Kaolin is a form of clay. Clays are formed by the weathering of rocks. Weathering of rocks can be classified as physical or chemical process. In case of kaolin, the chemical reaction between water, carbon dioxide and feldspar (rock) gives rise to kaolin clay deposits.
Feldspar (Rock) + Water + Carbon Dioxide (Gas) -> Kaolin (Clay)
The kaolin deposits are classified as primary when the chemical reaction takes place in-situ. The deposits are classified as secondary when the reaction has already taken place to form kaolin; after which the kaolin is washed away and deposited elsewhere