**2.1 Classification of clay minerals**

Clay minerals are classified into different groups as follows; Kaolinite, Smectite, Vermiculite, Illite and Chlorites.

Kaolinite is the purest of all clays, with little variation in composition. It also does not absorb water or expand when it comes into touch with it. As a result, kaolinite Al2Si2O5(OH)2 is the ceramic industry's chosen clay. Kaolinite clays have long been used in the ceramic industry, especially in fine porcelains, because they can be easily molded, have a fine texture, and are white when fired. These clays are also used as a filler in making paper. Clay minerals such as kaolinite, hallosite, nacrite, and dickite belong to the Kaolinite group, which is a 1:1 type clay mineral. It is made up of one layer of silica and one layer of alumina, which is created by advanced weathering processes or hydrothermal modifications of feldspars and other alluminosilicates in

### *Classification of Clay Minerals DOI: http://dx.doi.org/10.5772/intechopen.103841*

acidic circumstances [5]. The chemical formula of kaolinite is Al2O3.2SiO2.2H2O (39% Al2O3, 46.5% SiO2 and 14.0% H2O) and its structure possesses strong binding forces between the layers which resists expansion when wetted [1]. Kaolin is mostly white in color, has a very tiny particle size, is nonabrasive (hardness 2–2.5 on the Mohs scale), and is chemically inert in most applications. The individual kaolin particle is a thin, flat, pseudohexagonal platelet, so small that if 10,000,000 of them were distributed on a postage stamp, the layer would be thinner than a human hair [1]. In many applications, the thin, flat particle form is advantageous. The particle size and color, or brightness, of commercially available kaolin is used to determine the grade. Kaolin delivers strength, dimensional stability, and smooth surfaces to completed whitewares and sanitary wares by providing a white body, easy molding qualities, and adding strength, dimensional stability, and smooth surfaces. Kaolins are ideal for particular refractories because of their refractoriness, dimensional stability, and chemical inertness. Kaolin's outstanding dielectric qualities, in addition to the foregoing, make it ideal for porcelain electric insulators. Industrial uses of kaoline includes production of paper, paint, rubber, ceramic, plastic, and medicinal products, as well as a catalyst for petroleum cracking and vehicle exhaust emission control systems, and as a cosmetics foundation and pigment [6]. Furthermore, kaolin is also used as an anti-cracking agent in the manufacturing of fertilizer prills, as a pesticide carrier, in the production of white cement (where it supplies alumina without iron), and in the production of glass fiber as a low-iron, low-alkali alumina source. Kaopectate and Rolaids, for example, are the main ingredients in the original formulation of anti-diarrhea medication in pharmaceutical applications. Through firm and selective binding of aflatoxins, plant secondary metabolites, pathogenic microorganisms, heavy metals, and other poisons in animal diets that could be harmful to the digestive system, kaolin can be used to decontaminate aflatoxins, plant secondary metabolites, pathogenic microorganisms, heavy metals, and other poisons that could be harmful to the digestive system [7]. Naturally, kaolin may be accompanied by other mineral impurities such as feldspar and mica, quartz, titanoferous, illite, montmorillonite, ilmenite, anastase, hematite, bauxite, zircon, rutile, silliminate, graphite, attapugite, halloysite and carbonaceous materials [8] thus reducing its industrial usefulness.

Smectite, which includes montmorillonite, beidellite, nantronite, saponite and hectorite, are 2:1 layer clay minerals formed from the weathering of soils, rocks (mainly bentonite) or volcanic ash and belongs to a group of hydroxyl aluminosilicate [1]. The most common smectite is montmorillonite, with a general chemical formula: (1/2Ca,Na)(Al,Mg,Fe) 4(Si,Al)8O20(OH)4.nH2O Smectites are a category of dioctahedral 2:1 expandable minerals having a charge of 0.2–0.6 per formula unit. The octahedral substitution of Mg2+ for Al3+ gives Montmorillonite, the most prevalent member of this group, its charge. Tetrahedral replacements provide much of the charge in beidellite and nontronite, which are less common in soils. The presence of iron in the octahedral sheet distinguishes nontronite from beidellite. Van der Waals connections and weak cation-to-oxygen links hold the 2:1 layers in smectites together. The presence of exchangeable cations in the interlayer between water molecules causes the crystal lattice to expand as the mineral hydrates. The basal spacing between layers can exceed 2 nm when the material is saturated with water, but it can be lowered to less than 1 ran when the mineral is dry. The major component of bentonite is montmorillinite, which is formed by weathering volcanic ash. When water comes into contact with montmorillinite, it expands by many times its original volume. It may be used as a drilling mud (to keep drill holes open) as well as to seal leaks in soil, rocks, and dams because of this. This expansion and contraction trait found in

