Variation in the mineralogical, chemical and physical properties of the Cretaceous Clay Spur bentonite in Wyoming and Montana (U.S.A.)

Abstract

The Creataceous Clay Spur bentonite exhibits unique physiochemical properties which make it suitable for a variety of industrial applications. The bentonite is comprised of sodium smectite altered from rhyolitic ash deposited in the Mowry Sea which occupied the Western Interior seaway from Late Albian to Early Cenomanian time. The seaway was zoned with respect to depth, salinity and degree of oxygenation. The ash, therefore, altered under diverse chemical conditions. Bentonite rheological properties vary locally and regionally. Variation in bentonite quality may be attributed, in part, to variations in the structural chemistry and consequently the surface chemistry, of the smectite. Although the bentonite is mineralogically homogeneous, the smectite is zoned, on a regional scale, with respect to major element, trace element, and exchangeable cation chemistry. Inhomogeneities in the chemical and mineralogical composition of the parent ash, conditions of deposition, the environment of alteration, weathering reactions, and diagenesis are responsible for this chemical zonation. Smectite chemistry is most significantly affected by weathering. Weathering generally improves bentonite quality. Associated with weathering is a decrease in pH, loss of exchangable Ca 2+, decrease in CEC, and a slight reduction in smectite particle size. Changes in surface chemistry are a response to the oxidation of octahedral iron. The plastic viscosity of the Clay Spur bentonite is sensitive to pH, exchangable Ca 2+ concentration and the presence of micron-sized non-clay constituents. Plastic viscosity is weakly dependent on exchangable Mg 2+ and organic matter content. Filter cake permeability is influenced by organic matter content. These factors determine bentonite rheology by enhancing or disrupting the formation of a three-dimensional flocculation card-house network.