MONTMORILLONITE

    Class : Silicates
    Subclass : Phyllosilicates
    Crystal system : Monoclinic
    Chemistry : (Na,Ca)0.3(Al,Mg)2Si4O10(OH)2 nH2O
    Rarity : Ubiquitous

Montmorillonite is a hydrated phyllosilicate from the smectite group. It is a silicate alteration mineral that generally forms by alteration of basic igneous rocks in an alkaline environment. It can also result from the hydrothermal alteration of plagioclase near hydrothermal veins, or from the alteration of volcanic ash in a marine environment. It owes its name to its location of discovery : Montmorillon (Vienne, France). It is a naturally white clay that is often variously colored gray, greenish or pink. Like most clays, it appears as earthy, compact and creamy masses. Montmorillonite is, along with nontronite, beidellite and saponite, one of the main constituents of bentonite.


Main photo : Montmorillonite from Sanders-Defiance Plateau, Apache County, Arizona, USA © Andrew Hogdson

Montmorillonite in the World

Widespread worldwide, montmorillonite never forms macroscopic crystals or aesthetically velvety specimens.

Montmorillonite in France

Montmorillonite is present almost everywhere in the French underground.

Twinning

No twin known for this mineral species.

Fakes and treatments

No fakes recorded for this mineral species.


Hardness : 1 to 2
Density : 2 to 3
Fracture : Irregular
Streak : White

TP : Translucent
RI : 1.485 to 1.550
Birefringence : 0.020
Optical character : Biaxial -
Pleochroism : Visible
Fluorescence : None

Solubility : -

Magnetism : NoneRadioactivity : None

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