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Shock-induced color changes in nontronite: Implications for the Martian fines

Weldon, Ray J. and Thomas, Warren M. and Boslough, Mark B. and Ahrens, Thomas J. (1982) Shock-induced color changes in nontronite: Implications for the Martian fines. Journal of Geophysical Research B, 87 (B12). pp. 10102-10114. ISSN 0148-0227. doi:10.1029/JB087iB12p10102.

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Riverside nontronite, a candidate for the major mineral in the Martian fines, becomes both redder and darker upon shock loading between 180 and 300 kbar. The change from olive-yellow (2.5 Y 6/6) to strong brown (7.5 YR 4/6) in the 300-kbar sample brackets the range of color observed at the Viking lander sites. Optical microscopy, X-ray diffraction, optical, infrared, and ^(57)Fe Mössbauer spectroscopy were applied to understand the physical basis of the color change. The Riverside nontronite experienced partial dehydroxylation, probably due to shock-induced heating, that changed the coordination of the Fe3+ in the octahedral layer of the clay to a mixture of 4- and 6-fold or a distorted 5-fold coordination. These changes in the clay cause the O^(2−)-Fe^(3+) charge transfer absorption edge to shift from the near ultraviolet into the visible, producing a redder and darker phase. The absorption spectra of both impacted and nonimpacted Riverside nontronite contains the basic features of the reflectance spectra of the bright regions of Mars: a steep drop in absorption from the near UV into the visible and a featureless near IR region. Calculations indicate that significant impact induced color changes (and dehydration) can occur on Mars, though it seems likely that the mechanism would be more effective, volumetrically, at producing variations in color rather than affecting the absolute color.

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Additional Information:Copyright 1982 by the American Geophysical Union. (Received December 15, 1981; revised April 27, 1982; accepted June 18, 1982.) Paper number 2B0926. We appreciate the experimental assistance of J. Long, E. Gelle, and M. Long in the shock experiments. We thank A. Baird for samples of nontronite and helpful discussion. We are indebted to G. Rossman, R. Aines, and S. Mattson for assistance in optical and IR sample analysis and interpretation. We thank E> Stolper and D. Van Alstine for performing the heating and magnetometer experiments. M. Lange provided informative discussions regarding cratering calculations. S. Yamada typed the final manuscript. We appreciate the efforts of the reviewers. This research was supported by NASA (NGL-05-002-105), contribution 3726, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125.
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Caltech Division of Geological and Planetary Sciences3726
Issue or Number:B12
Record Number:CaltechAUTHORS:20141103-152831902
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Official Citation:Weldon, R. J., W. M. Thomas, M. B. Boslough, and T. J. Ahrens (1982), Shock-induced color changes in nontronite: Implications for the Martian fines, J. Geophys. Res., 87(B12), 10102–10114, doi:10.1029/JB087iB12p10102.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:51200
Deposited By: George Porter
Deposited On:04 Nov 2014 00:01
Last Modified:10 Nov 2021 19:07

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