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Texas Geology and Minerals |
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The
Llano Uplift The oldest rocks exposed in Texas formed over a
billion years ago when thick sequences of sediment were deposited in an ancient
sea that bordered the North American craton. Approximately one billion
years ago, the southern edge of this craton collided with another continent or
an oceanic plate in an tectonic event that metamorphosed the sediments into
gneisses and schists and partially melted more deeply buried rocks producing
granitic magmas. The metamorphic and igneous rocks produced in this
orogeny, the Packsaddle Schist, Lost Creek and Valley Spring Gneisses, Town
Mountain and finer-grained granites, are now exposed in the
Llano Uplift
in central Texas. The ultramafic rocks and serpentinites that make up the Coal
Creek igneous complex are though to represent the remnants of the island arc
that collided with the North American craton.
A wide variety of minerals are found
in the Llano Uplift. Quartz, microcline feldspar, plagioclase, biotite,
and hornblende amphibole are common constituents of granites and gneisses.
Garnet, muscovite, biotite, staurolite and other metamorphic minerals occur in
schists produced through the metamorphism of clay mineral-rich sediments.
Tourmaline occurs in some schists in Llano county and in quartz veins in
the Town Mountain granite near Llano. Serpentinite, an ultramafic rock
composed of serpentine group minerals, occurs in the Coal Creek igneous complex.
One of the most interesting of the Llano Uplift minerals is topaz, the Texas
state mineral). Colorless to pale blue gemstone-quality topaz crystallized
from fluorine-rich gases in Llano Uplift pegmatites. A photograph of one of
these colorless topazes is shown in the center photograph at the top of this
page. These coarse-grained pegmatites are the last vestiges of water-rich
granitic magma. Coarse-grained crystals of smoky quartz, microcline,
beryl, scheelite, and cassiterite also occur with topaz in the pegmatites.
 Terlingua
Texas is the site of one of the world's largest mercury deposits. At one
time this mine produced approximately one forth of the mercury used in the
United States. The mercury ore, cinnabar, occurs as powdery red veins in
the layered limestones and volcanic lavas and tuffs in Brewster County.
Liquid mercury also occurs in Terlingua. Industrial use of mercury
declined when it became known just how toxic mercury was, and mining ceased at
Terlingua in the 1970's.
Solution cavities and small caves in
Brewster County contain beautiful curving crystals of satin spar gypsum (the
white crystals shown at the left side of the top of the page). These
wave-like forms of satin spar gypsum are not unique to Brewster County, Texas.
Mammoth Cave in Kentucky contains similar curving crystals of satin spar.
Beautiful calcite scalenohedral crystals of calcite also precipitated in
water-filled solution cavities in Brewster County. The calcite
scalenohedrons in the specimen pictured above right occur as tiny, delicate
transparent and colorless and brick-red and opaque crystals indicating that the
trace-element composition of the calcite varied over time.
 The
Eagle Mountains fluorite deposits occurs as fracture fillings and
replacement deposits in Tertiary rhyolite tuffs and dikes that accumulated over
or intruded Lower Cretaceous limestones, sandstones, and quartzites of the Eagle
Mountains of Hudspeth County. Fluorite typically occurs with quartz,
calcite, and iron oxides in veins in fractured limestones. The Eagle
Mountains fluorite occurs as masses of intergrown cubes, coarse-grained massive
material, and color-banded and layered masses. The replacement deposits
formed where limestones were most extensively fractured. Fluorite appears
to have formed through the reaction of limestone and gaseous fluorine from the
magma that produced the rhyolite tuffs and dikes.
Pure fluorite is colorless.
Natural fluorite occurs in huge range of colors due to natural irradiation of
trace quantities of rare-earth elements in the fluorite. Dr. George
Rossman, a mineralogist at Cal Tech, has shown experimentally that bombarding
colorless fluorite with gamma rays will make the colorless fluorite turn purple.
The Eagle Mountains fluorite deposits
were discovered because of the need for fluorine during World War II. Glenn
Evans was enlisted by the United States Government during World War II to
prospect for minerals required for the war effort. After the war, Glen
Evans became a geologist with the Texas Memorial Museum and some of the Eagle
Mountains fluorite was acquired by the museum.
Fluorite also occurs in pegmatites
north of Mason in the Llano Uplift and in the igneous rocks in Chinati Peak in
Presidio County.
 The
Edwards Plateau was formed about ten million years ago when movement
along the Balcones fault zone thrust the Cretaceous rocks of the plateau
approximately two thousand feet above see level. The carbonate rocks in
the Edwards Plateau contain abundant solution cavities, many of which are lined
with well-formed crystals of calcite and celestite (shown at top of page, far
right photograph). Slightly acidic groundwater percolated through the
rocks of the Edwards Plateau dissolving fine-grained carbonate and sulfate
minerals disseminated through the limestones. These aqueous solutions
later precipitated calcite and celestite as large, well-formed crystals along
the walls of open solution cavities. The calcite and celestite occur as
well-formed crystals because crystallization occurred in solution-filled
cavities. If the cavities been mostly air-filled, calcite would
crystallize in a botryoidal form that resembles a mass of grapes.
The
Texas Coastal Plain is the surface expression of a thick wedge of
sediment deposited in the Gulf of Mexico by Texas rivers during the Cretaceous
(144 - 66 million years ago) and Tertiary (66 - 2 million years ago) periods.
The Cretaceous sediments consist primarily of thick sequences of limestone
produced by the accumulation of calcareous shells of organisms. The
Tertiary sediments consist of layers of sand and mud that may reach approach
50,000 feet in thickness.
The sediments of this wedge were
deposited on older thick sequences of halite, gypsum, and anhydrite that had
precipitated from the evaporating waters of a shallow sea in what is now the
Gulf coast about 200 million years ago during the late Jurassic. As the
pile of younger sediments accumulated over the Louann salt layer, the salt was
buried to greater depths under conditions of increasing temperature and
pressure. In time, the salt layer plastically deformed into salt diapirs
or domes that rose through the overlying sediment column due to their lower
density.
 When
the top of a salt dome approaches a few thousand feet of the Earth's surface,
ground water circulates through the salt dome and the surrounding sediments.
The circulating groundwater dissolves halite and precipitates native
sulfur, calcite, gypsum, and anhydrite in open solution cavities in and around
the crest of the salt dome (A sample of sulfur and calcite is shown at the top
of the page at the far left). Large volumes of native sulfur and gypsum
have been mined from the tops or caps of Texas salt domes.
Petrified wood is chalcedony (an aggregate of very fine-grained
cryptocrystalline quartz) that has replaced the organic material that formerly
composed the cells of a tree. Petrified wood still looks like wood, having
the texture or grain of wood, but it is entirely composed of silica (SiO2)
and trace quantities of iron.
Petrified wood forms when trees are
felled and rapidly buried by sediments before decay can occur. Typically
this rapid burial is the result of catastrophic events such as earthquakes,
landslides, and flooding. After burial, silica-bearing aqueous solutions
circulate through the sediments replacing the organic material in the wood with
silica. Petrified wood is found primarily in the unconsolidated sediments
of east Texas in Armstrong, Sabine, Newton, Jasper, Washington, Lee, Fayette,
Gonzales, McMullen, Live Oak, Webb, and Duval Counties.
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