smectites, often referred to as shrink-swell potential, is problematic to engineers and farmers alike due to the propensity for crack formation and general instability of the soil surface. Montmorillinite, however, is a dangerous type of clay to encounter if it is found in tunnels or road cuts. Because of its expandable nature, it can lead to serious slope or wall failures. Differences in the degree of chemical substitution within the smectite structure, the nature of the exchangeable cations present, and the type and quantity of impurities present induce variation in the physical and chemical characteristics of bentonites within and across deposits [1]. Quartz, cristobalite, feldspars, zeolites, calcite, volcanic glass, and other clay minerals such as kaolinite are all minerals found in smectites [9]. Differences in chemical composition due to replacements of Al3+ or Fe3+ for Si4+ in the tetrahedral cation sites and Fe2+, Mg2+, or Mn2+ for Al3+ in the octahedral cation sites define the groups of smectite clays. Smectites contain very thin layers and microscopic particle sizes, resulting in a large surface area and hence a high degree of absorbency for a variety of compounds such as oil, water, and other chemicals [1]. Because of their high cation exchange capabilities, surface area, surface reactivity, adsorptive capacity, and catalytic activity, smectites are important minerals for industrial purposes. Bonding foundry sands, drilling fluids, iron ore pelletizing, agriculture (as a carrier material for pesticides, fertilizers, and seed coating), paper making, paints, pharmaceuticals, cosmetics, plastics, adhesives, decolorization, and ceramics are just a few of the applications for this group of clays [10]. The substance is also employed as a clarifier for oils and fats, as well as a chemical barrier, a liquid barrier, and a catalyst [11]. Purification and physicochemical changes of pure smectite are required for the preparation of several high-tech materials such as pillared clays, organoclays, and polymer/smectite-nano ccomposites [1]. Also they are used in many industries; the most important uses are as drilling muds and catalysts in the petroleum industry, as bonding clays in foundries, as bonding agents for taconite pellets, and as adsorbents in many industries. However, the commercial bentonites should contain not less than 60% smectite.

Vermiculite is a high-charge 2:1 phyllosilicate clay mineral. It is generally regarded as a weathering product of micas. Vermiculite is also hydrated and somewhat expansible though less so than smectite because of its relatively high charge. It has a layer charge of 0.9–0.6 per formula unit, and contains hydrated exchangeable cations primarily Ca, and Mg in the interlayer [12]. In soils, vermiculite exists as an Al3+ dominated dioctahedral and, to a lesser extent, Mg2+ dominated trioctahedral mineral. Water molecules and exchangeable cations—primarily Mg2+ and Ca2+—are highly adsorbed within the interlayer region of vermiculites due to the tetrahedral charge origin. In vermiculites, unlike smectites, the strong bonding of the interlayer cations binds the 2:1 layers together, restricting basal spacing expansion to 1.5 nm. Vermiculite has a high cation exchange capacity due to its high charge per formula unit, and this clay type has a strong affinity for weakly hydrated cations including K+ , NH4 + , and Cs<sup>+</sup> . The water in raw flakes vermiculite flashes into steam and the flakes expand into accordion-like particles when heated rapidly to 900°C or higher [13], a phenomenon known as exfoliation [14]. Exfoliation, liberates bound water from between the mica-like layers of the mineral and literally expands the layers apart at right angles to the cleavage plane. The expanded or exfoliated material is low in density, chemically inert and adsorbent has excellent thermal and acoustic insulation properties, is fire resistant and odorless. Granular clay absorbents, such as vermiculite, have been used for over 75 years to clean up minor drips, spills and over sprays in factories and garages. Vermiculite is used to loosen and aerate soil mixes. Mixed with soil, it improves water retention and fertilizer release, making it ideal for starting seeds.

