Formation of Clay Minerals in Nature

In any geological environment the clay minerals may come from the following three sources (Millot, 1970 Eberl, 1984). 

The genesis, transformation and destruction of clay minerals occur in a cyclic process in nature, as shown in Fig. 1.5. Clay minerals in any natiual setting are inherited mostly from fine grained sedimentary rocks and less commonly from coarser sedimentary rocks and low grade metamorphic rocks  

When the unconsolidated, plastic clays are deposited in a sedimentary basin as pelitic sediments, they are transformed to rigid, brittle pelitic rocks by diagenesis. Claystones and mudstones are non-laminated pelitic rocks, and shales are laminated pelitic rocks. During diagenesis, some new clay minerals may appear through transformation of pre-existing clay minerals and neoformation of some non-clay silicate minerals. 

At high pressme and temperature, pelitic rocks are metamorphosed to form metapelites. Slate, phyllite and schist are produced successively as the grade of metamorphism increases. Clay minerals are not stable in the high pressure and temperature condition of metamorphism and are gradually transformed to other minerals. 

Fmlher increase of temperature melts the rocks to produce magma, which upon cooling and crystallization produce the igneous rocks. The rocks are weathered to form elay again. 

---