1. n. 
A group of compounds containing iron in +2 (ferrous) and +3 (ferric) valence states and sulfur in –2 (sulfide) valence states. Examples are ferric sulfide [Fe2S3], ferrous sulfide [FeS], and iron disulfide [FeS2], which is found in the minerals marcasite and pyrite. Fe2S3 and FeS2 are insoluble, but FeS dissolves in alkaline muds to form a gelatinous solid, Fe(OH)2, and soluble S–2 ions. FeS is the primary component in the mineral pyrrhotite, one of many possible sources of sulfides in water mud. Pyrrhotite might also coexist with BaSO4 in some barite ores. Even in trace amounts, FeS in a barite supply could generate enough sulfides in a weighted mud system to cause stress-corrosion cracking of drillpipe.