Crushed stone

Crushed stone is any rock that has been broken by mechanical means into smaller fragments. The output from the crushing process is usually screened to separate the material into different size categories, ranging from dust to boulders. The use of crushed stone in construction depends on the type of stone and its physical characteristics as determined by using standard engineering tests. Although crushed stone is utilized in a wide variety of applications, the construction industry utilizes some 80 percent of mine output.

Nepheline syenite, limestone/dolostone, sandstone, quartzite, novaculite, slate, and volcanic tuff are the major types of stone that have been quarried and utilized in Arkansas as crushed stone. Of this list, novaculite is no longer used due to its highly abrasive effects on crushing equipment and problems concerning its use as an aggregate in asphalt. Transportation costs to a specific job site are a major economic consideration when determining whether to use rock from any given quarry. Currently, all of these rock types are mined from open pits.

In 2005 total production and value of crushed stone construction aggregates from Arkansas amounted to 35.4 million metric tons and $223 million, up significantly in recent years. Each particular rock type is discussed briefly below.

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Nepheline Syenite

Much of Arkansas's nepheline syenites have high strength and weather-resistant properties and are crushed for use as roofing granules, road materials, riprap, and asphalt and concrete aggregate. Fines are used as a colorizing and fluxing agent in the manufacture of brick and as compaction fill. Historically, this rock has been used as a building, monument, and paving stone and for railroad culvert and bed construction. Syenite was also used extensively as riprap for the protection of river banks and road embankments. Arkansas's syenite deposits have been studied as a potential source of the mineral nepheline, which is used in the manufacture and fabrication of glass. However, the inclusion of various fine-grained iron-bearing minerals in this mineral results in too high of an iron value to manufacture low-iron glass. The development of new separation technology may allow the use of Arkansas nepheline concentrates for specialty markets. Several syenite deposits have been examined for potential use as low free-silica (minimal quartz) sand-blast abrasive. Federal government form listings term the rock as granite for regulators.

Nepheline syenite and its associated igneous rocks are exposed in 4 areas of the state: south-central Pulaski County between Little Rock and Sweet Home, Saline County in the vicinity of Bauxite, Garland County at Potash Sulphur Springs, and Hot Spring County at Magnet Cove. The total surface exposure of syenite in Arkansas is about 13 square miles.

Nepheline syenite is quarried at Granite Mountain in Pulaski County by several companies. It is crushed and sized for several aggregate uses. Crushed, sized roofing granules, colorized to builder’s specifications, are produced at a plant east of Little Rock, using syenite from a nearby quarry. Nepheline syenite has also been quarried near Bauxite in Saline County and at the Diamond Jo quarry in Magnet Cove, Hot Spring County. Presently, one company is producing nepheline syenite from a quarry near Bryant in Saline County. Mining of nepheline syenite exceeds 5 million short tons annually.

Limestone/Dolostone

All of the dolostone and most of the limestone in Arkansas are Paleozoic in age and are present in the Ozark region. A small amount of Paleozoic limestone in the Ouachita Mountains has been quarried.

Often, little distinction is made between limestone and dolostone because they are often interchangeable in their uses. Both are frequently sold under the name of limestone. Perhaps no other mineral resource has as many uses as limestone and dolostone. These two rocks are the basic building blocks of the construction industry. The principal aggregate uses are as crushed stone, riprap, asphalt fillers, and road fill material.

Crushed limestone, used largely as concrete and asphalt aggregate, is the major product of limestone/dolostone mining in Arkansas. Several companies in Benton, Independence, and Lawrence Counties mine and crush the stone. The bulk of the crushed material is used in road construction and concrete aggregate. In 2005, 1 dolostone and 27 limestone quarries were active, with total production of 13.5 million short tons, valued at 81.7 million.

Sandstone/Quartzite

Most of the sandstone quarried in Arkansas is crushed and used for aggregate in concrete and asphalt. Large blocks (riprap) are used for fill and in dike and jetty construction.

There are practically unlimited quantities of sandstone in the Paleozoic Highland area of Arkansas. An almost unlimited amount of this resource is present in the Boston Mountains and, to a lesser extent, in the Springfield and Salem Plateaus. The important sandstone units are principally in the lower Atoka Formation, Bloyd Shale, Hale Formation, Batesville Sandstone, St. Peter Sandstone, and Everton Formation. These sandstone-bearing units range in age from Ordovician to Pennsylvanian. The Arkansas Valley contains vast quantities of sandstone in the Savanna Formation, Hartshorne Sandstone, Atoka Formation, and Hale Formation (all Pennsylvanian). In the Ouachita Mountain region, sandstone is abundant in the Atoka Formation, Jackfork Sandstone, Stanley Shale, Blaylock Sandstone, Blakely Sandstone, and Crystal Mountain Sandstone. These formations range in age from Ordovician to Pennsylvanian.

Major aggregate quarries produce sandstone-based products near the larger cities and at other strategic sites in the Paleozoic Highlands of Arkansas.

Future demands for sandstone aggregate sources should continue to expand, notably near our larger communities, near and along the Arkansas River, and in the southern Ouachita Mountains. The nearby states of Louisiana, Mississippi, and Texas have been areas of major markets for high-quality Arkansas sandstone. In 2005, 18 quarries produced 10.5 million metric tons of crushed sandstone and quartzite, valued at $67.4 million.

Slatey Shale/Metamorphosed Shale and Sandstone

Slaty shale is mined from the core area of the Ouachita Mountains by Certain Teed Corporation. Early operations in the 1920s centered on Ordovician age slatey shales for roofing tiles, but they did not hold up to weather conditions in their markets and closed a few years after startup. Recent "slate" mining has been mostly restricted to Montgomery County. Slaty shale in the Stanley Shale (Mississippian), north of Glenwood, is hauled from Certain Teed’s open pit to a preparation plant where it is crushed and ground into granules for roofing. The final product is black.

Martin Marietta Materials Company produces aggregates for asphalt mix at their Jones Mill quarry near Magnet Cove in Hot Spring County. The rock is contact metamorphosed sandstone and shale of the Stanley Shale (Mississippian), adjacent to the Magnet Cove intrusive complex. The company also has an asphalt plant on site. Production is withheld by the USGS to avoid disclosing proprietary company data.

Tuff

In Arkansas, the Hatton tuff lentil of the Stanley Shale (Mississippian) is exposed in Polk County. Southwest of the community of Hatton, the tuff has a maximum thickness of 300 to 400 feet, but 90 feet is more common. Increased thickness is due to repetitive reverse faulting at the quarry location. The tuff is massive, homogeneous, and jointed so that determination of bedding is difficult. The unweathered fine-grained rock is dark greenish gray and may appear spotted due to light-colored feldspar crystals. Under the microscope, numerous broken volcanic glass fragments (shards) compose much of the rock. The unweathered rock is tough, compact, and may contain Late Pennsylvanian milky quartz veins.

The Hatton tuff is now used in Arkansas as an aggregate as fresher portions of the deposits are mined by Martin Marietta Materials Company. It was previously shipped to and continues to be an available concrete aggregate resource for east Texas. The Hatton also has potential as a cementing agent. Annual production is included in miscellaneous crushed stone by the USGS to avoid disclosing company proprietary data.

References

U. S. Geological Survey in cooperation with the Arkansas Geological Commission, 2000, Arkansas 1999 annual estimate: U. S. Geological Survey Mineral Industry Surveys, 6 p.

White, D. H., Jr., and Bush W. V., 1991, The mineral industry of Arkansas, in US Geological Survey Mineral Industry Surveys, Arkansas: p. 89-98.