Montmorillonite
3 in the gallery
Montmorillonite is a hydrated aluminum silicate clay mineral with a layered, platy habit and a soft, earthy luster. It forms in weathered, altered rocks and is a major constituent of bentonite clays, known for its swelling and high cation-exchange capacity when hydrated. Typical colors range from white to light gray or tan, and its microscopic platelets give the material a smooth, plastic feel in hand sample and a white-to-powdery appearance on weathered surfaces.
- Formula
- (Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2 · nH2O
- Rarity
- Common
- Stability
- Moderately Stable
- Collector interest
- Moderate Interest
- Classification notes
- Phyllosilicate mineral; SMectite group (silicate, phyllosilicate)
Where Montmorillonite is found
Top localities
- White Queen Mine3 specimens
About Montmorillonite
Significant specimens come from hydrothermal and sedimentary environments where clays accumulate, including locations in the United States and Brazil. In many localities, Montmorillonite occurs with other clay minerals and can form crusts or fine-grained aggregates that flatten light and exhibit a dull to pearly luster. Collectors value these clays for their textural qualities, thepm characteristics of alteration halos, and the broader context they provide for soil formation and clay mineralogy.