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.

Bentonite usually forms from weathering of volcanic ash, most often in the presence of water. However, the term bentonite, as well as a similar clay called tonstein, have been used for clay beds of uncertain origin. For industrial purposes, two main classes of bentonite exist: sodium bentonite and calcium bentonite.

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. Other common clay species, and sometimes dominant, are montmorillonite and kaolinite. Kaolinite dominated clays are commonly referred to as tonsteins and are typically associated with coal.

List of producing countries

This is a list of countries by bentonite production (tonnes) in 2006 mostly based on British Geological Survey accessed in June 2008.

* indicates "Natural resources of COUNTRY or TERRITORY" links.

Bentonite production (tonnes) per country in 2006

Country (or area)	Production
World	14,600,000
United States	4,620,000
China *	3,200,000
Greece	1,100,000
India *	[1] 1,081,000
Turkey *	600,000
Russia *	500,000
Italy *	470,000
Mexico	435,273
Brazil *	419,214
Germany	363,998
Argentina *	256,165
Czech Republic	220,000
Iran *	186,323
Spain	160,000
Cyprus	150,620
Australia	135,000
Bulgaria	134,500
Slovakia	95,700
Poland *	93,880
South Korea *	61,137
Azerbaijan *	40,600
South Africa *	32,878
Algeria	27,110
Ukraine *	25,000
Bosnia and Herzegovina *	24,050
North Macedonia *	20,353
Romania *	20,229
Pakistan *	20,000
Vietnam *	20,000
Denmark *	19,011
Croatia	16,410
Uzbekistan *	15,000
Peru *	14,590
Colombia *	8,500
Hungary *	6,635
Egypt *	6,300
Nicaragua	6,300
Indonesia *	5,000
Georgia	4,487
Philippines *	3,600
New Zealand	3,028
Thailand *	1,200
Burma *	800
Armenia *	720
Mozambique *	610