Iron(II) sulfide

Iron(II) sulfide is a chemical compound with the formula FeS. In practice, iron sulfides are often non-stoichiometric. Powdered iron sulfide is pyrophoric.

FeS can be obtained by reacting iron and sulfur.
 * S8 (s) + 8Fe(s) → 8FeS(s)

Forms of iron sulfide
"Iron sulfide" exists in several distinct forms which differ in the stoichiometry and properties:
 * Pyrrhotite, Fe1-xS, a  mineral as well as the waste product of the Desulfovibrio bacteria, pyrrhotite displays ferrimagnetism.
 * Troilite, FeS, which has similar material to pyrrhotite, but does not possess ferromagnetic properties.
 * Mackinawite, Fe1+xS, the least stable from of iron sulfide, mackinawite has a layered structure.
 * Pyrite, mineral form of FeS2 with a gold-like appearance, making it also known as "fool's gold."

Pedagogical uses
Certain chemical principles are nicely illustrated using the chemistry of iron sulfides. The methods of chemical composition can be shown by the production of iron(II) sulfide from iron and sulfur (see equation above).
 * This reaction is exothermic as more energy is given out than taken in.
 * Iron shows ferromagnetism; iron sulfides do not.
 * Iron sulfide with hydrochloric acid (HCl), the acidic properties of hydrochloric acid and the pungent fume of hydrogen sulfide can both be demonstrated:
 * FeS(s) + 2HCl(aq) → FeCl2 (aq) + H2S(g)

Biological and biochemical uses
The presence of ferrous sulfide as a visible black precipitate in the growth medium peptone iron agar can be used to distinguish between microorganisms that produce the cysteine metabolizing enzyme cysteine desulfhydrase and those that do not. Peptone iron agar contains the amino acid cysteine and a chemical indicator, ferric citrate. The catalysis of cysteine releases hydrogen sulfide gas that reacts with the ferric citrate to produce ferrous sulfide.