sedimentary
Bentonite Clay
Montmorillonite (Al,Mg)2Si4O10(OH)2·nH2O
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
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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.
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