1. n. 
A mineral containing ferrous sulfide, FeS, that typically contains inclusions of free sulfur and other minerals. It is commonly present in shales, and may occur as a trace mineral in some barite ores. Pyrrhotite can liberate sulfides in alkaline muds, with adverse consequences for safety and corrosion.
Reference: Binder GG, Carlton LA and Garrett RL: "Evaluating Barite as a Source of Soluble Carbonate and Sulfide Contamination in Drilling Fluids," Journal of Petroleum Technology 33, no. 12 (December 1981): 2371-2376.