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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MONTMORILLONITE specimen mnt-1
$ 30.00
Dims: 5-1/2" x 2-1/2" x 1-1/4"
Wt: 7.0 oz
California, U.S.A.
What we appear to have here is a shard of a large quartz crystal with 3 discernible but incomplete faces(they look like termination faces to me). The quartz is relatively clear, with 2 faces showing a substantial amount of pink Montmorillonite close to the faces, in a layer that could be described as a "phantom". The Montmorillonite seems to be just a bunch of pink smudges in the crystal, but closer examination reveals that the smudges are made up of fibrous sprays of "crystals", or what appear to be. The third face of the quartz seems to be fogged over with milky quartz, which is concentrated uniformly along the same plane as the Montmorillonite. The first picture gives an view of the entire specimen, and the second is a close-up of the junction of the three faces, with some of the inclusions.
no photo
mnt-1 ($ 30.00)
California, U.S.A.
MONTMORILLONITE specimen mnt-2
$ 50.00
Dims: 2.9 x 2.7 x 1.3" (7.4 x 6.8 x 3.2 cm)
Wt: 5.0 oz. (141 g)
White Queen Mine, Pala, California, U.S.A.
The Montmorillonite in this specimen exists mostly as fibers included in a partly-cut piece of quartz. They are in excellent condition, being protected through inclusion, and reach lengths of about 0.9" (2.3 cm). Though they are difficult to study, they may have some appreciable monoclinic form. Like other specimens from this locality, this material has a pale pink color and likely a dull luster, and is translucent to opaque. A large flat face has been cut into the quartz so that one can easily see it, and part of a natural crystal faces serves as a base.
no photo
mnt-2 ($ 50.00)
White Queen Mine, Pala, California, U.S.A.


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