Quartz-(Industrial)
Quartz, or silica (SiO2), is a hard, brittle, usually colorless or white, nonmetallic mineral that exhibits considerable resistance to weathering. Quartz is composed of the two most abundant elements (silicon and oxygen) in the earth's crust, making it common on the earth's surface. Hydrothermal quartz crystals and milky "bull" quartz veins are a common and striking geologic feature of the Ouachita Mountain region of Arkansas. Quartz as single crystals and groups or clusters of clear rock crystal from Arkansas are widely known for their aesthetic beauty. The mineralogical profession has recognized Arkansas quartz crystal as some of the best and purest in the world. Because of this, and the popularity of quartz with many tourists who visit Arkansas each year, the Arkansas General Assembly of 1967 established Act 128, which designated quartz crystal as the official State Mineral.
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Recently, large quantities of quartz crystals have been mined from open pits in two districts in Garland and Montgomery Counties and, to a lesser degree, in Saline and Pulaski Counties. Specimens are sold mostly to tourists, museums, schools, and private collectors domestically and abroad. Collecting in mines is a popular recreational activity, although a fee is required. The U.S. Forest Service has a free quartz crystal collecting site named Crystal Vista in the Ouachita National Forest south of Mount Ida in Montgomery County.
Most of the crystal deposits mined in the Ouachita Mountains formed as veins which filled cavities or fractures in the Crystal Mountain and Blakely Sandstones (both Ordovician). Individual quartz crystals up to 5 feet long and weighing over 500 pounds and clusters up to 15 feet long by 10 feet wide, weighing over 10 tons, have come from Arkansas's mines. Exquisitely developed large single crystals and quartz clusters may be wortj thousands of dollars.
Radiometric dating of adularia, a hydrothermal potassium feldspar present in some quartz veins, yields Late Pennsylvanian to Early Permian ages for the mineralization, placing the major period of quartz vein formation at the end of the Ouachita Mountain orogenic (mountain building) cycle. In the Ozark region of north Arkansas, minor deposits of clear to white, stubby quartz crystals are present, associated with lead and zinc deposits and lining small cavities, nodules ,or concretions in some Ordovician and Mississippian sedimentary units.
Clear, colorless, untwinned quartz crystals had important uses as oscillators in radio equipment and in periscopes, gun sights, and other optical equipment, particularly during World War II. More recently, quartz has been mined for use in electronics, fiber optics, and as the source of silica in the production of synthetic quartz crystals. One company produces the chemical feedstock (lasca) for synthetic quartz growth. Annual production varies from 450 to 570 tons. The milky quartz is mined from veins in an open pit near Paron, Saline County, and trucked to the processing plant near Jessieville, Garland County. There it is crushed, washed, cleaned in an acid bath, then hand sorted to 4 different grades. The quality of lascas produced from Arkansas is equal to or slightly superior to lascas produced in Brazil, and are usually less variable. Some crushed milky quartz was mined in Saline County and used as decorative surface aggregate in precast concrete.
Milky "bull" quartz veins in the Ouachita Mountain region are up to 60 feet thick and hundreds of feet in length. They may strike in any direction, but generally trend across the structure of the host rock. These veins occur in highly deformed shale sequences in the Ouachita Mountains region. Milky quartz veins have been investigated by several companies and individuals for their industrial quartz potential. With the increasing demand by tourists, collectors, museums, and the new commercial applications of synthetic quartz, prices of clear quartz crystal and processed milky quartz should rise.
References
Engel, A. E. J., 1952, Quartz crystal deposits of western Arkansas: U. S. Geological Survey Bulletin 973-E, p.173-260.
Howard, J. M., and Stone, C. G., 1988, Quartz crystal deposits of the Ouachita Mountains, Arkansas and Oklahoma, in Colton, G. W., ed., Proceedings of the 22nd Forum on the Geology of Industrial Minerals: Arkansas Geological Commission Miscellaneous Publication 21, p. 63-71.