## 2 Clays and Organoclays
### Classifications of Clays  
The basic structure of minerals contains silica tetrahedra, a combination of four oxygen atoms and one silicon atom. This is the most basic structure, where the \(+4\) charges of silicon are capped by the \(-2\times 4\) charges of four oxygen atoms, giving a net charge of \(-4\)[11]. Now these tetrahedrons are assembled into frameworks in simple to complex ways. Figure 2 shows the basic classification of silica clay, which is broken down into six distinct categories, namely, orthosilicate (single unit), disilicate (two units), ring silicate (closed unit), chain silicate, layered silicate (typically referred to as phyllosilicates), and framework silicates (i.e., quartz, feldspars, and zeolite).  
Figure 2: The basic classification of silicates and clay minerals.  
The clay material that we most commonly refer to as phyllosilicate, a layered structure, comprises silicon tetrahedrons (SiO\({}_{4}\))\({}^{4-}\) and aluminum octahedrons [Al(OH)\({}_{6}\)\({}^{3-}\)], which exist in various combinations [16; 17]. To understand its combination, it can be divided into five groups based on the octahedron-tetrahedron ratio. The only octahedral structure without a tetrahedral structure is called octahedral clay, which is further subdivided into dioctahedral and trioctahedral clays (see Figure 2). The clay falls into this category and is referred to as gibbsite or brucite [18].  
The second layered silicate category is based on the 1:1 ratio of tetrahedrons to octahedrons, represented as TO. These TO clays are also divided into dioctahedral and trioctahedral clays. The kaolinite clay group and the serpentine clay group fall into this category. Kaolinite, dickite, nacrite, and halloysite belong to the kaolin group, which has a lower cation exchange capacity (CEC) and surface area and is widely used in the cosmetics industry.  
Meanwhile, the most important and most commonly used phyllosilicate is the 2:1 group, consisting of two tetrahedra and one octahedron (TOT) [18]. In this structure, an octahedral sheet is sandwiched between two tetrahedral sheets. This group of clays is divided into two groups, namely, non-interlayer charge and interlayer charge. Non-interlayer charges are further classified into dioctahedral (pyrophyllite clay) and trioctahedral (talc). The currently most applicable clay category is the TOT interlayer charge group. This TOT group is further divided into expandable clays and non-expandable clays [16]. TOT-expandable layers with an interlayer charge of <0.6 belong to the so-called smectite group, containing dioctahedral clays, such as beieldlite, montmorillonite, and nontronite, and trioctahedral clays, such as hectorite, saponite, and sauconite. On the other hand, TOT-expandable clays with an interlayer charge of >0.6 are referred to as vermiculite clay. Now the TOT interlayer charge group clays with non-expandable layers are classified into three groups, with respect to their layer charge. The expandable TOT group with an interlayer charge of <0.5-0.75 is called the dioctahedral illite clay. Meanwhile, the expandable TOT group with an interlayer charge of 1 is called true mica, which is further classified into muscovite and paragonite and biologic and phlogopite on the basis of them being dioctahedral and trioctahedral. Finally, the third group of the expandable TOT group with an interlayer charge of 2 is called brittle mica, which is further classified into margarine and clintonite based on being dioctahedral and trioctahedral. The above-mentioned TOT groups of clays have a wide application due to their higher CEC and surface area properties.  
The fourth category is 2:1:1, i.e., TOT-O-TOT [16; 18]. Chlorite is the only clay belonging to this structural group. Finally, the fifth class of clays is referred to as inverted ribbons; it contains attapulgite and sepiolite clays. Based on the merits of the clays, they may now be suitable for application. Some clays are commonly used for landfills and the storage of toxic waste.