MINERALOGIC FEATURES OF BENTONITE BEDS OF THE BELLE FOURCHE AND MOWRY SHALES OF THE BLACK HILLS DISTRICT

The bentonite deposits of the Black Hill District are located in the tri-corner region of South Dakota, Montana, and Wyoming. These deposits are notable because of the large volume of sodium-type bentonite present in the Cretaceous sediments. Montmorillonite is the dominant clay mineral with contents ranging from 65-90 %. Non-clay minerals found in bentonite include quartz, gypsum, selenite, small flecks of mica, and grains of feldspar. Efflorescent alkali salts may occur where bentonite has been exposed to water. Key physical properties of a bentonite-water mixture such as dilatancy (swelling capacity), viscosity, gel strength, and bonding strength dictate how it is marketed. Bentonite from this region is generally of the highly dilatant, gel-forming, sodic variety that is desirable for use in drilling mud, kitty litter, foundry sand bonding, and iron ore pelletizing.

Detailed characterization of these clay beds yields insight into the effects of weathering on bentonite, the geochemical composition and likely sources of the volcanic ash beds from which the bentonite was derived, and the inter-layered migration of elements within the bentonite that may or may not be an environmental concern. This study compares the vertical variation in mineralogy and mineral morphology of bentonite beds. Of particular interest is the morphology of silica, the mineralic residence of iron, and the variability in exchangeable cations and trace elements along horizons in the bedding.