those of large quartz crystals, which are wide and rounded [1]. We find this spectral effect is primarily related to the surface roughness or texture of the sample. In most cherts, we find "pointed" doublet features occur for both natural and cut surfaces. However, these features become more rounded with well-polished versions of these samples. Conversely, the rounded doublet peaks in macrocrystalline quartz become pointed simply by cutting the sample. The drastic change in spectral shape may be caused by transmission through optically thin edges of small quartz crystals protruding from the surface. We speculate the prevalence of pointed features in natural cherts is due to the size of the microcrystalline quartz crystals influencing the scale at which the surface is roughened by weathering and/or erosional processes. We also speculate that the act of cutting samples creates grooves on the surface where individual grooves have a thickness on the scale of typical chert crystals, resulting in "pointed" quartz features. SEM cross sections for several samples will provide insight into the relationship between surface texture and the shape of quartz spectral features. [1] McDowell, M.L., (2009) PhD Diss., U. Hawai'i
Hardgrove Craig J.
Rogers David
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