**2.1 Clay minerals structure**

Clay minerals belong to phyllosilicates family and are constituted by a stacking of sheets. **Figure 1** illustrates the terminology used to define the layer silicates structure [19]. So, it can be established:


Every layer is constituted of two fundamental units:

**The tetrahedral sheet**, designed as (T), whose basic building unit is a "silica tetrahedron." In each unit, one silicon atom is surrounded by four oxygens. The resulting silica tetrahedra units are connected horizontally by sharing oxygens anions, *via* covalent bonding, to produce a sheet of Si2O6 (OH)4 structure.

**The octahedral sheet**, designed as (O), is built by the horizontal association of numerous octahedra, by covalent bonding. Each unit is composed of Al or Mg ions (and occasionally Fe ion) surrounded by six oxygen atoms or hydroxyl groups. This arrangement allows an eight-sided configuration labeled "octahedron." When the (O) is dominated by aluminum, two Al3+ cations are so needed to maintain the electrical neutrality and the sheet is so designated as "dioctahedral sheet." When magnesium predominates the (O), three Mg2+ cations are required, and the sheet is termed as "trioctahedral sheet" [15, 20–22].

The multiple associations between (T) and (O) and the numerous chemical substitutions induce different clay minerals with diverse physicochemical characteristics [16]. Consequently, three clay minerals categories are discerned:


**Figure 1.** *A schematic illustration of the general structure of phyllosilicates.*

• The space between two successive 1:1 and 2:1 layers is the "interlayer," which is empty if the layers are electrostatically neutral. If the layer takes an overabundance charge, defined as "layer charge," so it is neutralized by diverse interlayer elements, such as cations (Ca, Na, Mg, K), hydrated cations, and hydroxide octahedral groups. The hydroxide interlayer often forms an additional octahedral sheet yielding a 2:1:1 or TOTO layer. This structure is typical of chlorite, chamosite, and donbassite [25].

Clay minerals can be identified according to the assembly method and their shape. It is distinguished by the following:

