Question

Which of the following represents the correct sequence of plasticity of minerals in soil in an increasing order?
1. Kaolinite, silica, mantorillonite, illite
2. Kaolinite, silica, illite, montmorillonite
3. Silica, kaolinite, illite, montmorillonite
4. Silica, kaolinite, montmorillonite, illite

Answer 1

Correct Answer - Option 3 : Silica, kaolinite, illite, montmorillonite

Explanation: 

Plasticity: The property of a material to be deformed repeatedly without rupture by the action of a force, and remain deformed after the removal of force. The plasticity of the soil refers to the property of soil to possess plasticity, i.e. get deformed without rupture/breaking under external force. The plasticity of a soil is measured in terms of its Plasticity Index, which is the difference between Liquid Limit and Plastic Limit.

Clay Mineralogy: 

1) Kaolinite: The basic structure of this mineral is formed when the octahedron unit is joined to the silica tetrahedron unit by the oxygen present at the tip of the tetrahedron having a total thickness of 7 Angstrom. 

2) Montmorillonite: It is also known as Smectite, it is the most common clay mineral which is formed when the octahedron unit is sandwiched in between two silica tetrahedron units resulting in the structure having a total thickness of 10 Angstrom.

3) Illite: The structure of Illite is the same as that of montmorillonite with the only difference that substantial replacement of silica in the tetrahedron unit by the aluminum atom takes place and net negative charge is balanced by potassium ion.

The sequence of plasticity:

Silica < kaolinite < illite < montmorillonite