Essay on Soil Mineralogy

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Essay on Soil Mineralogy

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Essay # 1. Introduction to Soil Mineralogy:

The solids in soils, excluding organic matter, are termed as minerals. Mineral is a naturally occurring inor­ganic compound of various elements and has a fixed chemical composition, internal structure, and properties.

As per International Mineralogical Association, a mineral is an element or chemical compound that is normally crystalline and has been formed as a result of geological processes. Some minerals may not have any regular crystal structure and are thus amorphous. The branch of geology that deals with the study of minerals is called mineralogy.

Crystalline minerals comprise the greatest proportion of most soils encountered in engineering practice, and the amount of non-clay material usually exceeds the amount of clay. Inspite of this, clay and organic matter in a soil usually influence properties in a manner far greater than their abundance.

The structure of the minerals tells us a great deal about their surface characteristics and their potential interactions with the adjacent liquid phase. Mineralogy not only controls the size and shape of soil particles but also helps to understand soil properties and the nature of response of a soil, in terms of deformation and stress, to external loads.

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Essay # 2. Primary and Secondary Minerals:

Soil-forming minerals may be broadly classified into the following two groups:

A. Primary Minerals:

Primary minerals in soils originate from the parent rock and do not undergo any change in their chemical composition and internal structure, except a change in their particle size and shape. These minerals either have high resistance to chemical weathering or are present in soils that are formed by phys­ical weathering. Primary minerals are usually present in the gravels, sand, or non-plastic fine (silt) fraction of soils.

B. Secondary Minerals:

Secondary minerals are formed from the decomposition of primary minerals, which have undergone change in chemical composition and internal structure due to chemical and biological weathering of rocks and/or soils. Minerals present in clays, called clay minerals, are examples of secondary minerals.

Quartz is abundant in sand and silt. Feldspar is abundant in soils that are not subjected to extensive leaching. Amphibole, pyroxene, and olivine are easily weathered to form soils. Allophone is abundant in soil derived from volcanic ash deposits. Gibbsite is abundant in leached soils.

Chemical weathering is minimal in desert regions, where the water availability is limited. Soils in this region contain primary minerals such as gypsum, calcite, biotite, glauconite, nontronite, albite, and orthoclase, and are subjected to limited weathering. Fine silt and clay fractions of temperate regions mainly contain minerals, such as quartz, muscovite, illite, vermiculite, hydrous mica, and montmorillonite, developed under grass or trees. Soils in warm and humid equatorial regions contain minerals such as kaolinite, gibbsite, hematite, and zircon that are formed by intense chemical weathering.

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Essay # 3. Bonding in Soils:

The following are the various types of bonding in soils:

I. Primary Valence Bonds:

i. Ionic bond.

ii. Covalent bond.

iii. Heteropolar bond.

II. Hydrogen bond.

III. van der Waals’ Forces.

I. Primary Valence Bonds:

The atoms in the basic mineral units of soils are held together by primary valence bonds in which only the outer shell or valence electrons participate in the formation of bonds. In all primary bonds, the bonding energy is high. As soil is a particulate system, the external loads are resisted by a relative movement between the particles.

The shear stress at failure is much less compared to the stress required to break the primary bonds within the limits of stresses to which soils are subjected to in the usual construction practice.

II. The Hydrogen Bond:

Hydrogen atom, with atomic number 1, has only one electron. The atomic number of oxygen is 8 and it requires two electrons for a stable configuration. Two atoms of hydrogen combine with one oxygen atom and form a water molecule. Each electron of the two hydrogen atoms in the water molecule is more attracted to the oxygen atom than the other hydrogen atoms because of more protons in the oxygen atom.

The electrons of the hydrogen atom, therefore, revolve around the nucleus of oxygen atom for most of the time, giving a positive charge to the hydrogen atoms and a negative charge to the oxygen atom. Thus, the water molecule forms a dipole with a positive charge at one end and a negative charge at the other end with a bonding angle of 105°. Hydrogen bond is formed due to the attraction between the positive end (hydrogen) of one dipole and the negative (oxygen) end of another dipole.

If a hydrogen ion forms the positive end of a dipole, then its attraction to the negative end of an adjacent molecule is termed as hydrogen bond. When the electron is detached from a hydrogen atom, such as when it combines with oxygen to form water, only a proton remains.

As the electrons shared between the oxygen and hydrogen atoms spend most of their time between the atoms, the oxygen end acts as the negative ends of dipoles and the hydrogen ends act as the pos­itive ends. The strength of the hydrogen bond is much greater than that of other secondary bonds because of the small size of the hydrogen ion. Hydrogen bonds are important in the bonding between basic mineral units of clay minerals.

III. Van der Waals’ Forces:

Permanent dipole bonds, such as hydrogen bonds, are directional. Fluctuating dipole bonds, commonly termed as van der Waals bonds, also exist because at any one time there may be more electrons on one side of the atomic nucleus than on the other.

