Weathering of rocks and formation of soil

The exposed rocks of the earth's surface are subject to continuous decay, disintegration and degradation by certain physical, chemical and biological agents. This phenomenon is called rock crumbling. The physical/mechanical weathering of rocks is due to temperature fluctuations due to cycles of water freezing and thawing opening the rock mass in ancient humid climates and thermal effects in hot arid (arid) regions. Rainwater causes chemical weathering of rocks due to the chemical action of dissolved atmospheric gases (carbon dioxide, hydrogen, nitrogen, etc.). Organisms (nesting animals such as earthworms, ants and rodents) and plants also cause rock to break down through their physical activity. People also break rocks in different ways. 

Weathered rocks become more porous, individual grains weaken, and bonds between mineral grains disappear. Therefore, rocks lose strength and change shape more, and their permeability can change depending on the nature of the rocks, the presence and type of weathering, and the degree of weathering. The degree of weathering can be reflected by changes in index properties such as dry density, void ratio, clay content, and seismic velocity. The technical usability of stones depends mainly on two main forms of weathering: physical/mechanical weathering (degradation) and chemical weathering (degradation). Degradation of stones produces satisfactory technical materials that can be used as paving material and concrete filler, because the physical decomposition of stones occurs without dramatic changes in the mineral content of the stone and thus without a significant decrease in their durability. Weathering, on the other hand, involves chemical changes in rocks and results in the transformation of the most important minerals into some form of clay (Weinert, 1974). Estimating rock weathering has been a difficult problem for engineering geologists and geotechnical engineers. For convenience, researchers have classified rock weathering into different types/quality; Following table shows the classification proposed by Little (1969).

Little, A.L., Proceeding of the 7th International Conference on Soil Mechanics and Foundation Engineering, Vol. 1, pp. 1–10, 1969

Figure: Bowen’s reaction series

Soil-forming processes are complex and directly affect the engineering properties of the resulting soil mass. Soils are formed by the interaction of five soil-forming factors: source materials, topography, climate, organisms and time. The weathering of rocks as precursors plays an important role in the formation of soil and sediments. Minerals in rocks have different resistance to weathering. The above figure shows Bowen's (1922) reaction series, which lists some minerals in order of decreasing temperature of crystallization during their formation as the magma cools. This list also follows the order of increasing weather resistance once they are formed. Olivine, which crystallizes earlier, ie. at a higher temperature during the formation of rocks from magma, there is a more weather-resistant mineral in rocks. Quartz, which crystallizes later, i. at a lower temperature than when rocks form from magma, is the least weathered mineral. Quartz is the most common mineral in soils and sediments as a residue of weathering processes. Weathering of feldspar produces clay minerals (kaolinite or illite). In tropical weathering environments, clay minerals are further degraded, resulting in bauxite and laterite profiles.

It is important to note that most of the rocks and soils on or near the Earth's surface were formed during the last eighth of its geological time (4,600 million years - the age of the Earth and its Moon). About seven-eighths of the geological history of the Precambrian is poorly known. Based on the method of formation, soils are basically classified as follows:
1. Sedimentary soils
2. Residual soils
3. Fills
4. Organic soils

The formation of sedimentary soils consists of three stages: formation of sediments as a result of the erosion of rocks; the transport of sediments by water, wind, ice, gravity, and organisms called transporters, and the deposition of sediments in different environments. The orientation and distribution of particles within the soil mass, often referred to as soil structure, are controlled by the depositional environment. There are two types of soil structure viz. flocculated structure and dispersed structure. 

Soil structure: (a) flocculated and (b) dispersed

In the first structure, the particles have edge-to-edge or edge-to-edge contacts and have a net attraction, while in the second, the particles tend to adopt a face-to-face orientation, which has a net repulsion. The type of structure largely regulates the technical behavior of the soil. In general, a flocculated soil element has a higher strength, lower compressibility and higher permeability than a soil material with the same void ratio but in a dispersed state. When a flocculated soil is subjected to horizontal shear, the particles tend to align in a dispersed structure.

Residual soil originates from the in situ weathering of bedrock. Soils are usually above the water surface; therefore, they are often dissatisfied. The fill is artificial soil; the process of its formation is called filling. The fill is actually sedimentary soil whose processes are created by humans. Organic soils, such as peat, are derived from the composition of organic soils such as decaying vegetation, including leaves and tree roots.

Clay minerals are a group of complex aluminosilicates that form mainly due to the chemical weathering of primary minerals. For example, the clay mineral kaolinite is formed by the decomposition of feldspar under the influence of water and carbon dioxide. Most clay mineral particles are "plate-like" and have a high specific surface area (surface area per unit mass of material), so their properties are affected significantly more by surface forces than by body gravity. Long "needle-shaped" particles (eg Halloysite) may also be present but are rare.

The basic structural units of most clay minerals consist of silica tetrahedra and alumina octahedron. The main units are connected to plate structures. Silicon and aluminum can be partially replaced by other elements in those units, this is called isomorphic substitution, which can have two effects: a net deficit of unit charge causes an exchange, resulting in a negative net charge and a small distortion. crystal Lattice occurs because the ions are not the same size.

Many clay minerals form by stacking combinations of basic plate structures between plates connected by different bonds. There are three main clay minerals, namely kaolinite, illite, and montmorillonite (Grim, 1968). 

The basic structure of kaolinite consists of an octahedral layer of alumina over a tetrahedral layer of silica; this mineral is known as a "double layer" mineral. The thickness of the main device is approximately 7.2 Å. Isomorphic replacement of kaolinite is very limited. The sheets of combined silica and alumina are tightly held together by hydrogen bonding. A kaolinite particle can consist of more than 100 stacks. 

The basic structure of illite consists of an octahedral sheet of alumina sandwiched between two silica tetrahedra. The thickness of the main device is about 10 Å. In the octahedral sheet, aluminum is partially replaced by magnesium and iron, and in the tetrahedral sheet, silicon is partially replaced by aluminum. The connected plates are connected to each other by a rather weak bond, which is due to the non-exchangeable potassium ions located between them. 

Montmorillonite has the same basic structure as illite. In the octahedral plate, aluminum is partially replaced by magnesium. The thickness of the main device is about 9 Å. Between the connected plates are water molecules and exchangeable cations except potassium. Because of these ions, there is a very weak connection between the connected plates. Significant swelling of montmorillonite can occur due to the addition of adsorbed water between the bonded sheets. 

The surfaces of clay mineral particles have a net negative charge, which may be due to the following five factors or a combination of them: isomorphous substitution, surface dissociation of hydroxyl ions, absence of cations from the crystal lattice, adsorption of anions, and the presence of organic matter. Of these five factors, the isomorphic replacement of aluminum or silicon atoms by atoms of lower valence is the most important. 

In nature, a soil particle attracts ions to neutralize its net charge. Since these removed ions usually remain weakly on the particle's surface and can easily be replaced by other ions, they are called exchangeable ions and the phenomenon of cation exchange. Calcium is a very common exchangeable ion in soil. The cations are attracted to the clay mineral particle due to negative surface charges, but at the same time tend to move away from each other due to their thermal energy. The net effect is that the cations form a diffuse layer next to the particle. The concentration of cations decreases with increasing distance from the surface until the concentration is equal to that of normal water in empty space. The surface of a layer of negatively charged particles and dispersed cations is usually described as a bilayer.