2.1.2 Important properties of clay minerals
Some of the important properties that influence the behaviour of clay minerals are presented below:
High surface area
Specific surface area (SSA) is defined as the surface area of soil particles per unit mass (or volume) of dry soil. Its unit is in m2/g or m2/m3. Clay minerals are characterized by high specific surface area (SSA) as listed in Table 2.2. High specific surface area is associated with high soil-water-contaminant interaction, which indicates high reactivity. The reactivity increases in the order Kaolinite< Illite< Montmorillonite. For the purpose of comparison, SSA of silt and sand has also been added in the table. There is a broad range of SSA values
of soils, the maximum being for montmorillonite and minimum for sand. As particle size increases SSA decreases.
| Table 2.2 Typical values of SSA for soils (modified from Mitchell and Soga 2005) |
Soil |
SSA (m2/g) |
| Kaolinite | 10-30 |
| Illite | 50-100 |
| Montmorillonite | 200-800 |
| Vermiculite | 20-400 |
| Silt | 0.04-1 |
| Sand | 0.001-0.04 |
For smectite type minerals such as montmorillonite, the primary
external surface area amounts to 50 to 120 m2/g. SSA inclusive of both primary and secondary surface area (interlayer surface area exposed due to expanding lattice) and termed as total surface area would be close to 800 m2/g. For kaolinite
type minerals there is possibility of external
surface area where in the interlayer surface area
does not contribute much. There are different methods available for determination of external or total specific surface area of soils (Cerato and Lutenegger 2002, Arnepalli et al. 2008).
Plasticity and cohesion
Clay attracts dipolar water towards its surface by adsorption. This induces plasticity in clay. Therefore, plasticity increases with SSA. Water in clays exhibits negative pressure due to which two particles are held close to each other. Due to this, apparent cohesion is developed in clays.
Surface charge and adsorption
Clay surface is charged due to following reasons:
Isomorphous substitution (Mitchell and Soga 2005): During the formation of mineral, the normally found cation is replaced by another due to its abundant availability. For example: when Al+3 replace Si+4 there is a shortage of one positive charge, which appears as negative charge on clay surface. Such substitution is therefore the major reason for net negative charge on clay particle surface.
O-2 and OH- functional groups at edges and basal surface also induce negative charge.
Dissociation of hydroxyl ions or broken bonds at the edges is also responsible for unsatisfied negative or positive charge. Positive charge can occur on the edges of kaolinite plates due to acceptance of H+ in the acid pH range (Berkowitz et al. 2008). It can be negatively charged under high pH environment.
Absence of cations from the crystal lattice also contributes to charge formation.
In general, clay particle surface are negatively charged and its edges are positively charged.
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Due to the surface charge, it would adsorb or attract cations (+ve charged) and dipolar molecules like water towards it. As a result, a layer of adsorbed water exists adjacent to clay surface, the details of which are presented in section 2.2.1.