Bentonite is a colloid-rich soft rock which consists of clay minerals. It is produced by the weathering of volcanic tuff or ash. The main component of bentonite is the clay mineral montmorillonite, together with associated minerals including quartz, tridymite, cristobalite, mica, feldspar, pyrite and calcite.
The mineral is mined from various locations across the world before being ground to form a powder. Various additives may be added to the bentonite to enhance its performance.
Bentonite expands when wet, absorbing as much as 17 times its dry mass in water. This allows bentonite to permeate cracks and crevices, forming a self-sealing low permeability barrier – this is known as a bentonite slurry wall.