Authigenic kaolinite in sandstone occurs most commonly as fragile aggregates of crystals between sand grains, in vugs, along fractures, and as grains. It may form by crystallization from solution, by alteration of a parent mineral in place, and by recrystallization of fine-grained detrital kaolinite clay. Occurrences are in Ordovician to Miocene sandstones of various types and environments; these are tabulated by age for localities mostly in North America. Garnet, staurolite, feldspar, and sometimes quartz are unstable under kaolinite-forming conditions. As formation of kaolinite is favored by fresh or brackish ground water moving through a sandstone, its occurrence in a reservoir can be predicted from salinity maps.