*2.2.1 Kaolin minerals*

**Kaolinite** (Kaol) is the main element of this class. Its structure results from a combination of one tetrahedral sheet and one dioctahedral sheet. The basal spacing is about 7.1 Å [20]. It is differentiated by the presence of strong bonds between the layers, which opposes to its expansion and swelling [27]. Kaol is also characterized by negligible cation exchange capacity (CEC) and small surface area [28].

**Halloysite clay minerals**, with an empirical formula Al2Si2O5(OH)4nH2O, belong to the Kaolin class, with the same chemical structure as kaolinite [29]. Based on crystalline and geographical conditions, multiple morphologies are defined. The most abundant one in nature is the tubular form resulting from the rolling of the kaolin sheets with the presence of a water molecules layer in the interlayer space [30, 31].

The interlayer water in halloysite nanotubes (HNTs) is one of the principal properties discerning HNTs from Kaol. The dehydration action causes irreversibly the change of the d001 spacing from 10 to 7 Å [32].

HNTs are composed of two kinds of hydroxyl groups according to their position. The inner hydroxyl groups are located between layers and the outer hydroxyl groups that are located on the surface of the HNTs. The most ones are inner groups due to the tubular form [33]. HNTs diameter is of 40–70 nm with inner lumen diameter of 10–15 nm and length of 1000–2000 nm [34].

HNTs are characterized by a relatively high specific surface area and a total pore volume that is much higher than that of platy Kaol. This should be attributed to the rich pores in the structure of halloysite [30].
