Wyoming Bentonite

Bentonite includes any natural material dominantly composed of clay minerals in the smectite group. 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, of which its end market percent uses are: kitty litter (25 percent), drilling mud (20 percent), binder in foundry molds (19 percent), iron ore pelletizing (13 percent), and other applications (23 percent). 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 non-swelling variety, is relatively unimportant in Wyoming production.

American Colloid Company, Colony, Wyoming plant. [Credit: Wayne Sutherland, WSGS]

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. Currently 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.

According to the Wyoming Mining Association, bentonite deposits in Wyoming comprise as much as 70 percent of the world's known supply, making the state the nation's leader in bentonite production. 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, Southern Black Hills (Clay Spur) District, Kaycee District west of Kaycee, and Eastern Big Horn Basin. Bentonite from pits in these areas is blended, ground, dried, and processed into various products at 15 active mills in Wyoming.

Summary Report

Bentonite soil from the Mowry Formation. [Credit: Wayne Sutherland, WSGS]


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 pl., 45 p.

Virta, R.L., 2005, Clays: U.S. Geological Survey Mineral Commodity Summaries 2005, 2p.


Patty Webber, patty.webber@wyo.gov