Kaolinization

Kaolinization is a process of chemical weathering and hydrothermal alteration in which feldspar-rich rocks and other aluminosilicates are transformed into kaolinite, a 1:1 layer silicate mineral. In this transformation, alkalis and alkaline earth elements are removed and silica may be dissolved and/or re-precipitated as quartz. The result is a clay-rich alteration zone commonly referred to as a kaolinized rock or a kaolin deposit.

The process typically occurs under near-surface conditions with acidic, well-drained waters, particularly in tropical and subtropical climates where prolonged leaching intensifies weathering. The degree of kaolinization reflects the intensity and duration of alteration and is commonly observed in soils and saprolites as kaolinite-rich horizons. In some settings, hydrothermal fluids associated with granitic or volcanic rocks can also drive kaolinization, producing kaolinite along fractures and contact zones. The mineralogy of the alteration assemblage can include kaolinite, halloysite, and, at varying conditions, dickite, with quartz or illitic clays often present as companions.

Significance and applications: kaolinite clays are economically important for ceramics, paper, coatings, and various industrial uses. From a geological perspective, the extent and texture of kaolinization help characterize weathering intensity, soil formation processes, and alteration histories of granitoid and volcanic terranes. Analytical methods such as X-ray diffraction are used to identify kaolinite-group minerals and to assess the degree of alteration.