Montmorillonite is a member of the general mineral group the clays. It typically forms microscopic or at least very small platy micaceous crystals. The water content is variable, and in fact when water is absorbed by the crystals they tend to swell to several times their original volume. This makes montmorillonite a useful mineral for several purposes. It is the main constituent in a volcanic ash called bentonite, which is used in drilling muds. The bentonite gives the water greater viscosity("thickness" of flow), which is very important in keeping a drill head cool during drilling and facilitating removal of rock and dirt from within a drill hole. Another important use of montmorillonite is as an additive to soils and rocks. The effect of the montmorillonite is to slow the progress of water through the soil or rocks. This is important to farmers with extended dry periods, engineers of earthen dams or levees or perhaps to plug up old drill holes to prevent leakage of toxic fluids from bottom levels to higher aquifers used for drinking water.

As a mineral specimen, montmorillonite does not get much consideration. Usually, pure samples of montmorillonite are massive, dull and not very attractive. However, as with all minerals, there are those exceptional specimens that defy the norm. Montmorillonite has been found as attractive pink inclusions in quartz crystals, and these make for interesting specimens.


  • Color is usually white, gray or pink with tints of yellow or green.
  • Luster is dull.
  • Transparency crystals are translucent and masses are opaque.
  • Crystal System is monoclinic; 2/m.
  • Crystal Habits: never in large individual crystals, usually found in compact or lamellar masses. Also seen as inclusions in quartz as fibers and powder-like masses.
  • Cleavage is perfect in one direction, basal; not seen in massive specimens.
  • Fracture is uneven to lamellar.
  • Hardness is 1- 2 (can sometimes leave marks on paper)
  • Specific Gravity is variable from 2.3 - 3 (average)
  • Streak is white.
  • Other Characteristics: crystals expand to many times their original volume when added to water.
  • Associated Minerals include other clays, garnets, biotite and quartz.
  • Notable Occurances: include sources in France, Italy, USA and many other locallities world wide.
  • Best Field Indicators softness, color, soapy feel, luster and expandability when added to water.
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