Bentonite

clay rock, mainly montmorillonite

Bentonite is a kind of clay. Bentonite usually forms from weathering of volcanic ash. It is volcanic ash mixed with water under pressure.

The different types of bentonite have different proportions of sodium, calcium or potassium. They can absorb quite a lot of water, and are used as fillers and/or lubricators in various industries. The clay is used for many practical purposes.