Consider the following clay minerals:
1. Kaolinite
2. Montmorillonite
3. Illite
What is the correct sequence in an increasing order of their plasticity index ?
Kaolinite has hydrogen bond which is fairly strong hence mineral lattice is stable.
Montmorillonite mineral is a 2: 1 clay mineral. Bonding force between units is weak Van
derWaal's forces. Hence large quantity of water and exchangeable cations can easily enter
between the layers causing the separation of layers. This adsorbed water results in high
plasticity index of the clay mineral.
Illite has bonds of non-exchangeable potassium K+ ions though stronger than that in
montmorillonite but is considerable weaker than hydrogen bond in kaolinite. The swelling of
Illite is more than that of Kaolinite but less than that of montmorillonite.
Hence, correct option is (c).
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