Montmorillonite

Montmorillonite
A sample of montmorillonite (unknown scale)
General
Category	smectite, phyllosilicate
Chemical formula	(Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2·nH2O
Crystal symmetry	Monoclinic prismatic H-M symbol: 2/m Space group: C2/m
Unit cell	a = 5.17 Å, b = 8.94 Å, c = 9.95 Å; β = 99.54°; Z = 1
Identification
Color	White, pale pink, blue, yellow, red, green
Crystal habit	compact masses of lamellar or globular microcrystalline aggregates
Crystal system	Monoclinic
Cleavage	[001] perfect
Fracture	Uneven
Mohs scale hardness	1-2
Luster	Dull, earthy
Diaphaneity	Translucent
Specific gravity	1.7-2
Optical properties	Biaxial (-)
Refractive index	nα = 1.485 - 1.535 nβ = 1.504 - 1.550 nγ = 1.505 - 1.550
Birefringence	δ = 0.020
2V angle	Measured: 5° to 30°
References	[1][2][3]

Montmorillonite is a very soft phyllosilicate group of minerals that typically form in microscopic crystals, forming a clay. It is named after Montmorillon in France. Montmorillonite, a member of the smectite family, is a 2:1 clay, meaning that it has 2 tetrahedral sheets sandwiching a central octahedral sheet. The particles are plate-shaped with an average diameter of approximately one micrometre. Members of this group include saponite.

Montmorillonite is the main constituent of the volcanic ash weathering product, bentonite.

The water content of montmorillonite is variable and it increases greatly in volume when it absorbs water. Chemically it is hydrated sodium calcium aluminium magnesium silicate hydroxide (Na,Ca)_0.33(Al,Mg)₂(Si₄O₁₀)(OH)₂·nH₂O. Potassium, iron, and other cations are common substitutes, the exact ratio of cations varies with source. It often occurs intermixed with chlorite, muscovite, illite, cookeite, and kaolinite.

Cave conditions

Montmorillonite can be concentrated and transformed within cave environments. The natural weathering of the cave can leave behind concentrations of aluminosilicates which were contained within the bedrock. Montmorillonite can form slowly in solutions of aluminosilicates. High HCO₃ concentrations and long periods of time can aid in the formation. Montmorillonite can then transform to palygorskite under dry conditions and to endellite in acidic conditions (pH 5 or lower). Endellite can further transform into halloysite by drying.^[4]

Uses

Structure of montmorillonite

Montmorillonite is used in the oil drilling industry as a component of drilling mud, making the mud slurry viscous which helps in keeping the drill bit cool and removing drilled solids. It is also used as a soil additive to hold soil water in drought prone soils, to the construction of earthen dams and levees and to prevent the leakage of fluids. It is also used as a component of foundry sand and as a desiccant to remove moisture from air and gases.

Similar to many other clays, montmorillonite swells with the addition of water. However, some montmorillonites expand considerably more than other clays due to water penetrating the interlayer molecular spaces and concomitant adsorption. The amount of expansion is due largely to the type of exchangeable cation contained in the sample. The presence of sodium as the predominant exchangeable cation can result in the clay swelling to several times its original volume. Hence, sodium montmorillonite has come to be used as the major constituent in non-explosive agents for splitting rock in natural stone quarries in order to limit the amount of waste, or for the demolition of concrete structures where the use of explosive charges is unacceptable.

This swelling property makes montmorillonite-containing bentonite useful also as an annular seal or plug for water wells and as a protective liner for landfills. Other uses include as an anti-caking agent in animal feed, in paper making to minimize deposit formation and as a retention and drainage aid component. Montmorillonite has also been used in cosmetics.

Sodium montmorillonite is also used as the base of some cat litter products, due to its adsorbent and clumping properties.

Calcined clay products

Montmorillonite can be (calcined) to produce arcillite, a porous, calcined clay sold as a soil conditioner for playing fields and other soil products such as for use as bonsai soil as an alternative to akadama.^{[citation needed]}

Use in medicine and pharmacology

Montmorillonite clay is widely used in medicine and pharmacology.^{[citation needed]}

Montmorillonite is effective as an adsorptive of heavy metals.^[5]

For external use, montmorillonite has also shown its effectiveness.^[6]

Discovery

Montmorillonite was first described in 1847 for an occurrence in Montmorillon in the department of Vienne, France,^[2] more than 50 years before the discovery of bentonite in the US. It is found in many locations world wide and known by other names.

Lipid organization

Montmorillonite is also known to cause micelles (lipid spheres) to assemble together into vesicles. These are structures that resemble cell membranes on many cells. It can also help nucleotides to assemble into RNA which will end up inside the vesicles and, under the right conditions, will replicate themselves.^[7] This process may have led to the origin of life on Earth.^[8]

See also

• Dispersion (soil)
• Sodification
• Emulsion dispersion

References

1. ^ Handbook of Mineralogy
2. ^ ^{a b} Mindat
3. ^ Webmineral data
4. ^ Hill, Carol; Paolo Forti (1997). "Deposition and Stability of Silicate Minerals". Cave Minerals of the World (Second ed.). National Speleological Society. pp. 177. ISBN 1-879961-07-5. 
5. ^ Adsorption of a few heavy metals on natural and modified kaolinite and montmorillonite: a review. Bhattacharyya KG, Gupta SS. Adv Colloid Interface Sci. 2008 Aug 5;140(2):114-31. Epub 2008 Jan 17.
6. ^ A systematic review of contact dermatitis treatment and prevention. Saary J, Qureshi R, Palda V, DeKoven J, Pratt M, Skotnicki-Grant S, Holness L. J Am Acad Dermatol. 2005 Nov;53(5):845. Review.
7. ^ Clays May Have Aided Formation of Primordial Cells
8. ^ Clay's matchmaking could have sparked life

• Papke, Keith G. Montmorillonite, Bentonite and Fuller’s Earth Deposits in Nevada, Nevada Bureau of Mines Bulletin 76, Mackay School of Mines, University of Nevada-Reno, 1970.
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