The kaolin group minerals kaolinite, halloysite, dickite and nacrite have the same chemical composition, Si2Al2O5(OH)4, but different crystal structures. Supergene alteration is responsible for the formation of many economically operating kaolin deposits worldwide. Most supergene kaolin is formed by the incongruent dissolution of alkali feldspar in the presence of slightly acidic solutions; a combination of geochemical (chemical leaching) and biochemical (microbial nitrification and acidification) processes work together in almost all situations. The most important deposits have formed by complex multi-stage processes including (i) in situ weathering of granitic rocks under temperate to tropical climate conditions that produce deep soil profiles and lateritic regoliths containing residual kaolin, (ii) transport and deposition of sedimentary rocks, with synsedimentary and early- to late-diagenetic alteration of kaolin, and (iii) mixed-type kaolinization, including redeposition of kaolin and/or geologically recent weathering under temperate to tropical climatic conditions. Type (iii) deposits are the richest in kaolin group minerals. The largest and highest quality kaolin deposits known in the world have formed by supergene alteration of sedimentary rocks deposited on granite.