text: Sulfide minerals such as pyrite (FeS) and pyrrhotite (FeS) occur in many ore deposits. Pyrite and its polymorph marcasite also are important in many coal deposits and shales. These sulfide minerals form in environments more reducing than that of the Earth's surface. When in contact with oxidizing surface waters, sulfides react: sulfate (SO) forms, and the water becomes acidic and charged with a variety of elements, some potentially toxic. Consequences can be environmentally harmful, as discussed in the entry for acid mine drainage.
  Sulfur oxidation to sulfate or sulfur dioxide also is important in generating sulfur-rich volcanic eruptions, like those of Pinatubo in 1991 and El Chichon in 1982. These eruptions contributed unusually large quantities of sulfur dioxide to the Earth's atmosphere, with consequent effects on atmospheric quality and on climate. The magmas were unusually oxidizing, almost two log units more so than the NiNiO buffer. The calcium sulfate, anhydrite, was present as phenocrysts in the erupted tephra. In contrast, sulfides contain most of the sulfur in magmas more reducing than the FMQ buffer.