Iron(III) sulfide, also known as ferric sulfide or sesquisulfide (), is one of the several binary iron sulfides. It is a solid, black powder that degrades at ambient temperature.
precipitates from solutions containing its respective ions:
The resulting solid decays at a temperature over 20 ðC into iron(II) sulfide (FeS) and elemental sulfur:
With hydrochloric acid it decays according to the following reaction equation:
Greigite, with the chemical formula , is a mixed valence compound containing both Fe(III) and Fe(II). It is the sulfur equivalent of the iron oxide magnetite (Fe<sub>3</sub>O<sub>4</sub>). As established by X-ray crystallography, the S anions form a cubic close-packed lattice, and the Fe cations occupy both tetrahedral and octahedral sites.