# A Review of the Evolution Characteristics and Argillization of Clay Interbeds in Rockslides
## 3 Parameter Changes during the Argillization Process
### Clay Minerals in the Process of Argillization
#### 3.5.2 Changes in Clay Minerals during the Argillization Process  
Clay minerals are essential components in the argillization process. However, scholars are still studying whether there are necessary requirements for the types and quantities of minerals that are needed. Based on the alteration zones at different depths, [PERSON] [47] suggested that the argillization process consists of three successive clay transformation processes. They are illitization, montmorillonite, and kaolinitization (Figure 8). By comparing the clay mineral compositions of the mud-sandstone in the sliding body to those in the argillization zone, it was observed that the content of chlorite and illite increased gradually. The montmorillonite content remained unchanged, and the main reactions vary slightly depending on the pH values of the external aqueous solutions. When the acidity was high, it would mainly result in dissolution. Otherwise, it would mainly involve hydration, hydrolysis, and ion exchange. Overall, the higher the acidity level, the more intense the reaction will be. But in all cases, the reaction will reach a state of dynamic equilibrium after a certain period of time [13]. Figure 7: Transformation of clay minerals (modified from [PERSON], B.A., 1981 [43]). The reason that montmorillonite played a controlling role in argillization was first due to the fact that it disintegrated rapidly. Then, it would result in positive ion exchange and enrichment, which would influence the physical and chemical transformation of clay minerals [48], and the swelling of the montmorillonite crystal could be described through the thermodynamic relationship between the swelling pressure and suction [49]. The diffusion bilayer model was used to explain the osmotic swelling mechanism. It should be noted that the properties of liquids differ significantly between the surface of clay and the bulk phase. Moreover, it was determined that the macroscopic theories, such as DLVO, were not applicable to these unique microscopic layer-by-layer swelling phenomena. [PERSON] [50] argued that argillization only required a specific quantity of clay minerals rather than specific types. Based on sampling from various locations, the investigation of clay interbeds and adjacent soft rocks revealed the presence of clay minerals in both. At the same time, even if the sandstone or siltstone were broken by tectonism. The argillization phenomenon has not yet occurred due to a lack of sufficient clay minerals. In fact, the presence or absence of montmorillonite was not the decisive factor in mudstone argillization. Sufficient amounts of illite and kaolinite could also induce argillization [51]. Kaolinite can promote the argillization process. The electrostatic stress on the surface of kaolinite clay particles could generate a gravitational field. This attracts water molecules near to the surface of kaolinite clay particles and arranges them into a tight and orderly manner, producing a \"water absorption\" effect. Moreover, it could also enhance water-rock interaction and facilitate the formation of clay interbeds in fractured rock masses [52]. Illite can promote the formation of clay interbeds [53]. Clay minerals in the red strata of Sichuan Province are primarily non-swelling illite, which is distinct from the interlayer expansion mechanism observed in montmorillonite (Figure 9). It belongs to the intra-molecular swelling mechanism because the cohesive force between the cells was weakened after encountering water. Then, the water molecules were able to enter the cells, increasing the spacing between the cells and resulting in the expansion of the particles. The particles would then form the stress field due to electrostatic attraction. The stress field would lead to the macroscopic phenomenon of the \"water film effect on the surface of clay minerals in mudstone\", further promoting the argillization process. Different compositions and fabrics of clay minerals can result in varying degrees of argillization.