There are about a dozen major hydrogeochemical processes that can account for the chemical composition of most natural waters. One of these is the oxidation of pyrite, a process at least as important a source of sulfate in natural waters as seawater and sea spray, gypsum dissolution, and atmospheric emissions. The natural process of pyrite oxidation is fundamental to the super-gene alteration of ore deposits, the formation of acid-sulfate soils, and the development of acidity and metal mobilization in natural waters.