Bentonite (/ ˈ b ɛ n t ə n ʌ ɪ t /) is an absorbent aluminium phyllosilicate clay consisting mostly of montmorillonite.One of the first findings of bentonite was in the Cretaceous Benton Shale near Rock River, Wyoming.The Fort Benton Group, along with others in stratigraphic succession, was named after Fort Benton, Montana, in the mid-19th century by Fielding Bradford Meek and F. V ...