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Proceedings Paper

Making silica rock coatings in the lab: synthetic desert varnish
Author(s): Randall S. Perry; Vera M. Kolb; Ajish I. Philip; Bridget Y. Lynne; Nicola McLoughlin; Mark Sephton; David Wacey; Owen R. Green
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Paper Abstract

Desert varnish and silica rock coatings have perplexed investigators since Humboldt and Darwin. They are found in arid regions and deserts on Earth but the mechanism of their formation remains challenging (see Perry et al. this volume). One method of researching this is to investigate natural coatings, but another way is to attempt to produce coatings in vitro. Sugars, amino acids, and silicic acid, as well as other organic and (bio)organic compounds add to the complexity of naturally forming rock coatings. In the lab we reduced the complexity of the natural components and produced hard, silica coatings on basaltic chips obtained from the Mojave Desert. Sodium silicate solution was poured over the rocks and continuously exposed to heat and/or UV light. Upon evaporation the solutions were replenished. Experiments were performed at various pH's. The micro-deposits formed were analyzed using optical, SEM-EDAX, and electron microprobe. The coatings formed are similar in hardness and composition to silica glazes found on basalts in Hawaii as well as natural desert varnish found in US southwest deserts. Thermodynamic mechanisms are presented showing the theoretical mechanisms for overcoming energy barriers that allow amorphous silica to condense into hard coatings. This is the first time synthetic silica glazes that resemble natural coatings in hardness and chemical composition have been successfully reproduced in the laboratory, and helps to support an inorganic mechanism of formation of desert varnish as well as manganese-deficient silica glazes.

Paper Details

Date Published: 22 September 2005
PDF: 11 pages
Proc. SPIE 5906, Astrobiology and Planetary Missions, 59060U (22 September 2005); doi: 10.1117/12.626546
Show Author Affiliations
Randall S. Perry, Univ. of Oxford (United Kingdom)
Planetary Science Institute (United States)
Vera M. Kolb, Univ. of Wisconsin-Parkside (United States)
Ajish I. Philip, Univ. of Wisconsin-Parkside (United States)
Bridget Y. Lynne, Univ. of Auckland (New Zealand)
Nicola McLoughlin, Univ.of Oxford (United Kingdom)
Mark Sephton, Imperial College (United Kingdom)
David Wacey, Univ. of Oxford (United Kingdom)
Owen R. Green, Univ. of Oxford (United Kingdom)

Published in SPIE Proceedings Vol. 5906:
Astrobiology and Planetary Missions
Richard B. Hoover; G. Randall Gladstone; Gilbert V. Levin; Alexei Yu. Rozanov, Editor(s)

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