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Geology; July 2006; v. 34; no. 7; p. 537-540; DOI: 10.1130/G22352.1
© 2006 Geological Society of America
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Baking black opal in the desert sun: The importance of silica in desert varnish

Randall S. Perry1, Bridget Y. Lynne2, Mark A. Sephton3, Vera M. Kolb4, Carole C. Perry5 and James T. Staley6

1 Impacts and Astromaterials Research Centre, Department of Earth Science and Engineering, South Kensington Campus, Imperial College, London SW7 2AZ, UK, and Planetary Science Institute, Seattle, Washington 98115, USA
2 Department of Geology, University of Auckland, Private Bag 92019, Auckland, New Zealand
3 Impacts and Astromaterials Research Centre, Department of Earth Science and Engineering, South Kensington Campus, Imperial College, London SW7 2AZ, UK
4 Department of Chemistry, University of Wisconsin-Parkside, Kenosha, Wisconsin 53141-2000, USA
5 Chemistry Division, School of Biomedical Natural Sciences, Nottingham Trent University, Clifton Lane, Nottingham NG118NS, UK
6 Department of Microbiology, School of Medicine, University of Washington, Seattle, Washington 98195-1310, USA

Desert varnish, a widespread black manganese-rich rock coating, contains labile organic compounds, but a mechanism for its formation and for their preservation remains unproven. Using Raman spectroscopy, X-ray diffraction, and scanning transmission electron microscopy, we analyzed varnish and found amorphous hydrated silica (opal) and the silica mineral moganite, similar to findings we have reported from siliceous hot-spring deposits. We suggest that the slow dissolution of silica from anhydrous and hydrous minerals, and its subsequent gelling, condensation, and hardening, provides a simple explanation of a formation mechanism for desert varnish and silica glazes and the incorporation of organic material from local environments. These chemical signatures, sequestered in silica, provide valuable information about terrestrial and extraterrestrial paleoenvironments.

Key Words: desert varnish • silica glaze • silica • rock varnish • rock coatings • moganite • paleoenvi-ronments




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