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LAND OF THE SLEEPING RAINBOW
The Capitol Reef area was called the “Land of the Sleeping Rainbow”
by the Navajo because of the brilliantly colored canyon walls.
The rocks of Capitol Reef are mostly sedimentary rocks which are
a drab off-white color unless the rocks contain small amounts
of impurities which act as pigments. Iron (“Nature’s Paintbrush”)
in its various chemical states is the most common coloring agent
found in rocks.
COMMON ROCK COLORS
Red to reddish brown to purplish rocks contain hematite
(ocher) which is simply rust or iron oxide (Fe2O3). Less than
half of a percent by weight of hematite is enough to color a rock
brilliantly red. Red rocks were deposited under oxidizing conditions.
A Capitol Reef example: the Moenkopi Formation.
Yellowish to orangish to rusty brown rocks are colored
by limonite (rust containing water: FeO OH nH2O). Limonite forms
under oxidizing and hydrating conditions such as in well-drained
nonmarine or transitional environments that are barren of vegetation.
Geothite, which is a mineral similar to limonite, forms brown
concretions in some sandstone layers such as in the Navajo Sandstone.
A Capitol Reef example: the Dakota Sandstone.
Light blue, greenish gray, and off-white rocks show
the true colors of the sedimentary particles themselves. These
persist in environments with neutral to slightly reducing conditions.
A Capitol Reef example: the Shinarump Member of the Chinle Formation.
Dark green rocks contain minerals which contain reduced
(ferrous) iron, and were deposited in stagnant marine basins,
swamps, bogs, and lakes. A Capitol Reef example: the Morrison
Formation.
Dark gray, brownish gray to black rocks contain incompletely
decomposed organic matter which is preserved under reducing
conditions such as in stagnant marine basins. A Capitol Reef
example: the Mancos Shale.
Bright white rocks may consist of the mineral gypsum
(CaSO4 2H2O) which is an evaporite mineral found in some rock
layers including the Moenkopi Formation and the Carmel Formation.
Gypsum also occurs as clear crystals known as selenite. Thin
white veins of gypsum were deposited by circulating groundwater
and are common in Capitol Reef.
LESS COMMON COLORS
Canary yellow to yellowish green rocks may contain carnotite,
an ore of uranium and vanadium. Carnotite has a strong pigmenting
power and can color a sandstone yellow in concentrations of even
less than one percent. Carnotite may be found in the Shinarump
Member of the Chinle Formation and in the Salt Wash Member of
the Morrison Formation in Capitol Reef.
Bright green and azure blue rocks are colored by the
copper minerals malachite (green) and azurite (blue). These
rocks are found only in small quantities associated with uranium
ores in the Capitol Reef region.
Green to greenish gray splotches in red rocks are caused
by bits of organic matter preserved in the rocks producing local
areas of reducing conditions in rocks otherwise colored red
by oxidized iron. These green areas are commonly caused by plant
roots, animal burrows, or fecal pellets.
COLORED SURFACE COATINGS
The red to dusky brown to black coating on rock surfaces
is desert varnish. The distinctive components of varnish are oxides
and hydroxides of manganese (Mn) and iron (Fe); however, varnish
consists primarily of clay minerals (70%). The color of desert
varnish depends on the relative amounts of manganese and iron
in it: manganese-rich varnishes are black; manganese-poor, iron-rich
are red to orange. The clays and other particles which make up
the bulk of varnish are cemented to rock surfaces by manganese
emplaced and oxidized by bacteria living there. These microorganisms
are able to take manganese out of the environment, and then oxidize
it, and emplace it onto rock surfaces.
These microorganisms thrive in deserts and appear to fill
an environmental niche unfit for faster growing organisms which
feed only on organic material.
Hard white coatings on soils, exposed rock surfaces,
and in fractures are generally made up of calcite (calcium carbonate).
In soils, this coating is sometimes called caliche, calcrete,
or hardpan. Caliche is deposited by groundwater solutions moving
upward and then evaporating at the surface under arid conditions.
White calcite coatings on slickrock surfaces may be caliche
formed at the time of deposition.
SEEING COLORS
White light is made up of a spectrum of electromagnetic radiation
with wavelengths and energies that produce the colors of the rainbow
and which our eyes can detect. When white light strikes an object
(such as a rock) some wavelengths of light are absorbed, the rest
are scattered, transmitted, or reflected. We see the wavelengths
that are not absorbed. For example, if a rock absorbs all wavelengths
of light, it appears black. If a rock absorbs no light, it is
white or colorless. If a rock absorbs all wavelengths of light
except red, it appears red.
A group of elements called the transition elements are the
most important coloring agents in rocks. These elements can
have variable numbers of electrons. Oxidation states, such as
reduced or oxidized, mean that an element has different numbers
of electrons associated with it. Different numbers of electrons
in the electron orbitals cause different energies (and wavelengths)
of light to be absorbed. Therefore, different colors are perceived.
Iron is the most abundant transition element in the earth’s
crust and is thereby the most common pigmenting agent in rocks,
hence “Nature’s Paintbrush.”
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