Bentonite includes any natural material dominantly composed of clay minerals in the smectite group (Hosterman and Patterson, 1992). Wyoming bentonite consists of hydrous silicate of alumina, commonly referred to as montmorillonite clay. Also known as sodium bentonite, Wyoming bentonite is high-swelling sodium montmorillonite clay used in kitty litter (25%), drilling mud (20%), binder in foundry molds (19%), iron ore pelletizing (13%), and other applications (23%) (Virta, 2005). Wyoming Bentonite can swell up to 16 times its original size and absorb up to 10 times its own weight in water. Calcium bentonite, a low or nonswelling variety, is relatively unimportant in Wyoming production.
Bentonite, originally known as ‘mineral soap’ or ‘soap clay’, was named ‘bentonite’ in 1898 by Wilbur C. Knight for deposits in the Benton Shale near Rock River, Wyoming. The Benton Shale included Upper and Lower Cretaceous units lying between the Niobrara and Cloverly Formations and their equivalents. Currenty named equivalent stratigraphic units in Wyoming include the Frontier Formation, Carlile Shale, Greenhorn Formation, Belle Fourche Shale, Mowry Shale, Aspen Shale, Muddy Sandstone, Newcastle Sandstone, Thermopolis Shale, Skull Creek Shale, and Bear River Formation. The best Wyoming bentonite is found primarily in the Upper Cretaceous Mowry Shale (Hosterman and Patterson, 1992).
According to the Wyoming Mining Association, bentonite deposits in Wyoming comprise as much as 70 percent of the world's known supply. In 2005, Wyoming bentonite producers mined 5.2 million tons and milled 4.6 million tons of bentonite. Surface mining to depths no greater than about 50 feet accounts for all bentonite production. Bentonite is mined in Wyoming from numerous pits in various areas of the state. Major producing districts are the Northern Black Hills (Colony) District, the Southern Black Hills (Clay Spur) District, the Kaycee District west of Kaycee, and the Eastern Big Horn Basin. Bentonite from pits in these areas is blended, ground, dried, and processed into various products at fifteen active mills in Wyoming.
Black Hills Bentonite Worland plant, eastern Bighorn Basin.
Hosterman, J.W., and Patterson, S.H., 1992, Bentonite and fuller’s earth resources of the United States: U.S. Geological Survey Professional Paper 1522, 1 plate, 45 p.
Virta, R.L., 2005, Clays: U.S. Geological Survey Mineral Commodity Summaries 2005, 2p.
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