As desert varnish (English desert varnish a dark, glossy coating is sometimes) mainly composed of clay minerals referred to, that of iron - and manganese - oxides is held together and, above all on sandstone in deserts occurs. It only occurs on rock surfaces that are not exposed to wind erosion or other types of erosion , which means that they are physically stable.
In addition, the varnish- like matt sheen on rocks caused by wind grinding ( corrosion ) , which is often removed by precipitation, is also referred to as desert varnish. This species is not discussed here.
Origin and occurrence
Desert varnish arises from the superficial evaporation of capillary water solutions with iron and / or manganese content in connection with clay particles that are blown from the outside. Water from night dew plays a role. Desert varnish can also contain trace elements and organic components; its color can take on various shades of brown up to deep black.
Although the term "desert varnish" indicates an occurrence in the desert regions of the world, this phenomenon also occurs in the temperate latitudes of the world. With the sandstone deposits in the Hohe Mark-Westmünsterland Nature Park, Germany in particular has similarly colored rocks with the same coloring mechanisms. The black color of the "rind sandstone" found there is always on the side facing the sun. This phenomenon is perhaps only possible in Germany because this rock is on the one hand very porous and on the other hand is downright cemented by the iron compounds. The places with the appearance of the desert varnish are, for example, in the Haard on the Stimberg under strict nature protection.
Desert varnish occurs mainly on rocks such as basalt , quartz and harder metamorphic rocks, as these are very stable. Desert varnish can rarely form on limestone , the material is very porous and contains a lot of water, which is why it is physically unstable.
The high proportion of manganese in the desert varnish is striking . The earth's crust contains only about 0.12 percent manganese, but in the desert varnish its concentration is increased 50 to 60 times. So far, this phenomenon has been explained by bacterial activity in the desert varnish and by a biochemical enrichment of the material.
So-called petroglyphs were popular with various tribes of the Indians . By processing - scratching, scraping - the desert varnish, it is removed in places, making the lighter stone underneath visible and thus depicting motifs.
- scinexx.de: Mineral crusts turn light into electricity. April 23, 2019
- Where to Look for Life on Mars. On: Wissenschaft.de , December 18, 2008.