This creates weak instantaneous dipoles whose oppositely charged ends attract each other. Although individual van der Waals bonds are weak, typically an order of magnitude weaker than a hydrogen bond, they are non-directional and additive between atoms. Consequently, the influence of van der Waals bonding forces extends over larger distance than those of primary valence and hydrogen bonds when there are large groups of atoms. They are strong enough to determine the final arrangements of groups of atoms in some solids (e.g., many polymers), and they may be responsible for small cohesion in fine-grained soils.

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Essay # 4. Mineralogy of Fine-Grained Soils:

The fine-grained soil consists of mainly the silt and clay fraction. Mineralogy of the clay fraction has pronounced influence on the engineering behavior of fine-grained soils as well as coarse-grained soils containing fine fraction.

The term clay usually refers to clay minerals formed by chemical weathering of primary minerals of rocks. Clay minerals have distinct properties such as – (a) small particle size, (b) net negative electrical charge, and (c) plasticity when mixed with water. Clay minerals are primarily hydrous aluminum silicates and have individual particles of size smaller than 2 µm.

It may be noted that all clay particles are neither smaller than 2 µm nor all of them are coarser than 2 µm; how­ever, the amount of clay mineral in a soil is often closely approximated by the amount of material finer than 2 µm. In reality, the individual coarse- grained particles such as sand, gravel, boulders, or even the silt particles or a com­bination of these particles are held together in a dry state by the clay fraction forming large-size particle groups due to cohesive forces. The appropriate way of determining the clay fraction is wet sieve analysis followed by sedimentation analysis.

In particle size classification, size less than 2 µm is referred to as clay, which may include both clay and non-clay minerals. Where the term clay refers to particle size, it is recommended to refer to it as clay-size. Some primary min­erals that have undergone physical weathering by glaciers are found to be of clay size, that is, less than 0.002 mm.  

i. Kaolinite:

Kaolinite is by far the most common clay mineral found in soils and most clay deposits contain at least some amount of kaolinite. Clay deposits will frequently be nearly 100% kaolinite pure. Rocks that are rich in kaolinite are known as kaolin or china clay. The name kaolinite is derived from Kao-ling, a town in China, where the occurrence of the kaolinite mineral is first reported.

ii. Montmorillonite:

Montmorillonite is a soft mineral in the form of microscopic crystals belonging to the smectite group of clay miner­als. It is named after the place Montmorillon in France, where it was first found.

iii. Illite:

The name illite was proposed by Grim, Bray, and Bradley in 1937 for the mica-type mineral occurring in argilla­ceous sediments.

iv. Chlorite:

Chlorite mineral is formed when two silica sheets combine alternately with two alumina or brucite sheets. The mineral is formed by stacking of several such basic structural units one over the other. A single structural unit of chlorite is thus equal to two units of kaolinite mineral, with the thickness of the repeating unit being equal to about 14 Å.

The mineral may be dioctahedral or trioctahedral or both. That is, either two-thirds or all octahedral positions of the gibbsite sheet are occupied by aluminum or all octahedral positions are occupied by magnesium. There is a substitution of silicon in the silica sheet by aluminum and of aluminum in the brucite sheet by magnesium.

Chlorite is found in metamorphic rocks such as schist and is referred to as 2:2 clay mineral. The proportion of chlorite in clays is generally less, and it particularly occurs in marine sediments. The Chlorite mineral is formed by isomorphous substitution resulting in smectite by Mg²⁺ with the replacement of interlayer water by brucite layer. Chlorite is also formed by weathering of low-to-intermediate-metamorphic rocks.

The chemical formula of chlorite is (Mg, Fe)₃(Si, Al)₄O₁₀(OH)₂. It is usually light to dark green, grayish-green, or black in color. The specific gravity of the mineral varies from 2.6 to 3.3. The mineral can be identified by the XRD peaks of about 14.2 Å, 7.1 Å, and 2.36 Å. The first-order reflection of 14.2 Å is rather weak, and to distinguish the mineral from kaolinite, the mineral may be treated with an acid. The peaks will disappear for the acid-treated chlorite due to dissolution of brucite layer in the acid, whereas the XRD pattern of kaolinite remains unchanged with acid treatment.

v. Vermiculite:

Vermiculite resembles in structure to montmorillonite, with the difference that the thick­ness of water layers between structural units restricted to two water molecules. The mineral is usually trioctahedral with all the octahedral positions of the alumina sheet filled with aluminum. Thus, the ver­miculite mineral can be identified by XRD with the first-order reflection of 15 Å, which is equal to the thickness of one mineral layer plus two water molecules. The water can be removed by heating at 700°C, and the mineral will not rehydrate in the presence of water similar to halloysite. Vermiculite has the highest CEC among all the clay minerals.

The chemical formula of chlorite is (Mg, Fe++, Al)₃(Al, Si)₄O₁₀(OH)₂ .4(H₂O) and its molecular weight is 504.19 g. It is usually colorless, or green, gray white, or yellow brown in color. The specific gravity of the mineral varies from 2.3 to 2.7.The SEM image of typical vermiculite, which is somewhat similar to that of a montmorillonite in terms of particle thickness, lateral dimensions, and surface area.

