Bentonite is an absorbent aluminium phyllosilicate generally impure clay consisting mostly of montmorillonite.
There are a few types of bentonites and their names depend on the dominant elements, such as potassium, sodium, calcium, and aluminium.
As noted in several places in the geologic literature, there are some nomenclatorial problems with the classification of bentonite clays.
In stratigraphy and tephrochronology, completely devitrified (weathered volcanic glass) ash-fall beds are commonly referred to as K-bentonites when the dominant clay species is illite.
This is a list of countries by bentonite production (tonnes) in 2006 mostly based on British Geological Survey accessed in June 2008.