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Bentonite identification and geology

Bentonite rock reference specimen

Soft, swelling clay rock formed from alteration of volcanic ash. Composed mainly of montmorillonite/smectite clays. Texture: Fine-grained, waxy when wet. Rock type: Sedimentary Altered volcanic. Grain size: Clay. Minerals present: Montmorillonite, Illite, Quartz, Feldspar relict. Formation: Forms from diagenetic alteration of volcanic ash deposits in marine or lacustrine environments. Common geological settings: Wyoming USA , Turkey, Greece, India; found as altered ash beds in sedimentary sequences. Common look-alikes and related entries: Mudstone, Claystone, Fuller Earth.

Sources, scope & corrections

These authoritative resources support category-level nomenclature, terminology, identification methods, and safety. When a specimen-specific safety warning appears, the public-health source listed here directly supports that precaution. They are not line-by-line citations for every other property on this entry, and natural specimens vary.

Editorial owner
GeoMiner
Last updated
2026-03-30T14:39:39.156916+00:00
Review status
Published; not independently peer-reviewed
  • Rocks — U.S. National Park Service. Authoritative overview of rock groups, the rock cycle, and geologic processes.
  • Can you identify my rock or mineral? — U.S. Geological Survey. Identification limitations and routes to in-person professional verification.

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Common questions

What is Bentonite?
Soft, swelling clay rock formed from alteration of volcanic ash. Composed mainly of montmorillonite/smectite clays.
How can you identify Bentonite?
Compare several properties rather than relying on colour alone. This entry records rock type: Sedimentary Altered volcanic; texture: Fine-grained, waxy when wet; grain size: Clay; common minerals: Montmorillonite, Illite, Quartz, Feldspar relict. Confirm an important identification with additional field tests or qualified laboratory work.
How does Bentonite form?
Forms from diagenetic alteration of volcanic ash deposits in marine or lacustrine environments.
Where is Bentonite commonly found?
Wyoming USA , Turkey, Greece, India; found as altered ash beds in sedimentary sequences.
What is Bentonite commonly confused with?
Bentonite may be confused with Mudstone, Claystone, Fuller Earth. Compare multiple diagnostic properties because colour and surface appearance can overlap.

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