Exchangeable cations and cation exchange capacity
            Due to negative charge, clay surface attracts cations towards it to make the charge neutral. These cations can be replaced by easily available ions present in the pore solution, and are termed as exchangeable ions. The total quantity of exchangeable cations is termed as cation exchange capacity, expressed in milliequivalents per 100 g of dry clay. Cation exchange capacity (CEC) is defined as the unbalanced negative charge existing on the clay surface. Kaolinite exhibits very low cation exchange capacity (CEC) as compared to montmorillonite. Determination of CEC is done after removing all excess soluble salts from the soil. The adsorbed cations are then replaced by a known cation species and the quantity of known cation required to saturate the exchange sites is determined analytically.
Flocculation and dispersion
           When two clay particles come closer to each other it experiences (a) interparticle attraction due to weak van-der-Waal’s force (b) repulsion due to –ve charge. When particles are sufficiently close, attraction becomes dominant active force and hence there is an edge to face configuration for clay particles as shown in Fig. 2.4a. Such a configuration is termed as flocculant structure. When the separation between clay particles increase, repulsion becomes predominant and hence the clay particles follows face to face configuration called dispersed structure (Fig. 2.4b). A lot of micro and macro level behaviour of clays are associated with these arrangement of clay particles (Mitchell and Soga 2005).

Fig. 2.4 Different arrangement of clay particle

Swelling and shrinkage
            Some clay minerals when exposed to moisture are subjected to excessive swelling and during drying undergo excessive shrinkage. A lot of engineering properties of soil is affected by this behaviour and the stability of structures founded on such soils become detrimental. The swelling of clay minerals decreases in the order montmorillonite> illite> kaolinite.

2.1.3 Minerals other than silica and clay
            Other than silica and clay, subsurface contains a variety of minerals such as oxides and carbonates that governs the reactivity of soil and its interaction with the environment. Some of the abundant metal oxide minerals present are iron oxides (hematite, magnetite, goethite etc.) and aluminium oxides (gibbsite, boehmite). Other oxide minerals (such as manganese oxide, titanium oxide) are far less than Fe and Al oxides, but because of small size and large surface area, they would affect very significantly the geochemical properties of subsurface. These oxides are mostly present in residual soils of tropical regions.  Other major components include soluble calcium carbonate and calcium sulphate, which has relatively high surface area. In most soils, quartz is the most abundant mineral, with small amount of feldspar and mica present. Carbonate minerals such as calcite and dolomite are found in some soils in the form of bulky particles, precipitates etc. Sulphate minerals mainly gypsum are found in semiarid and arid regions.

2.1.4 Soil mineralogy characterization
           One of the very well established methods for mineralogy characterization of fine-grained soils is by using X-ray diffraction (XRD) analysis. Majority of the soil minerals are crystalline in nature and their structure is defined by a unique geometry. XRD identifies minerals based on this unique crystal structure. In XRD, characteristic X-rays of particular wave length are passed through a crystallographic specimen. When X-ray interacts with crystalline specimen it gives a particular diffraction pattern, which is unique for a mineral with a particular crystal structure. The diffraction pattern of the soil specimen (according to its crystal structure), which is based on powder diffraction or polycrystalline diffraction, is then analyzed for the qualitative and quantitative (not always) assessment of minerals. Sample preparation method for XRD should be done with great care as the XRD reaches only a small layer (nearly 50 µm) from the surface of the sample. Hence, homogeneity is very important. Soil sample is initially dried and sieved through 2 mm sieve. Sieved sample is homogenized in a tumbler mixer for 30 min. A control mix of 30 g was taken and ground in lots of 15 g in a gyratory pulverizer. 15 percent by weight of KIO₄ (internal standard) was added to 5 g of specimen and again homogenized in a mixer. The prepared specimen is then subjected to analysis. .
           X-ray wave of monochromatic radiation (K_α) is commonly obtained from copper radiation, which is commonly known as Cu- K_α. A typical XRD output is represented by Fig. 2.5. It can be noted from the figure that ordinate represent relative intensity of X-ray diffraction and abscissa represents twice of angle at which a striking X-ray beam of wave length λ makes with parallel atomic planes. Based on this diffraction pattern, the minerals can be identified by matching the peak with the data provided by International Centre Diffraction Data (ICDD) formerly known as Joint Committee on Powder Diffraction Standards (JCPDS).