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Desert
Varnish
Desert varnish is the thin red to black coating
found on exposed rock surfaces in arid regions.
Varnish is composed of clay minerals, oxides
and hydroxides of manganese and/or iron, as
well as other particles such as sand grains
and trace elements. The distinctive elements
are Manganese (Mn) and Iron (Fe).
The
color of rock varnish depends on the relative
amounts of manganese and iron in it: manganese-rich
varnishes are black; manganese-poor, iron-rich
varnishes are red to orange; those intermediate
in composition are usually a shade of brown.
Varnish surfaces tend to be shiny when the varnish
is smooth and rich in manganese.
Desert
varnish consists of clays and other particles
cemented to rock surfaces by manganese emplaced
and oxidized by bacteria living there. It is
produced by the physiological activities of
microorganisms which are able to take manganese
out of the environment, then oxidize and emplace
it onto rock surfaces. These microorganisms
live on most rock surfaces and may be able to
use both organic and inorganic nutrition sources.
These manganese-oxidizing microorganisms thrive
in deserts and appear to fill an environmental
niche unfit for faster growing organisms which
feed only on organic materials.
The
sources for desert varnish components come from
outside the rock, most likely from atmospheric
dust and surface runoff. Streaks of black varnish
often occur where water cascades over cliffs.
No major varnish characteristics are caused
by wind.
Thousands
of years are required to form a complete coat
of manganese-rich desert varnish so it is rarely
found on easily eroded surfaces. A change to
more acidic conditions (such as acid rain) can
erode rock varnish. In addition, lichens are
involved in the chemical erosion of rock varnish.
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