Iron sulfide is the chemical compound FeS, a black solid. It is made of iron and sulfide ions. FeS has iron in its +2 oxidation state. It reacts with acids such as hydrochloric acid to make hydrogen sulfide gas.
In plural, "iron sulfides" may refer to a range of chemical compounds composed of iron and sulfur. For example to FeS2 (iron disulfide) found in the ground as the mineral pyrite. FeS2 is shiny like a metal and has a bright golden color. That's why it is sometimes called "fool's gold".
By increasing order of stability:
- Iron(II) sulfide, FeS, the less stable amorphous form;
- Greigite, a form of iron(II,III) sulfide (Fe3S4), analog to magnetite, Fe3O4;
- Pyrrhotite, Fe1−xS (where x = 0 to 0.2), or Fe7S8;
- Troilite, FeS, the endmember of pyrrhotite;
- Mackinawite, Fe1+xS (where x = 0 to 0.1);
- Marcasite, or iron(II) disulfide, FeS2 (orthorhombic);
- Pyrite, or iron(II) disulfide, FeS2 (cubic), the more stable endmember, known as fool's gold.