Bentonite is an absorbent clay formed by the chemical weathering of volcanic ash in water. It is known for its remarkable ability to swell significantly when wetted, making it highly valuable for sealing and industrial binding applications.
Is this bentonite?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch bentonite with a known reference. Bentonite sits at Mohs 1-2, softer than the next harder reference, harder than the previous one.
- 2Read the textureLook at a fresh break with a hand lens. Texture: Very soft and porous; feels soapy or greasy. Grain size: Microscopic platy grains.
- 3Pick out the mineralsComposition: Mostly montmorillonite (smectite) clay, usually formed from altered volcanic ash. Identify the two or three most abundant grains.
- 4Read the lusterHold the specimen under a strong light. Luster: dull.
- 5Match the color rangeCompare against the expected color range: white, cream, gray, yellow, green.
Often confused with
Bentonite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Luster reads dull on Bentonite and pearly to dull earthy on Kaolinite.
How to tell apart: Luster reads dull on Bentonite and earthy on Illite.
How to tell apart: Luster reads dull on Bentonite and pearly, greasy, dull on Talc.
Often found alongside bentonite
Minerals reported to co-occur with bentonite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Mohs hardness
- 1-2
- Density
- 2.0-2.7 g/cm³
- Luster
- Dull
- Transparency
- Opaque
- Rarity
- Common
- Uses
- Industrial, Drilling fluid, Cat litter, Binder
- Host rock
- altered volcanic ash deposits
- Texture
- Very soft and porous; feels soapy or greasy
- Grain size
- Microscopic platy grains
- Composition
- Mostly montmorillonite (smectite) clay, usually formed from altered volcanic ash
Where rockhounds find bentonite
2 mapped spotsClassic worldwide localities
- Wyoming, USA
- Montana, USA
- Greece
- Turkey
- Italy
Field-hunting tip
Where it forms: altered volcanic ash deposits. If you start seeing quartz, calcite, gypsum in float, you are in the right ground. In the U.S., the densest reported localities are in California, Montana. Start trip planning there.
Common questions
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Find bentonite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
