Desert varnish

As desert varnish, a dark coating of iron and manganese oxides is called, which occurs typically on rock surfaces in deserts. It occurs only on the rock surfaces, which are exposed to the wind erosion, or other types of erosion, which are therefore physically stable.

In addition, the wind cut ( corrasion ) is caused varnish- like, called matte sheen on rocks, which is often already eliminated by precipitation as desert varnish.

Desert coating formed during the evaporation of capillary rising solutions of these metals from the interior of the rock in conjunction with clay, which is blown in from outside. These substances react by the high temperatures in the desert, where obviously the moisture of morning dew plays a role. By the shares of several other trace elements and organic compounds, the color can take various shades of brown to deep black.

Although the term " desert varnish " suggests an occurrence in the desert areas of the world, this phenomenon also occurs in the temperate zones of the earth. Especially Germany has achieved with the sandstone deposits in the Hohe Mark Nature Park - Westmünsterland similarly colored rocks with the same coloring mechanisms. The blackening of the encountered there " rind sandstone " is always on the side facing the sun. This phenomenon is in Germany, perhaps only possible because this rock on the one hand is very porous and the other hand is downright cemented by iron compounds. The sites with the occurrence of desert varnish are for example in the Haard on the Stimberg strictly protected.

Desert varnish is mainly produced on rocks such as basalt, quartz rocks and metamorphic rocks harder, because they are very stable. Made of limestone, no desert varnish can form, the material is very porous and contains a lot of water, so it is physically unstable.

Puzzling is the high percentage of manganese in the desert varnish. Manganese occurs in the earth's crust in a proportion of only about 0.12 per cent, in the desert varnish, the concentration is, however, at the 50 - to 60-fold increases. So far one explain this phenomenon by bacterial activity in desert varnish and by a biochemical enrichment of the material.

Although desert varnish has a high content of iron and manganese, there is no commercial use of the material. So-called petroglyphs were popular among various tribes of Indians. These are images that have been scratched into the paint and so among which are clearly visible through the lighter rock.

• Chemical sedimentary rock
• Diagenesis