What does bentonite mean?
Bentonite is an absorbent aluminium phyllosilicate clay consisting mostly of the mineral montmorillonite, formed by the chemical alteration of volcanic ash over long periods. It takes its name from the Benton Shale formation near Rock River, Wyoming, where large deposits were identified in the nineteenth century. What makes bentonite remarkable is its capacity to swell dramatically when it absorbs water — sodium bentonite in particular can expand to many times its dry volume — giving it an exceptional ability to seal, bind, and thicken. These properties explain its wide industrial use: it stabilises the mud used in oil-well drilling, binds sand moulds in foundries, waterproofs foundations and pond liners, suspends solids in paints and pharmaceuticals, and forms the clumping agent in many cat litters. In everyday language it appears most often as "bentonite clay," especially on labels for cosmetics and wellness products.
nounA soft, porous absorbent clay formed chiefly from altered volcanic ash, composed mainly of the mineral montmorillonite, used in drilling muds, foundry binders, cat litter, and as a sealing and adsorbent material.
- An absorbent swelling clay formed from volcanic ash, composed mainly of montmorillonite, used industrially in drilling fluids, foundry casting, construction sealing, cosmetics, and cat litter.
"The drilling crew mixed bentonite into the mud to seal the borehole walls."
"Engineers pumped a bentonite slurry into the trench to support its walls during excavation."
"Many natural deodorants rely on bentonite for its ability to draw out moisture and impurities."
Rare; used mainly in geology and materials science when distinguishing varieties such as sodium and calcium bentonite. In ordinary usage bentonite is uncountable.
"Geologists distinguish several bentonites, each characterised by its dominant exchangeable cation."
Named after a single rock formation in Wyoming, bentonite can swell up to several times its volume when wet — which is why it plugs leaks, lines ponds, and clumps in your cat's litter box.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of bentonite
The word bentonite was coined in American English in 1898 by geologist William Taylor, who named it after the Benton Shale formation near Fort Benton, Montana (the rock unit itself honours Senator Thomas Hart Benton of Missouri). It is thus a relatively recent technical coinage rather than an inherited word from Latin or Greek. Its only true relatives are place-derived terms such as Benton and related geological names; there are no classical cognates.
How bentonite is actually used
A technical term common in geology, civil engineering, drilling, and personal-care product labelling; in consumer contexts it almost always appears as "bentonite clay." The plural is rare because the substance is usually treated as an uncountable material noun.
Easily confused with bentonite
Kaolin is a white clay of different mineral origin (kaolinite) used in ceramics, whereas bentonite is a swelling clay derived from volcanic ash and dominated by montmorillonite.