The bentonite used for drilling fluids, grout and sealants is primarily mined in the area encompassing Wyoming and surrounding areas in the western United States. This sodium-based bentonite is an ore whose major constituent is the clay mineral, sodium montmorillonite. Montmorillonite is a three-layer mineral consisting of two tetrahedral silicate layers sandwiched around a central octahedral aluminate layer. This clay “sandwich” is held together by the bonds between shared oxygen atoms, with each metal atom gaining a partial electron from the multiple oxygen atoms directly surrounding it. This bentonite was formed by the deposition of volcanic ash in a marine environment during the Cretaceous period. Over time, this ash was highly compressed and changed chemically to produce this dense platelet structure. This characteristic is what makes bentonite such an essential tool when drilling wells. In fact, each cubic inch of bentonite has enough platelet surface area to cover 66 football fields.
It is important to realize that all bentonite removed from the ground is not the same. In one deposit there may be several grades of bentonite that can be used for different purposes. The most sought after is a high-yield bentonite referred to as yellow bentonite. It is soft to the touch, has a yellowish tinge and typically has a very high yield, or expansion coefficient, that is required to produce bentonite drilling fluids. While this is the most sought after bentonite, it is also the most limited in its supply. To add to that, there are several grades of this “yellow” bentonite. Typically, underneath this vein of yellow bentonite is a blue bentonite that is much harder and has a bluish color profile. This bentonite is used for many products, including the exceedingly popular bentonite chips.