Knowledge of soil-water interaction and soil-water-contaminant interaction is very important for solving several problems encountered in geoenvironmental engineering projects. The following section introduces soil mineralogy and various mechanisms governing soil-water-contaminant interaction.
2.1 Soil mineralogy characterization and its significance in determining soil behaviour
Soil is formed by the process of weathering of rocks which has great variability in its chemical composition. Therefore, it is expected that soil properties are also bound to the chemical variability of its constituents. Soil contains almost all type of elements, the most important being oxygen, silicon, hydrogen, aluminium, calcium, sodium, potassium, magnesium and carbon (99 percent of solid mass of soil). Atoms of these elements form different crystalline arrangement to yield the common minerals with which soil is made up of. Soil in general is made up of minerals (solids), liquid (water containing dissolved solids and gases), organic compounds (soluble and immiscible), and gases (air or other gases). This section deals with the formation of soil minerals, its characterization and its significance in determining soil behaviour.
2.1.1 Formation of soil minerals
Based on their origin, minerals are classified into two classes: primary and secondary minerals (Berkowitz et al. 2008). Primary minerals are those which are not altered chemically since the time of formation and deposition. This group includes quartz (SiO2), feldspar ((Na,K)AlSi3O8 alumino silicates containing varying amounts of sodium, potassium), micas (muscovite, chlorite), amphibole (horneblende: magnesium iron silicates) etc. Secondary minerals are formed by the decomposition and chemical alteration of primary minerals. Some of these minerals include kaolinite, smectite, vermiculite, gibbsite, calcite, gypsum etc. These secondary minerals are mostly layered alumino-silicates, which are made up of silicon/oxygen tetrahedral sheets and aluminium/oxygen octahedral sheets. Primary minerals are non-clay minerals with low surface area (silica minerals) and with low reactivity (Berkowitz et al. 2008). These minerals mainly affect the physical transport of liquid and vapours. Secondary minerals are clay minerals with high surface area and high reactivity that affect the chemical transport of liquid and vapours (Low 1961).
Silica minerals are classified as tectosilicates formed by SiO4 units in frame like structure. Quartz, which is one of the most abundant minerals comprises up to 95 percent of sand fraction and consists of silica minerals. The amount of silica mineral is dependent upon parent material and degree of weathering. Quartz is rounded or angular due to physical attrition. The dense packing of crystal structure and high activation energy required to alter Si-O-Si bond induce very high stability of quartz. Therefore, the uncertainty associated with these materials is minimal. In the subsurface, quartz is present in chemically precipitated forms associated with carbonates or carbonate-cemented sandstones.
Clay minerals, which can be visualized as natural nanomaterials are of great importance to geotechnical and geoenvironmental engineers due to the
more complex behaviour it exhibits. Therefore, this chapter emphasize more on understanding clay mineral formation and its important characteristics. Basic units of clay minerals include silica tetrahedral unit and octahedral unit depicted in Fig. 2.1.
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It can be noted from the figure that metallic positive ion is surrounded by non-metallic outer ions. Fig. 2.2 shows the formation of basic layer from basic units indicated in Fig. 2.1. There are 3 layers formed such as (a) silicate layer, (b) gibbsite layer and (c) brucite layer.