COMPOSITION OF SOILS, Mineral component of soils, Types of bonds...


A soil is a mineral or organic, organo-mineral, multicomponent, multiphase dynamic system that is studied in engineering-geological terms. The constituent components of the soil system are: a solid component (mineral, ice and organic components), a liquid component (natural water contained in the voids of the rocks), a gaseous component (gases in rock voids) and a biotic living component (macro- and microorganisms , living in rocks) [50].

The ratio of components in the ground determines their state and properties. Depending on whether the pores are completely or partially filled with water (or gas) and live microorganisms are contained in them, the soils can be two-, three- and four-component systems. If the soil consists of solid particles and all the pores are filled with water, then it is a two-phase system. In most cases, in the ground, in addition to solid particles and water, there is air or another gas, either dissolved in water or in the form of bubbles. Such a soil is a three-phase system. The ice contained in the ground gives it specific properties that must be taken into account, especially when building in permafrost areas. The frozen soil is a four-phase system. In some soils, there are organic substances in the form of plant residues or humus. The presence of even a relatively small amount of such substances in the soil significantly affects its properties.

Mineral component of soils

Types of bonds, composition and properties of soil mineral matter

Types of bonds in solid soil components

In engineering and geological studies of soils, it is especially important to know the content of rock-forming minerals in them, which are in the predominant quantities and affect their properties. In most cases, the upper layers of the earth's crust are composed of large clastic, sandy, silty-clayey, organogenic and technogenic soils. Most of the dispersed soils were formed as a result of the accumulation of products of physical and chemical weathering. Some soils arose as a result of the deposition of organic substances, as well as as a result of artificial pouring or reclamation of various materials. In the process of physical weathering, coarse-fragmented and sandy soils formed. The result of chemical and partly biological weathering is the minerals that make up the finely dispersed part of the silty-clay soils.

Minerals of the class of primary silicates are of the greatest importance, in which intracrystalline ion-covalent bonds predominate; simple salts (carbonates, sulfates, halides) having an ionic type of bonds; clay minerals (hydromica, montmorillonite, kaolinite, etc.), characterized by a wide variety of intracrystallization bonds.

In addition, in soils a considerable amount of organic matter can be contained, in the structure of which there are hydrogen and molecular bonds.

Fig. 2.1. Ratio of ground components

The properties of minerals, which in turn determine the properties of soils, are related to the peculiarities of their chemical composition, internal structure and bonds that exist inside the minerals themselves (atoms, ions, radicals). The strength of the solid components themselves is determined by the strength and nature of the bonds within them. Minerals with the same chemical composition can have different strengths due to differences in their crystal structure and the nature of chemical bonds within the crystal itself, so it is more justified to subdivide the solid component into the predominant type of bonds within the component - the stronger the bonds prevailing in the solid component, the stronger it is.

By the nature of the bonds and certain spatial distributions of electrons in the solid components of the soil, five types of bond are distinguished: covalent, ionic, metallic. hydrogen and molecular.

Covalent bonds form atoms (or groups of atoms) on valent orbitals of which there are unpaired electrons, the socialization of which leads to the formation of a common electron pair for the binding electrons (one electron from each atom). In the formation of a covalent bond from identical atoms, the distribution of the electron density in the orbitals of the bonding electron pairs is strictly symmetric with respect to both atomic centers (nonpolar bonds). Atoms with different electronegativity (valency) form polar covalent bonds in which the electron bonding cloud is displaced towards the more electronegative atom. Covalent bonds are inherent in silicate minerals, they are very strong and characterized by directionality. Many crystalline compounds with covalent bonds are refractory and have high hardness (diamond) and strength. However, covalent bonds can exist not only in solid crystals, but also in liquids (in water molecules), as well as in organic compounds (in molecules of organic compounds, for example, C-C, CN, CN, etc., where the sign " - means a covalent bond.)

Ion bonds are formed by Coulomb forces of attraction of oppositely charged ions: cations and anions. Ion bonds due to their electrostatic nature do not have a directivity and saturation. The unsaturation of the ionic bond is manifested in the propensity of compounds with such a bond to form crystal lattices in whose nodes each ion is surrounded by the maximum possible number of ions of the opposite charge. Ionic bonds are characteristic for many salts (halides, sulfates, carbonates). The most characteristic property of compounds with an ionic type of bond, in contrast to covalent solubility, is their ability to dissociate ions in solutions in polar solvents. This is due to the fact that the solvation energy of ions by solvent molecules is higher than the ionic bond energy in the crystal.

Metal bonds are formed in the connections between elements that have free valence orbitals and low ionization energy. Such bonds are characteristic for all compounds related to metals - substances with high electrical and thermal conductivity, due to the considerable mobility of electrons in the crystal lattice.

The metal is a series of positive ions localized in certain positions of the crystal lattice and immersed in the "sea" free mobile electrons, which distinguishes the non-directional metallic bond from the directed covalent bond. The energy of bonds between electrons and nuclei in metal crystals is relatively small, it is minimal in alkali metals and increases with increasing nuclear charge, reaching the highest values ​​for transition metals.

Hydrogen bonds represent a special type of the so-called three-center bond: X HV (the symbol "-" means a covalent bond and the sign is "..." hydrogen), in which the central atom hydrogen, H, linked by a covalent bond to the electronegative atom X (for example, O, C, N, S, etc. atoms), forms an additional bond e atom V (for example, N, O, S, etc.) This connection is not a shared electronic pair. There are inter- and intramolecular hydrogen bonds. Hydrogen bonds are characteristic for hydrogen-containing solid components of soil - ice, crystalline hydrates, some clay minerals, etc.

The molecular bonds (van der Waals forces) are due to the mutual polarization of the molecules and, therefore, can arise between polar and between different neutral molecules. These are very weak bonds that can exist in solid molecular crystals (for example, hardened inert gases, gas hydrates, etc.), as well as in organic solid components and clay minerals. Solids with molecular bonds have a low melting point and are markedly sublimated.

Based on the allocation of the predominant type of bond, the solid components of the soils are divided into the following groups, which differ significantly in their properties:

• minerals with a predominance of covalent bonds - primary silicates;

• with the predominance of ionic bonds - ionic minerals and salts;

• metal bonds - metal connections;

• with a predominance of covalent and the presence of molecular and hydrogen bonds - clay minerals;

• components with a predominance of molecular and the presence of covalent bonds - organic matter and organo-mineral complexes;

• components with a predominance of hydrogen, molecular and the presence of covalent bonds - ice and gas hydrates [50].

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