sedimentary

Bentonite Clay

Montmorillonite (Al,Mg)2Si4O10(OH)2·nH2O

AI-generated identification · may be incorrect

Hardness: 1-2 (Mohs); Color: Gray, tan, or brown; Luster: Dull/Earthy; Structure: Monoclinic; High plasticity and swelling ability when wet.

Hardness: 1-2 (Mohs)
Color: Gray, tan, or brown
Luster: Dull/Earthy

Identified July 17, 2026More sedimentary →

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Physical properties

Hardness: 1-2 (Mohs); Color: Gray, tan, or brown; Luster: Dull/Earthy; Structure: Monoclinic; High plasticity and swelling ability when wet.

Formation & geological history

Formed primarily from the chemical weathering and alteration of volcanic ash in marine or lacustrine environments over millions of years.

Uses & applications

Used extensively in drilling mud, as a bonding agent in foundry sand, for iron ore pelletizing, and in water treatment or cosmetics.

Geological facts

Depending on the dominant element, it is classified as Sodium Bentonite (high swelling) or Calcium Bentonite (low swelling). It can expand up to 15 times its dry volume.

Field identification & locations

Identified in the field by its soapy or waxy feel when moist and its tendency to turn into a highly sticky, slippery mud when rained upon. Found in large deposits in Wyoming, USA.