The majority of observed photogeochemical reactions involve a mineral catalyst. Many naturally occurring minerals are semiconductors that absorb some portion of solar radiation. These semiconducting minerals are frequently transition metal oxides and sulfides and include abundant, well-known minerals such as hematite (FeO), magnetite (FeO), goethite and lepidocrocite (FeOOH), and pyrolusite (MnO). Radiation of energy equal to or greater than the band gap of a semiconductor is sufficient to excite an electron from the valence band to a higher energy level in the conduction band, leaving behind an electron hole (h); the resulting electron-hole pair is called an exciton. The excited electron and hole can reduce and oxidize, respectively, species having suitable redox potentials relative to the potentials of the valence and conduction bands. Semiconducting minerals with appropriate band gaps and appropriate band energy levels can catalyze a vast array of reactions, most commonly at mineral-water or mineral-gas interfaces.