Geology July 2006

Baking Black Opal in the Desert Sun: The Importance of

Silica in Desert Varnish

Randall S. Perry^{1 and 2}, Bridget Y. Lynne³, Mark Sephton¹, Vera M. Kolb⁴ Carole C. Perry⁵, James T. Staley⁶

¹ Impacts and Astromaterials Research Centre, Department of Earth Science and Engineering, South Kensington Campus, Imperial College, London SW7 2AZ, UK

²Planetary Science Institute, 6920 Roosevelt Way NE 177, Seattle, WA 98115 USA

³Department of Geology, University of Auckland, Private Bag 92019, Auckland, New Zealand

⁴Department of Chemistry, University of Wisconsin-Parkside, Kenosha, Wisconsin 53141-2000, USA

⁵Chemistry Division, School of Biomedical Natural Sciences, The Nottingham Trent University, Clifton Lane, Nottingham NG118NS, UK

⁶Department 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 their preservation remains unproven. Using Raman spectroscopy, X-ray diffraction and scanning transmission electron microscopy, we analysed the varnish and found amorphous hydrated silica (opal) and the silica mineral moganite, similar to those 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 extra terrestrial paleo-environments.