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Selenite Gypsum

Texas Geology and Minerals

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.


CalciteTerlingua 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.


FluoriteThe 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.


CalciteThe 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.

Sulfur (yellow) and Calcite (white)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.



Frequently used abbreviations: NPL  Non-vertebrate Paleontology Laboratory | TNSC Texas Natural Science Center | UTDGS Department of Geological Sciences | BEG  Bureau of Economic Geology | VPL Vertebrate Paleontology Laboratory | JSG  Jackson School of Geosciences | SUPPORT | VOLUNTEER | GLOSSARY

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