What is Bentonite?


Bentonite is an absorbent aluminium phyllosilicate, essentially impure clay consisting mostly of montmorillonite. There are different types of bentonite, each named after the respective dominant element, such as potassium (K), sodium (Na), calcium (Ca), and aluminium (Al). Bentonite usually forms from weathering of volcanic ash, most often in the presence of water. For industrial purposes, two main classes of bentonite exist: sodium 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.

Sodium bentonite

Sodium bentonite expands when wet, absorbing as much as several times its dry mass in water. Because of its excellent colloidal properties, it is often used in drilling mud for oil and gas wells and for geotechnical and environmental investigations.


The property of swelling also makes sodium bentonite useful as a sealant, especially for the sealing of subsurface disposal systems for spent nuclear fuel and for quarantining metal pollutants of groundwater. Similar uses include making slurry walls, waterproofing of below-grade walls, and forming other impermeable barriers, e.g., to seal off the annulus of a water well, to plug old wells, or to line the base of landfills to prevent migration of leachate. It is also used to form a barrier around newly planted trees to constrain root growth so as to prevent damage to nearby pipes, footpaths and other infrastructure.


Sodium bentonite can also be “sandwiched” between synthetic materials to create geo-synthetic clay liners (GCL) for the aforementioned purposes. This technique allows for more convenient transport and installation, and it greatly reduces the volume of sodium bentonite required.

Various surface modifications to sodium bentonite improve some rheological or sealing performance in geoenviromental applications, for example, the addition of polymers.




Calcium bentonite

Calcium bentonite is a useful adsorbent of ions in solution, as well as fats and oils, being a main active ingredient of fuller’s earth, probably one of the earliest industrial cleaning agents. Calcium bentonite may be converted to sodium bentonite (termed sodium beneficiation or sodium activation) to exhibit many of sodium bentonite’s properties by a process known as “ion exchange” (patented in 1935 by Germans U Hofmann and K Endell). In common usage, this means adding 5–10% of a soluble sodium salt such as sodium carbonate to wet bentonite, mixing well, and allowing time for the ion exchange to take place and water to remove the exchanged calcium Some properties, such as viscosity and fluid loss of suspensions, of sodium-beneficiated calcium bentonite (or sodium-activated bentonite) may not be fully equivalent to those of natural sodium bentonite.[1] For example, residual calcium carbonates (formed if exchanged cations are insufficiently removed) may result in inferior performance of the bentonite in geosynthetic liners.



Potassium bentonite

Also known as potash bentonite or K-bentonite, potassium bentonite is a potassium-rich illitic clay formed from alteration of volcanic ash.



Many of the world’s largest commercial bentonite deposits formed by alteration of fine-grained volcanic debris deposited over relatively large areas. Other bentonites formed by in situ hydrothermal alteration of coarser igneous rock. Because certain chemical trace-elements are usually retained when an igneous rock alters to clay, plots of ratios of these elements on a Geochemical grid can often indicate the composition of the parent igneous material. This may be of use in bentonite exploration.


  1. Eruption of ash and its airborne transport
  2. Water sorting and concentration of volcanic ash in shallow-marine environments to give thick accumulations.
  3. Conversion of the volcanic ash to smectite
  4. Protection of bentonite beds from subsequent erosion.
  5. No subsequent thermal alteration of the smectite to other clay mineral phases.




Bentonites range in color from black through white but most frequently are bluish-green when fresh, weathering to a yellowish-brown color at or near outcrop due to the oxidation of ferrous iron. Material from near outcrop often exhibits enhanced swelling properties.


LUSTER is dull

TRANSPARENCY CRYSTALS are translucent and masses are opaque

CRYSTAL SYSTEM is monoclinic; 2/m

CRYSTAL HABITS never in large individual crystals, usually found in compact or lamellar masses. Also seen as inclusion in quartz as fibers and powder-like masses


SPECIFIC GRAVITY is variable from 2.3 – 3 (average)

BEST FIELD INDICATORS softness, color, soapy feel, luster and expandability when added to water

Usage Areas


Foundry Moulding Sands:  Probably the largest use for bentonite. Sand is mixed with bentonite and water so that is can be formed around round a pattern. When this pattern is removed, hot metal is poured into the mould.


Drilling muds:  Dilute suspensions of bentonite lubricate and cool the drilling bit, help bring cutting to the surface and seal the walls of the hole. They also carry weighting media such as baryte or hematite to balance high hydrostatic pressures in the drill holes.


Civil Engineering:  Bentonite slurries seal porous strata and stop water movement in foundations for building, tunnels and dams.


Iron Ore Pelletizing: Bentonite is used to bind fine-grained iron ore into pellets before these are introduced into the blast furnace.


Bleaching Oils and Fats: Removes color and impurities from edible oils and fats and is also used extensively for filtering and cleaning hydrocarbon oils. It is usually activated with acid to increase the effectiveness of bentonite for this application.


Agriculture: Used as a carrier for insecticides and pesticides and as coating for seeds. Also used as a soil conditioner.


Wastewater: Used as adsorbents and coagulant in wastewater treatment facilities.


Other Industrial and Domestic Uses: Is a component of paints, dyes and poloshes, pharmaceuticals, medicines and cosmetics. Adsorbs oil and grease from floors.  Increasingly used as a pet litter, especially in Europe. Used also as a raw material in detergents.