Download Hydrated Sodium Calcium Aluminum Magnesium Silicate Hydroxide

Montmorillonite
By Dominic Papineau
General properties of montmorillonite
“Hydrated Sodium Calcium Aluminum Magnesium Silicate Hydroxide”
Montmorillonite is part of the smectite/montmorillonite group of clay
minerals (dioctahedral smectites). The dioctahedral smectites also
include beidellite and nontronite as other important end-members.
Trioctahedral smectites include saponite and hectorite as the most
important end-members.
The water content is variable: when water is absorbed by the crystals, they
tend increase several times their original volume.
It typically forms microscopic or at least very small platy micaceous crystals.
The physical properties of montmorillonite
Color: usually white, gray or pink with tints of yellow or green.
Luster: dull
Cleavage: perfect in one direction (not seen in massive specimens)
Hardness: 1 - 2
Specific gravity: 2.3 – 3 average
Crystal habit: never in large individual crystals, usually found in
compact or lamellar masses. Also seen as inclusions in quartz as fibers
and powder-like masses.
The optical properties of montmorillonite
Montmorillonite is biaxial negative
2VZ = 5 - 30°
 = 1.485 – 1.535
 = 1.504 – 1.550
 = 1.505 – 1.550
= 0.015 – 0.020 (usually first order yellow or lower interference colors)
Not pleochroic
The chemical properties of montmorillonite
Montmorillonite’s chemical formula: (My+ nH2O) (Al2-yMgy) Si4O10(OH)2
Common chemical substitution:
Al3+ and Fe3+ can substitute for Mg2+ or Fe2+
Al3+ can substitute for Si4+
Octahedral site
Tetrathedral site
These can result in layers of negative charges which need to be balanced by
interlayer cations (K+, Na+, Ca2+, Mg2+, etc.) and also organic ions.
These exchangeable cations can be hydrated by nH20. In turn, this
causes swelling of the crystals due to the interlayer water molecules.
The crystallographic properties of montmorillonite
Crystal system: monoclinic
Point Group: 2/m
Unit cell parameters: a = 5.17
b = 8.94
c = 9.95
 = 99.9°
Z=1
V = 453.04
Calculated density = 1.61
Space group: C2/m
The crystal structure of montmorillonite
Silicon tetrahedra
Aluminum octahedron
Magnesium octahedron
Sodium, potassium or
calcium ion
In brief: T – O – T sheets
The use of montmorillonite
The water absorption property of montmorillonite makes it an
important to farmland soils where extended periods of drought occur.
It is the main constituent in bentonite (a volcanic ash) which is used in
drilling muds. The bentonite gives the water greater viscosity and
thus keeps the drill head cool during drilling.
Montmorillonite may have promoted the catalysis of terrestrial polymeric
biomolecules and could do the same on other planetary bodies.
Origin of terrestrial life
The RNA world has been proposed by many scientists to have been the
precursor for terrestrial life because RNA molecules provide both the
capabilities to store genetic information and to catalyze chemical reactions.
RNA
Nucleic acids
O
O
O
Mechanism of polymerization catalysis
-
-
-
- - - - - -
Montmorillonite particle
Negative charges due to
chemical substitutions
+
-
-
-
- -
+
-
-
+
+
+
+
+
-
+
-
-
-
-
+
-
-
-
+
-
+
Montmorillonite particle
+
+
+
+
-
+
+
-
+
-
The cations include a wide range of possible cations among which organic
ions (such as nucleotides and amino acids) can be attracted and oriented.
The catalytic properties of montmorillonite
It has been shown that montmorillonite-catalyzed reaction are up to 19 times
more efficient than non-catalyzed biochemical polymerization reactions
(Joshi, Pitsch, and Ferris, (2000) Origin of Life).
Hence, montmorillonite is known to catalyze phosphodiester bonds and also to
influence the regiochemistry of the phosphodiester bond produced.
RNA molecule
In fact, montmorillonite favors
3’,5’phosphodiester bonds
O
But not
O
2’, 5’ phosphodiester bonds
O
Geological evidence of the origin of life: an hypothesis to test
Ammonium (NH4+) can result from the biogeochemical degradation
of biomolecules such as nucleic acids and proteins.
NH4+ has the same charge and a very similar ionic radius to K+ and can thus
substitute in K+ containing minerals (e.g. biotite, muscovite, K feldspar).
NH4+ is very stable in high temperature processes and survives through
crustal processes.
If the RNA of the RNA world was indeed polymerized on clay minerals
such as montmorillonite, then Hadean metapelites may contain high
concentrations of NH4+ as evidence of the earliest terrestrial life.
Such an hypothesis could also be tested on Martian
clay-rich sediments from a Mars Sample Return.
Occurrences of smectite/montmorillonite
Locations on Earth:
Montmorillonite deposits are located all over the globe
and important sites include:
1. Himalaya - China, Tibet
2. Ural - Pakistan
3. Caucasus - Georgia (Russia)
4. Andes - Peru, Ecuador
5. Wasatch - Utah (USA)
Present on Mars:
Orbital infrared detection: Esposito et al., (2000) JGR
Christensen et al., (1998) Science
Detection in Martian meteorites: Bridges and Grady, (2000) EPS-letters
Treiman and Lindstrom, (1997) JGR