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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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