Test: Soil Structure & Clay Mineralogy - Civil Engineering (CE) MCQ

Concept:
The atomic structure of clay minerals has two fundamental crystalline sheets:

1. Tetrahedral sheet or Silica sheet
2. Octahedral sheet or Alumina sheet

1. Tetrahedral sheet or Silica sheet: It consists of four oxygen atoms placed at tips of a tetrahedron enclosing a silicon atom. Each of the ‘O’ ion at the base is common to two adjacent units. The sharing of charges leaves three negative charges at the base per tetrahedral sheet. This, along with two negative charges at apex, makes total four negative charges with four positive charges of the silicon ion. Thus there is net charge of -1 per unit.

2. Octahedral sheet or Alumina sheet: It is combination of octahedral units. An octahedral unit consists of six hydroxyl ions at the tips of an octahedron enclosing an aluminum or magnesium or any other metallic atom. The rows of hydroxyl ions in the sheet are in two planes with each hydroxyl ion common to three octahedral units. Each hydroxyl ion divides its -1 charge with two other units, leaving a total of 2 negative charge per unit. Thus, net charge of an octahedral unit with aluminum ion having +3 charge at center is +1.  

Expansive soils are those having swelling and shrinkage characteristic.
These soils exhibits high volume change characteristic when environment conditions changes from dry to wet.
The chief constituent of these soil which provide swelling and shrinkage characteristics is Montmorillonite clay mineral.

Montmorillonite:

• Montmorillonite is a clay mineral belongs to the smectite group.
• It is 2:1 clay minerals, that is two tetrahedral sheet of silica sandwiching a central octahedral sheet of alumina.
• These minerals bonded with very weak Van der Waals forces thus subjected to swelling and shrinkage.

Illite: 

• The structure of illite is similar to that of montmorillonite except that there is always substantial (20% ±) replacement of silicons by aluminum in the tetrahedral layers and potassiums are between the layers serving to balance the charge resulting from the replacement and to tie the sheet units together.

Kaolinite:

• It is the most common mineral of the kaolin group.
• The kaolinite structural unit is made up of gibbsite sheets (with aluminum atoms at their centers) joined to silica sheets through the unbalanced oxygen atoms at the apexes of the silicas, (i.e., the apexes of the silica layer and one of the gibbsite form a combined layer).
• This structural unit is symbolized by which is about 7 Ȧ (one angstrom, Ȧ = 10^-7 mm = 10^-10 m) thick.
  

1. With reference to grain size:
   Kaolinite > Illite > Montmorillonite
2. With reference to swelling and shrinkage behavior
   Montmorillonite > Illite > Kaolinite
3. With reference to Plasticity Index
   Montmorillonite > Illite > Kaolinite

Kaolinite:

• The kaolinite structural unit is made up of gibbsite sheets(with aluminum atoms at their centers) joined to silica sheets through the unbalanced oxygen atoms at the apexes of the silicas.
• These basic units are then stacked one on top of the other to form a lattice of the mineral
• The successive 7 angstrom units are held together by hydrogen bonds.
• The strong bonding does not permit water to enter the lattice
• Thus, kaolinite minerals are stable and do not expand under saturation
• The total thickness of the structural unit of a Kaolinite mineral is 7 A°, where 1 A° = 10-10 meters.
  

Concept:
Soil structures:

• The geometrical arrangement of soil particles with respect to one another is known as soil structure. The various soil structures depend upon the particle size and the mode of formation.

Flocculent and dispersed structures:

• These are found in soils having a size of less than 0.0002 mm. The fine particles are mostly flaky, platelets, and not spherical, and contain surface electric charges.
• The formation of these soils is due to surface electric force and not due to the gravity force. Most of the clay has these structures.

Flocculent structure:

• In a flocculent structure, clay particles connect edge to edge or edges to face.
• These soils form due to attractive forces.
• Flocculent structure enclosed a large volume of voids

Dispersed structure:

• In dispersed structure, particles pint face to face and repulsive forces play an important role in the deformation of such soils.
• Dispersed structures develop in clays that have been remolded.
• The dispersed structure has fewer voids.

Kaolinite:
Kaolin, also called china clay, soft white clay that is an essential ingredient in the manufacture of porcelain china and is widely used in the making of paper, rubber, paint, and many other products.

Concept:
Hardness is the resistance of a material against scratching. For Example: If material A can scratch material B, then material A is harder than material B.
The hardness of any mineral is defined by the Mohs scale. As per this scale, hardness of minerals is ranked in the scale of 1 to 10. Higher the rank, higher will be the hardness.
Diamond is the hardest known natural mineral in earth according to Mohs scale.

It is clear from above table that Quartz is the maximum hardness among all other options.

• Black cotton soils have montmorillonite clay mineral which shows swelling and shrinkage behavior on wetting and drying.
• Under-reamed piles are used for the structures constructed on black cotton soils or expansive soil to resist the expansive nature of the soil through the bulb provided in the piles.
• Black Cotton Soil also consists of the excess of Montmorillonite mineral.
• Bentonite clay shows swelling and shrinkage characteristics due to moisture content variation due to the presence of Montmorillonite mineral.
• Bentonite clay is also called Montmorillonite clay.

Clay Minerals

• Clay minerals and oxide minerals (including quartz) are the most common byproducts of chemical weathering.
• Clay minerals are an important group of minerals because they are among the most common products of chemical weathering, and thus are the main constituents of the fine-grained sedimentary rocks called mudrocks (including mudstones, claystones, and shales). 
• Clay minerals make up about 40% of the minerals in sedimentary rocks.  In addition, clay minerals are the main constituent of soils. 
• Clay minerals are used extensively in the ceramics industry and are thus important economic minerals.  

Based on their structures and chemical compositions, the clay minerals can be divided into three main classes:

1. Kaolinite minerals
2. Illite minerals
3. Montmorillonite minerals

Diffused double-layer
The exchangeable ions are surrounded by water molecules and may thus be considered as forming a solution which is often a micellar solution or inner solution.
The solution containing free electrolytes is called the outer solution or inter-micellar solution.
The ionic conditions on the outside surface of a clay particle or a pocket of clay particles dispersed in water or an electrolyte solution are controlled by the proportion of the exchangeable cations that disperse into the solution.
Hence, the thickness of the diffuse double layer in pure clay will be maximum for the predominant clay mineral Montmorillonite.:
The thickness of the diffuse double layer depends upon the following factors:
(i) Concentration of Total Electrolyte:
(ii) Valence of the Counter Ion
(iii) Dielectric Constant: