[meteorite-list] Desert Varnish formation

[Kevin Forbes]

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Desert Varnish formation

http://waynesword.palomar.edu/ploct97.htm

One of the most remarkable biogeochemical phenomena in arid desert regions 
of the world is desert varnish. Although it may be only a hundredth of a 
millimeter in thickness, desert varnish often colors entire desert mountain 
ranges black or reddish brown. Desert varnish is a thin coating (patina) of 
manganese, iron and clays on the surface of sun-baked boulders. Desert 
varnish is formed by colonies of microscopic bacteria living on the rock 
surface for thousands of years. The bacteria absorb trace amounts of 
manganese and iron from the atmosphere and precipitate it as a black layer 
of manganese oxide or reddish iron oxide on the rock surfaces. This thin 
layer also includes cemented clay particles which help to shield the 
bacteria against desiccation, extreme heat and intense solar radiation. It 
has been estimated that up to 10,000 years are required for a complete 
varnish coating to form on boulders in extreme arid desert regions.

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Kevin, VK3UKF.

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