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A group of clay minerals that have the ability to absorb large quantities of water. Montmorillonite is a fine-grain, aluminum silicate compound with flat hexagonal crystals. They are formed by the decomposition of basic rocks such as basalt. Most secondary clays contain some montmorillonite. Some clays high in montmorillonite, such as bentonite, are highly plastic with high shrinkage rates. Other montmorillonite clays, such as fuller's earth, are aplastic.

Synonyms and Related Terms

smectite; beidellite; bentonite; fuller's earth; montmorillonita (Esp.); montmorillonite (Fr;); montmorilonite (Port.); Montmorillonit (Deut.); montmorilloniet (Ned.)

Other Properties

Broad sheets break into irregular fluffy masses.

Streak = white

Composition Al2O3-4SiO2-H2O
Mohs Hardness 1.5-2.0
Density 2.2-2.7
Refractive Index 1.480 - 1.630

Hazards and Safety

Inhalation can cause lung irritation.

Additional Information

Mineralogy Database: Montmorillonite

Sources Checked for Data in Record

  • G.S.Brady, Materials Handbook, McGraw-Hill Book Co., New York, 1971 Comment: p. 175
  • The Merck Index, Martha Windholz (ed.), Merck Research Labs, Rahway NJ, 10th edition, 1983 Comment: entry 6341
  • Encyclopedia Britannica, Comment: "Montmorillonite." Encyclopædia Britannica. 2004. Encyclopædia Britannica Premium Service. 15 Apr. 2004 .
  • Richard S. Lewis, Hawley's Condensed Chemical Dictionary, Van Nostrand Reinhold, New York, 10th ed., 1993
  • Random House, Webster's Encyclopedic Unabridged Dictionary of the English Language, Grammercy Book, New York, 1997