Fig. 2.2 Basic layer of mineral formation (modified from Mitchell and Soga 2005)

Gibbsite layer is otherwise termed as dioctahedral structure in which two-third of central portion is occupied by Al⁺³. Similarly, brucite layer is termed as trioctahedral structure in which entire central portion is occupied by Mg⁺². These basic layers stack together to form basic clay mineral structure. Accordingly, there is two and three layer configuration as indicated in Fig. 2.3. More than hundreds of these fundamental layer join together to form a single clay mineral.

Fig. 2.3 Fundamental layers of clay minerals (modified from Mitchell and Soga 2005)

Description on common clay minerals
Some of the important and common clay minerals are described below in Table 2.1.

Table 2.1 Summary of important clay minerals

Mineral	Origin	Symbol	Bond	Shape	Remark
Kaolinite	Orthoclase Feldspar (Granite rocks)	d	Strong hydrogen bond	Flaky and platy	Approximately 100 layers in a regular structure d =7.2A⁰
Halloysite (Kaolinite group)	Feldspar Tropical soil	H₂O	Less strong bond	Tubular or rod like structure	At 60⁰C it looses water and alter soil properties
Illite	Degradation of mica under marine condition Feldspar		K⁺ provides bond between adjacent layers	Thin and small flaky material	Bond is weaker than kaolinite d =10A⁰ High stability
Montmorillonite (Smectite group)	Weathering of plagioclase		H₂O molecules pushes apart mineral structure causing swelling Presence of cations	Very small platy or flaky particle	Exhibits high shrinkage and swelling Weak bond d >10A⁰
Vermiculite	Weathering of biotite and chlorite		Presence of H₂O and Mg⁺² predominantly Mg⁺²	Platy or flaky particle	Shrinkage and swelling less than montmorillonite

Kaolinite formation is favoured when there is abundance of alumina and silica is scarce. The favourable condition for kaolinite formation is low electrolyte content, low pH and removal of ions that flocculate silica (such as Mg, Ca and Fe) by leaching. Therefore, there is higher probability of kaolinite formation in those regions with heavy rainfall that facilitate leaching of above cations. Similarly halloysite is formed due to the leaching of feldspar by H₂SO₄ produce by the oxidation of pyrite. Halloysite formations mostly occur in high-rain volcanic areas. Smectite group of mineral formation are favoured by high silica availability, high pH, high electrolyte content, presence of more Mg⁺² and Ca⁺² than Na⁺ and K⁺. The formation is supported by less rainfall and leaching, and where evaporation is high (such as in arid regions). For illite formation, potassium is essential in addition to the favourable conditions of smectite.