Bentonite is a clay consisting mainly of smectite minerals, commonly formed by 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.
There are three main types of bentonite:
- High swelling (sodium)
- Low swelling (calcium)
- Moderate swelling (intermediate sodium-calcium) bentonite.
The sodium bentonites are the most useful because of their greater swelling capacity. They are used mainly as bonding clay in foundries, in drilling mud, in animal feed pellets, and for civil engineering applications