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Desert Rock Varnish
orinoco river rocks

The slowest accumulating terrestrial sedimentary deposit known, desert varnish is usually dark brown to black in color depending on the relative abundance of MnO2. Archaeologists often utilize desert varnish for relative age determinations. Although this application has proven unreliable, recent studies suggest that the chemical microstratigraphy in desert varnish can provide valuable information about past environmental fluctuations. Other applications of desert varnish have also been found. While some varnish deposits are uniform and continuous, most desert varnish appears in discontinuous patches of variable thickness and texture (Staley et al., 1991). The primary components which make up desert varnish are clays and iron and manganese oxides that are derived from air-borne dust and other sources external to the underlying rock. Clay minerals, such as montmorillonite and illite, comprise more than 70 percent of the varnish and are involved in manganese fixation. Iron (predominantly ferric) and manganese oxides constitute the bulk of the remainder (~30%) and are dispersed
throughout the clay layer.

river rocks

Desert varnish has been around for nearly 100,000 years, and can be found worldwide in arid to semiarid environments. For centuries, desert varnish has invoked scientific curiosity. Alexander von Humboldt first noticed desert varnish in the early nineteenth century while traveling through South America (Liu and Broecker, 2000); Charles Darwin observed it on his early expeditions as well (Staley et al., 1991). Defined as a distinct morphological entity which appears naturally as a thin, dark coating, desert varnish has an abrupt boundary with the underlying rock

river rock varnish

Varnishes occur on nearly all rock types, including quartzites, which have little or no iron, manganese, and clay minerals (Thiagarajan and Lee, 2004); however, varnishes are less common on limestone than on the less calcareous rocks (Hunt, 1954). Manganese-rich varnishes are found in both wet and dry environments. The term “desert varnish” refers specifically to varnishes which form in arid or semi-arid regions. Coated and polished surfaces found in alpine regions, glacial moraines, and in streams and intertidal marine environments are usually referred to by the general term “rock varnish.” These varnishes may appear superficially similar to desert varnish, but differ mineralogically (Staley et al., 1991).

desert rock varnish

The primary components which make up desert varnish are clays and iron and manganese oxides that are derived from air-borne dust and other sources external to the underlying rock. Although these varnishes are widespread, no comprehensive global comparisons of their occurrences and properties have been conducted. Most work on desert varnish has been directed towards its characterization and understanding its origin (Staley et al., 1991). Clay minerals, such as montmorillonite and illite, comprise more than 70 percent of the varnish and are involved in manganese fixation. Illite in particular is known to fix Mn under pH and oxidation-reduction conditions at which varnish forms. Small amounts of kaolinite and chlorite are present in some samples (Potter and Rossman, 1977). The average composition of the complex clay mixture in extracted varnish was determined by electron microprobe analysis to be as follows: Na0.2 K0.3(Al1.6Mg0.3Fe0.2)(Si3.5Al0.5O10)(OH)2 Iron (predominantly ferric) and manganese oxides constitute the bulk of the remainder (~30%) and are dispersed throughout the clay layer. These oxides are likely present as external coatings on the clay particles. It has been suggested that the clay and oxide phases in desert varnish may be mutually dependent: the clay depends on oxides for resistance to erosion, and oxides depend on clay for transport and deposition (Potter and Rossman, 1977).

rock varnish

In 1981, Dorn and Oberlander observed that manganese-rich varnish is often present in places where water intermittently streams over rock surfaces. These moistened surfaces are favorable for microbial colonization and development. Based on these observations, they suggested that natural desert varnish and other manganese rich rock varnishes in nondesert environments are products of microbial activity in which microorganisms concentrate ambient manganese that becomes greatly enhanced in brown to black varnish.


The growth rates of rock varnish vary from <1 to 40 µm per thousand years on subaerially exposed rock surfaces and rarely reach thicknesses >200 µm, regardless of age. The primary components which make up desert varnish are clays (nearly 70%) and iron and manganese oxides that are derived from air-borne dust and other sources external to the underlying rock

 

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