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Essay # 5. Mineralogy of Cohesionless Soils:

Cohesionless soils are formed mostly by physical weathering and therefore consist mainly of rock-forming pri­mary minerals. Common rock-forming minerals are feldspar, quartz, mica, pyroxene, amphibole, and clay min­erals apart from other minerals, such as calcite, muscovite, biotite, etc. Quartz is by far the most common and abundant of the primary minerals in most soils. Feldspar and mica are frequently present in small percentages. Amphibole, pyroxene, and olivine have crystal structures that are rapidly broken down by weathering.

Hence, these minerals are absent from most soils. Calcite (CaCO₃) and dolomite [Ca. Mg. (CO₃)₂] are found in some soils, especially in some deep sea sediments. Sulfates in various forms, specifically with gypsum (CaSO₄ × 2H₂O), are found primarily in soils of semi-arid and arid regions. Iron and aluminum oxides are abundant in residual soils of tropical regions.

1. Quartz:

Quartz (SiO₂) consists of continuous three-dimensional framework of silica tetrahedra, with all tetrahedra oxygen bonded to silicon with a stable hexagonal crystal structure. As the framework is continuous, all the oxygen atoms of each silica tetrahedron are shared by the neighboring silica tetrahedra units, resulting in an electrically neutral mineral. Quartz has no weakly bonded ions in the structure, and the mineral has high hardness. Collectively, these factors account for the high persistence of quartz in soils. Quartz usually has a specific gravity of 2.65 and a hard­ness of 7 on Moh’s scale.

2. Feldspar:

Feldspar consists of continuous three-dimensional framework of silica tetrahedra. All the four oxygen atoms of one silica tetrahedron are shared by the neighboring silica tetrahedral. Some of the silicon atoms may be replaced by aluminum atoms. The mineral acquires a net negative charge and cations such as potassium, cal­cium, and sodium enter the structure to balance the charge deficiency. If potassium is the exchangeable cation, the feldspar is called orthoclase feldspar (KAlSi₃O₈), and in case it is sodium or calcium, it is called plagioclase feldspar.

Orthoclase possesses monoclinic crystal structure and microcline has triclinic crystal structure. Both of them have a chemical formula of KAlSi₃O₈. All plagioclase feldspars have triclinic crystal structure.

Plagioclase feldspar is of three types – albite (NaAlSi₃O₈) is sodium plagioclase feldspar, anorthite (CaAl2Si₃O₈) is calcium plagioclase feldspar, and labradorite (NaCaAlSi₃O₈) consists of both sodium and calcium. As these cations are relatively large, their coordination number is also large. This results in an open structure with low bond strength between units. Consequently, there are cleavage planes, the hardness is only moderate, and feldspars are relatively easily broken down. This accounts for their lack of abundance in soils compared to their abundance in rocks.

Orthoclase and plagioclase feldspars have elongated particles and have a hardness of 6. The plagioclase feldspar contains slightly bigger particles and a higher specific gravity (2.6 – 2.7) than the orthoclase feldspar (2.57).

3. Mica:

The basic unit of mica consists of one aluminum octahedral sheet sandwiched between two silica tetrahedra sheets. The oxygen atoms at the tips of silica tetrahedral are common to the silica and aluminum sheets. The basic struc­tural units are stacked one above the other and held together primarily by potassium ions in 12-fold coordination that provide an electrostatic bond of moderate strength. One of every four silicon atoms of the tetrahedral sheets is replaced by aluminum atoms, by isomorphous substitution, resulting in a net negative charge. Potassium ions exist between basic units of mica to balance this negative charge.

When two-thirds of the octahedral positions are filled and one-third is left vacant (dioctahedral), the min­eral is called muscovite or white mica. When all the aluminum atoms at octahedral positions are replaced by magnesium or iron, the mineral is called biotite or black mica. The muscovite [KAl₃Si₃O₁₀ (OH)₂] and biotite [K (Mg, Fe)₃AlSi₃O₁₀ (OH)₂] minerals have a monoclinic crystal structure. They have a specific gravity of 2.8 – 3.1 and hardness of about 2.5. As a result of the thin-plate morphology of mica flakes, sand and silts, containing only a few percent mica, may exhibit high compressibility and large swelling.

4. Calcite:

Calcite commonly occurs with hexagonal crystal structure but may sometimes occur in fibrous, granular, or nodu­lar form. It has hardness of 3 and a specific gravity of 2.7. The chemical formula for calcite is CaCO₃.

5. Dolomite:

Dolomite has a hexagonal crystal structure and has a granular form. Its hardness is 3.5 – 4.0 and specific gravity is 2.7. Its chemical formula is CaCO₃.MgCO₃.

6. Gypsum:

Gypsum has a monoclinic crystal structure but may also occur in granular or fibrous form. Its hardness is 2 and specific gravity is 2.3. The chemical formula of gypsum is (CaSO₄.2H₉O).