Bentonite clay is a fine-grained rock composed mainly of montmorillonite minerals. The formation of bentonite is an in situ alteration of rhyolitic volcanic ash. Pyroclastic material was ejected into the atmosphere by volcanic activity and deposited as sediment in a marine environment.
Bentonite clay is composed of microscopic platelets consisting of layers of aluminum hydroxide held between layers of silicate atoms. These platelets are stacked one on top of the other. If a gram of bentonite were spread out only one particle layer thick, its billions of particles would cover an area greater than a regulation football field.
There are two basic types of bentonite, sodium type and calcium type.
Its color ranges from white to black and includes shades of gray, light olive green, cream, yellow, earthy red, brown, sky blue and purple. Bentonite feels and appears greasy or waxy and when wet it is highly plastic and slippery.