Bentonite - Wikipedia
Bentonite layers from an ancient deposit of weathered volcanic ash tuff in Wyoming Gray shale and bentonites ( Benton Shale; Colorado Springs, Colorado) Bentonite ( / ˈbɛntənaɪt /) [1] [2] is an absorbent swelling clay consisting mostly of montmorillonite (a type of smectite) which can either be Na-montmorillonite or Ca-montmorillonite.