A ‘mineral’ is an inorganic chemical compound formed in nature. As a solid, it may occur in an amorphous state or in a crystalline state. A ‘crystal’ is a homogenous body bounded by smooth plane surfaces. Soil particles are largely composed of mineral crystals. Molecules of minerals are composed of atoms of chemical elements. The atoms in a crystal are arranged in a definite orderly manner to form a three dimensional net-work, called a “lattice.” Earth is about 12,500 km in diameter and most geotechnical engineering work is confined to the top few hundred meters of the crust, which is comprised essentially of oxygen (49.2%), silicon (25.7%) and aluminum (7.5%) present in the form of oxides, with some Fe3+, Ca2+, Na+, K+, Mg2+, etc. The atomic structure of a clay mineral is made of one of the two structural units: tetrahedrons containing a silicon atom at the center surrounded by four oxygen atoms at the corners and octahedrons containing aluminum or magnesium ions at the center surrounded by six hydroxyl or oxygen ions at the corners.
Formation of Clay Minerals
A soil particle may be a mineral or a rock fragment. A mineral is a chemical compound formed in nature during a geological process, whereas a rock fragment has a combination of one or more minerals. Based on the nature of atoms, minerals are classified as silicates, aluminates, oxides, carbonates and phosphates.
Out of these, silicate minerals are the most important as they influence the properties of clay soils. Different arrangements of atoms in the silicate minerals give rise to different silicate structures. Basic structural units soil minerals are formed from two basic structural units: tetrahedral and octahedral. Considering the valencies of the atoms forming the units, it is clear that the units are not electrically neutral and as such do not exist as single units. The basic units combine to form sheets in which the oxygen or hydroxyl ions are shared among adjacent units. Three types of sheets are thus formed, namely silica sheet, gibbsite sheet and brucite sheet.
Isomorphous substitution is the replacement of the central atom of the tetrahedral or octahedral unit by another atom during the formation of the sheets. The sheets then combine to form various two-layer or three-layer sheet minerals. As the basic units of clay minerals are sheet-like structures, the particle formed from stacking of the basic units is also plate-like. As a result, the surface area per unit mass becomes very large.
Atomic and Molecular Bonds
Forces which bind atoms and molecules to build up the structure of substances are primarily of electrical nature. They may be broadly classified into “primary bonds” and “secondary bonds’’. Primary bonds combine the atoms into molecules. Secondary bonds link atoms in one molecular to atoms in another. They are much weaker than the primary bonds. Primary bonds are the ionic bond and the covalent bond. Secondary bonds are the hydrogen bond and the Van der Waals bond.
1) Ionic Bond
The ionic bond is the simplest and strongest of the bonds which hold atoms together. This bond is formed between oppositely charged ions by the exchange of electrons. Atoms held together by ionic bonds form “ionic compounds”’, e.g. common salt (sodium chloride) and a majority of clay mineral crystals fall into this group. Ionic bonding causes a separation between centres of positive and negative charge in a molecule, which tends the molecule to orient in an electric field forming a “dipole”. Dipole is the arrangement of two equal electro-static charges of opposite sign. A dipolar molecule is one which is neutral but in which the centres of positive and negative charges are separated such that the molecule behaves like a short bar magnet with positive and negative poles.
2) Covalent Bond
The covalent bond is formed when one or more bonding electrons are shared by two atoms so that they serve to complete the outer shell for each atom.
3) Hydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule. Thus when water molecules are close together, their positive and negative regions are attracted to the oppositely-charged regions of nearby molecules. The force of attraction, shown in Fig. 2 as dotted line, is called a hydrogen bond. Each water molecule is hydrogen bonded to four others. Hydrogen bond can link the oxygen from a water molecule to the oxygen on the clay particles surface. Hydrogen bonding between two oxygen atoms is responsible for some of the weaker bonds between crystal layers for holding water at the clay surface and for bonding organic molecules to the clay surface.
4) Van der Waals Bond
It is the sum of the attractive or repulsive forces between molecules (or between parts of the same molecule) other than those due to covalent bonds or ionic bond. The covalent bonds within the molecules are very strong and rupture only under extreme conditions. The bonds between the molecules that allow siding and rupture to occur are called Van der Waals forces.
When ionic and covalent bonds are present, there is some imbalance in the electrical charge of the molecule. The angle hydrogen atoms are bonded to oxygen atom in water produces a positive polarity at the hydrogen-rich end of the molecule and a negative polarity at the other end. As a result of this charge imbalance the water molecules are attracted to each other. This is the force that holds the molecules together in a drop of water, shown in Fig. 3. Heat can be used to break the Van der Waal forces between the molecules and change the form of the material from solid to liquid gas.
Basic Structural Units of Clay Minerals
The clay minerals are a group of complex alumino-silicates, i.e., oxides of aluminium and silicon with smaller amounts of metal ions substituted within the crystal. The atomic structures of clay minerals are built up of two basic units such as Silica tetrahedral units and Aluminium (or magnesium) octahedral unit. These units are held together by ionic bonds.
1) Silica Unit
The silica unit consists of a silicon ion surrounded by four oxygen ions arranged in the form of a tetrahedron. The basic units combine in such a manner as to form a sheet. In the silica sheet, the bases of the tetrahedrals are all in the same plane and the tips all point in the same direction. Each of the three oxygen at the base is shared by two silicon of adjacent units.
2) Aluminium (or Magnesium) Octahedral Unit
The octahedral unit has an aluminium ion or a magnesium ion endorsed by six hydroxyl radicals or oxygen arranged in the form of an octahedron. In some cases, other cations (e.g. Fe) are present in place of Al and Mg. Combination of octahedral units forms an Octahedral sheet, which is called a ‘gibbsite” sheet if the central action of the unit is aluminum or a “brucite” sheet if the central cation is magnesium.
Types of Clay Minerals
From an engineering point of view, three clay minerals of interest are kaolinite, montmorillonite and illite.
1) Kaolinite
This is the most common of the Kaolin group. Each structural unit of Kaolinite is a combination of two layers with a silica layer joined to one of a gibbsite layer. Successive layers of structural units are held together to form kaolite particles which occur as platelets joined by strong H-bond. Kaolinite is used for making paper, paint and in pharmaceutical industry.
2) Montmorillonite
The montmorillonite mineral is a stacking of basic sheet like structural units, with each unit made up of gibbsite sheet sandwiched between two silica sheets joined by weak Van der Waal’s bond. It is easily separated by water. Because of the fact that bonding by Van der Waals forces between silica sheet of adjacent structural units is weak and there is a net negative charge deficiency in octahedral sheet, water and exchangeable cations can enter and separate the layers. Thus soil containing montmoriillonite mineral exhibits high swelling and shrinkage characteristics.
3) Illite
The basic structural unit of illite is the same as that of montmorillonite except for the fact that there is some substitution of aluminium for silicon in the silica sheet and the resultant charge deficiency is balanced by potassium ions, which bond the layers in the stack. There is about 20% replacement of aluminium with silicon in the gibbsite sheet due to isomorphous substitution. The bond with the non-exchangeable K+ ions are weaker than the hydrogen bond in the Kaolite but is stronger than the water bond of montmorillonite. The illite crystal does not swell so much in the presence of water as does in montmorillonite particles.
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