Spectral Properties of Natural and Synthetic Allophanes, Imogolites, Hydrated Silica and Phyllosilicates and Applications to Mars

Details Spectral Properties of Natural and Synthetic Allophanes, Imogolites, Hydrated Silica and Phyllosilicates and Applications to Mars Spectral Properties of Natural and Synthetic Allophanes, Imogolites, Hydrated Silica and Phyllosilicates and Applications to Mars

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p34b..03b&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #P34B-03

Physics

[3672] Mineralogy And Petrology / Planetary Mineralogy And Petrology, [3694] Mineralogy And Petrology / Instruments And Techniques, [5464] Planetary Sciences: Solid Surface Planets / Remote Sensing, [6225] Planetary Sciences: Solar System Objects / Mars

Scientific paper

We have measured reflectance and emissivity spectra of a number of natural and synthetic poorly ordered materials for comparison with spectra of the martian surface. Here we focused on the near-IR spectra of these materials, the shape and character of the bands near 1.4, 1.9 and 2.2 μm and how these features change with Al/Si/Fe composition and relative humidity (RH) conditions. Spectra acquired under low RH conditions include the following general features: allophane exhibits a doublet at 1.38/1.41, and bands near 1.92 and 2.190-2.199 μm; imogolite has a doublet at 1.37/1.39 and bands near 1.92-1.93 and 2.188-2.197 μm; opal exhibits a doublet at 1.37/1.41 μm and bands near 1.91 and 2.20-2.26 μm. Observed variations depended on the chemistry and character of the sample, e.g., the OH bands for Fe-bearing allophanes shifted toward longer wavelengths and additional bands for Fe-bearing opal were observed at 2.26 and 2.30 μm. The spectra of allophane acquired under ambient conditions barely showed the ~1.4 μm bands that are masked by a much larger ~1.45 μm H2O band and the ~2.2 μm band broadened into a shoulder, likely due to H-bonding. Variations in H2O content cause structural changes in some phyllosilicates [1]. RH-controlled X-ray diffraction experiments indicated different trends: montmorillonite exhibited increasing low-angle intensity at elevated RH, no changes in intensity were observed for opal, and both allophane and imogolite exhibited decreasing low-angle intensity with increasing RH. Poorly ordered materials such as allophane and imogolite exhibit spectral features near 2.2 μm similar to those of Al-phyllosilicates and hydrated silica. Allophane and imogolite frequently form as volcanic materials alter and can be viewed as protoclays. We propose that such poorly ordered materials may be present on Mars in regions where phyllosilicates have been detected. [1] Bish,DL,etal.(2003)Icarus 164, 96-103.

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