### *Classification of Clay Minerals DOI: http://dx.doi.org/10.5772/intechopen.103841*

Also used as a medium for winter storage of bulbs and flower tubers. The common applications of exfoliated vermiculite include making of friction light weight aggregates, thermal insulator, brake linings, various construction products, animal feeds and in horticulture [1]. Vermiculite in fertilizers improves the efficiency with which nutrients are released, making fertilizers more cost-effective for customers [15]. Vermiculite's layered structure and surface qualities allow it to be employed in intumescent coatings and gaskets, as well as the treatment of hazardous waste and air freight. The internal pressure generated by the expansion of vermiculite when heated is sufficient to crush hard rock during tunneling activity [16]. Other minerals such as feldspars, pyroxenes, amphiboles, carbonates, and quartz, which develop alongside vermiculite in the rock and appear as major components, as well as minor components such as phosphates, iron oxides, titanium oxides, and zircon, can be found in vermiculite ores. Some impurities, such as asbestiform amphibole minerals present in vermiculite, have a negative impact on human health, as they can cause illnesses like malignant mesothelioma, asbestosis, or lung cancer; hence, clay characterization is necessary to detect such impurities [1].

Illite is the most frequent clay mineral, accounting for more than half of the claymineral suite in the deep sea. It is comparable to muscovite. They form in temperate climates or at high altitudes in the tropics, and they generally reach the ocean by rivers and wind transmission. Illite clays have a structure similar to muscovite, although they are generally low in alkalies and contain less Al substitution for Si. As a result, the typical formula for illites is KyAl4(Si8-y,Aly)O20(OH)4, with 1 < y < 1.5, but always with y < 2. Ca and Mg can occasionally be used in place of K due to a charge imbalance. The interlayer cations of K, Ca, or Mg prohibit H2O from entering the structure. As a result, the illite are non expanding clays. Clay micas are another name for the illite clay mineral group. Mica is a phyllosilicate mineral that may be split or delaminated into thin sheets that are platy, flexible, clean, elastic, transparent to opaque, robust, reflecting, refractive, dielectric, chemically inert, insulating, light weight, and hydrophilic [1]. Mica minerals' atoms are bound together into flat sheets, allowing for flawless cleavage of the minerals to generate durable sheets in a variety of colors, including brown, green, black, violet, and colorless, with a vitreous to pearly shine [17]. There are around 30 members of the mica group, but muscovite, biotite, phlogopite, lepidolite, fuchsite, and zinnwaldite are the six most frequent forms found in nature and employed in microscopy and other analytical applications [1]. Clay minerals are made up of three members (the illite group), which include illite, glauconite, and muscovite, and display clay-like characteristics, with illite being the most frequent. Illite is generated by alkaline weathering of potassium and aluminumrich rocks such as muscovite and feldspar. Illite is a 2:1 layer silicate clay mineral that is non-expansive due to poorly hydrated potassium cations or calcium and magnesium ions filling the gap between the crystals of individual clay particles, preventing water molecules from entering the clay structure. Illite's cation exchange capacity ranges from 20 to 40 meq per 100 g. Minerals range in color from gray white to silvery white to greenish gray. Because of their high potassium concentration, illites are used in the structural clay industry and in agro minerals [18]. Quartz, feldspar, kaolin, and pyroxene are among the impurities found in mica clay ores [19]. The presence of these minerals in mica ores will affect the industrial value of the deposits as well as the processing complexity, lowering or boosting their value depending on the uses [20].

Chlorites are hydrous aluminosilicates with an interlayer organized in a 2:1 configuration. Chlorites are fundamental minerals found in soils that weather to generate vermiculite and smectite. Interlayered hydroxy-Al vermiculites or

smectites, on the other hand, are regarded secondary minerals that develop as intermediate mineral weathering products or by the deposition of hydroxy-Al polymeric components inside the interlayer space of expanding minerals. There is no water adsorption within the interlayer space; thus, chlorites are considered nonexpansive minerals. These hydroxy-Al polycations balance a portion of the charge but they are not interchangeable. The CEC of the expandable 2:1 clays is lowered as a function of the quantity and valence of the hydroxy-Al polymer dwelling within the interlayer space because the level of hydroxy-Al occupancy within the interlayer space is varied. In the octahedral sheet inside the 2:1 layer and in the interlayer hydroxide sheet, they incorporate mostly Mg, Al, and Fe cations, with lesser amounts of Cr, Ni, Mn, V, Cu, and Li cations. In the tetrahedral sheet, they also show a substantial replacement of Si by Al cations [21]. Chlorites range in color from white to practically black or brown with a green tinge, and their optical qualities are linked to their chemical makeup [22]. In the study of phase interactions in low and intermediate grade metamorphic rock, understanding the chemical composition of chlorite is crucial [23]. The first mineralogical and chemical investigation of clay ores can be used to determine the appropriateness of the material for various uses, given the diversity of clay mineral groups in nature.
