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In this article we will discuss about:- 1. Structure of Montmorillonite 2. Electrical Charge and Isomorphous Substitution 3. Morphology and Surface Area 4. Formation and Occurrence 5. Commercial Uses.
Structure of Montmorillonite:
The basic structural unit of montmorillonite mineral consists of one alumina sheet sandwiched between two silica sheets. The oxygen atoms at the tip of each silica sheet combine with the OH ions of both sides of the alumina sheet by hydrogen bond. As the bond is fairly strong, the basic structural unit of montmorillonite mineral is also stable. The thickness of the basic structural unit of montmorillonite is 9.2 Å.
Montmorillonite mineral is formed by stacking of several structural units one over the other. The interface between the two structural units during this stacking is through the silica sheet of one structural unit and the silica sheet of the other unit.
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This bond is by van der Waals’ forces, which is rather weak. In dry condition, the van der Waals’ forces are rather strong so that it is difficult to break a dry lump of montmorillonite clay. However, water can easily enter between the layers, often dividing the montmorillonite particle into individual structural units.
One characteristic of van der Waals’ forces is that their magnitude decreases rapidly with distance. As water enters between the structural units, the distance between them increases, causing reduction in attractive forces. Thus, a montmorillonite clay particle is stable as long as it is dry but breaks into individual structural units in the presence of water.
The chemical formula of montmorillonite is (Na, Ca)0.33(Al, Mg)2Si4O10(OH)2.n(H2O) and its molecular weight is 549.07 g. It is white, gray white, yellow, brownish yellow, or greenish yellow in color. The specific gravity of the mineral varies from 2.0 to 2.7.
Electrical Charge and Isomorphous Substitution:
Some of the aluminum atoms in the gibbsite sheet of the montmorillonite mineral may be replaced by iron (Fe2+) or magnesium (Mg2+) by isomorphous substitution. Similarly, some of the silicon atoms in the silica sheet are also replaced by aluminum by isomorphous substitution. This gives a net negative charge to the surface of the mineral. In hectorite mineral of the group, a lithium atom is found to substitute for magnesium.
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The negatively charged outer surface of the silica sheet attracts cations present in pore water. There is thus electrical force of attraction between the negatively charged surface and the cations as well as between the cations and anions, which are attracted by the cations. The pore water is held to the surface under these attractive forces. This results in the formation of an adsorbed water layer on the mineral surface.
Morphology and Surface Area of Montmorillonite Mineral:
A typical particle of montmorillonite mineral clay is of 0.001-0.01 µm (or 0.000001 – 0.00001 mm) thickness. This means that a montmorillonite particle is made of 1 – 10 basic structural units, stacked one over the other.
Montmorillonite particles have lateral dimensions of the order of 0.1 – 0.5 µm (or 0.0001 –0.0005 mm). The specific surface of montmorillonite is about 800 m2/g. The SEM image clearly shows that the montmorillonite particles are sheet or plate-like and that their thickness is very small compared to the lateral dimensions.
The volume of a typical montmorillonite particle is about 1 ×10 –14 mm3, considering the particle as a rectangular prism. Taking the density of soil solids as 2.7 g/cc, the weight of a typical montmorillonite particle is of the order of 2.7 ×10 –17g. Due to the large surface area, the magnitude of total surface forces becomes more. This makes it clear, why surface forces of attraction and repulsion are dominant compared to the weight in clay soils.
Formation and Occurrence of Montmorillonite Mineral:
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The 2:1 minerals such as montmorillonite and illite are formed when silica is abundant, as they require two silica sheets for every alumina or gibbsite sheet. Montmorillonite is formed in areas of low rainfall, poor drainage, and leaching that occur in arid and semi-arid regions. Montmorillonite is formed in areas of high pH and electrolyte concentration. It is usually formed from basic and intermediate igneous rocks. Montmorillonite is formed by weathering of volcanic ash under poor drainage conditions or in saline environment.
Important minerals of the montmorillonite group are montmorillonite, nontronite, beidellite, hectorite, and saponite. Of these, montmorillonite, nontronite, and beidellite are dioctahedral and hectorite and saponite are trioctahedral. Montmorillonite is the most common mineral of the group.
Bentonite and montmorillonite clays are often used as synonyms and have similar properties. Bentonite was originally named for smectite clay found near Fort Benton Wyoming of the United States.
Commercial Uses of Montmorillonite Mineral:
Montmorillonite is used as a component of drilling mud in oil drilling and in other drilling operations such as in construction of pile foundations as well as in soil explorations by drilling. Addition of bentonite that contains almost 99% montmorillonite makes the drill mud viscous that helps in keeping the drill bit cool and removing drilled solids as well as in stabilization of walls of the bore holes. It is also used as an annular seal or plug for water wells.
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It is used as a principal component in clay liners in landfills and other waste containment systems to prevent the leakage of fluids. The negatively charged montmorillonite mineral attracts the positively charged toxins in hazardous wastes and helps to prevent their dispersion and migration, preventing the pollution of ground water resources. Sodium montmorillonite is used as a 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.
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