**2. Structure, mineralogical and physicochemical composition of clay**

The structure of the clay consists of phyllosilicate sheets that are arranged properly to form structural layers. Individual layer is made by a stack of tetrahedral and octahedral sheets that shape the frame of all the clay mineral [8, 9]. The tetrahedral (T) sheet consist of cations coordinated to four oxygen atoms and linked to adjacent tetrahedral by sharing three corners to form two dimensional hexagonal mesh [10]. The most common tetrahedral cations are Si4+, Al3+ and Fe3+.

The second sheet is called octahedral sheet (O), which is comprised of six oxygen atoms which are closely parked together and hydroxyl ions in which cations are arranged to form octahedral coordination and linked to neighboring octahedral by sharing edge. The edge of shared octahedral forms sheet of hexagonal or pseudo hexagonal symmetry and shows different topologies depending on octahedral hydroxyl position [8]. Cations in octahedral sheet are usually Al3+, Fe3+, Mg2+ or Fe2+. When cation with positive valence of three (Al3+ or Fe3+) is present in the octahedral sheet, only two-thirds of the possible positions are filled in order to balance the charges and the mineral is therefore termed dioctahedral. Conversely, when cation with positive charge of two (e.g. Mg2+ and Fe2+) is present, all three positions are filled to balance the structure and the mineral is termed trioctahedral. The phyllosilicate sheets are joined together by sharing the apical oxygen atom or hydroxyls to form hexagonal network with each sheet in a fundamental structure. **Figure 1** depicts structures of octahedron sheets and tetrahedron sheets proposed by Grim [11]. Based on the number and ratio of the sheets in the fundamental structural units, the existing cations substitutions in the octahedrons and tetrahedrons caused for resulting charge of the layers which can be descended into two main groups of clay mineral namely 1:1 (kaoline) and 2:1 (smectite and illite) [12, 13].

the octahedral sheet. With the layer charges close to zero, the kaolin mineral has essentially no interlayers and does not show interlayer expansion in the water because the contiguous layers within particles are strongly held together by Al-OH and O-Si-OH bonding supplemented by dipole–dipole and van der Waal interaction. **Figure 2** represents the typical structure of

**Figure 1.** Schematic diagrams of octahedron and tetrahedron sheets. Top: (A) an alumina octahedron in which the central aluminum ion is coordinated to six hydroxyls; (B) an alumina octahedral sheet formed by linking octahedra through edge-sharing. Bottom: (A) a silica tetrahedron in which the central silicon ion is coordinated to four oxygens;

Mineralogical and Chemical Characteristics of Raw and Modified Clays and Their Application…

http://dx.doi.org/10.5772/intechopen.74474

47

(B) a tetrahedral sheet formed by linking silica tetrahedra through corner-sharing [11].

Clay mineral groups such as smectite and vermiculite are part of 2:1 type and it constitutes minerals such as montmorillonite, saponite, nontronite and beidellite. The 2:1 minerals are composed of one octahedral sheet between two tetrahedral sheets. For which the interlayer thickness is 1 nm when the sheet is closed [16]. Generally in this group of clay minerals

kaoline mineral [15].

**Figure 2.** Typical structure of Kaoline mineral [15].

**2.2. Type 2:1**
