Desert Varnish - Composition

Composition

Originally scientists thought that the varnish was made from substances drawn out of the rocks it coats. Microscopic and microchemical observations, however, show that a major part of varnish is clay, which could only arrive by wind. Clay, then, acts as a substrate to catch additional substances that chemically react together when the rock reaches high temperatures in the desert sun. Wetting by dew is also important in the process.

An important characteristic of black desert varnish is that it has an unusually high concentration of manganese. Manganese is relatively rare in the Earth's crust, making up only 0.12% of its weight. In black desert varnish, however, manganese is 50 to 60 times more abundant. This significant enrichment is thought to be caused by manganese-oxidizing microbes (mixotrophs) which are common in environments poor in organic nutrients. A micro-environment pH above 7.5 is inhospitable for manganese-concentrating microbes. In such conditions manganese-poor (Fe-rich) orange varnishes develop. An alternative hypothesis for Mn/Fe fluctuation has been proposed that consider Mn-rich and Fe-rich varnishes to be related to humid and arid climates, respectively

Even though it contains high concentrations of iron and manganese, there are no significant modern uses of desert varnish. However, some Native American tribes created petroglyphs by scraping or chipping away the dark varnish to expose the lighter rock beneath.

Desert varnish often obscures the identity of the underlying rock, and different rocks have varying abilities to accept and retain varnish. Limestones, for example, typically do not have varnish because they are too water soluble and therefore do not provide a stable surface for varnish to form. Shiny, dense and black varnishes form on basalt, fine quartzites and metamorphosed shales due to these rocks' relatively high resistance to weathering.

As external links show, desert varnish is suggested as a hint for life on other planets.