Bentonite is a clay consisting mainly of smectite minerals, commonly formed by the decomposition of volcanic ash or tuff, or sometimes from other igneous or sedimentary rocks. Bentonite is a very plastic clay that shrinks (or swells) markedly in response to the removal (or addition) of water. The main uses of bentonite are in drilling mud and as a binder, purifier, absorbent, and carrier for fertilizers or pesticides. As of around 1990, almost half of the US production of bentonite was used as drilling mud. Minor uses include filler, sealant, and catalyst in petroleum refining.
