Bentonite is an industrial mineral, rich in a clay mineral called montmorillonite. Bentonites used for hydraulic barriers in North America are produced from the naturally occurring sodium bentonites mined in the state of Wyoming. These bentonites have a unique cation chemistry. The cations that balance electrostatic charge in the clay structure are “exchangeable” cations and consist of sodium (Na+), calcium (Ca++), and Mg (++) cations. The largest deposits of commercial sodium bentonite are found in Wyoming, Montana and South Dakota. Over the years, these special bentonites have become famous and commonly referred to as “Wyoming” bentonite.
The combination of the relative purity, its unusual geochemistry and structure is why “Wyoming” bentonite is used as a sealant. Purity is defined by the amount of sodium montmorillonite as compared to the other minerals present in the bentonite. A typical “Wyoming” bentonite contains ~90% montmorillonite while other bentonites produced around the world have only 70-80% montmorillonite.
The montmorillonite structure contributes to the usefulness of bentonite. Montmorillonite is a layered aluminum and silicate mineral, which is thought to occur as very small, two dimensional mineral form called a “platelet”. The thickness of this basic building block is approximately 1.0 – 1.5 nanometers, or about 50,000 times thinner than a human hair. These montmorillonite platelets have aspect ratios of 500:1 to over 1000:1 (width x height) which in turn creates a very high surface area per unit volume. These platelets physically occur in nanometer-scale stacks or “decks-of-cards”.