Iron(II,III) sulfide is a blue-black (sometimes pinkish) chemical compound of iron and sulfur with formula Fe<sub>3</sub>S<sub>4</sub> or FeS÷Fe<sub>2</sub>S<sub>3</sub>, which is much similar to iron(II,III) oxide. It occurs naturally as the sulfide mineral greigite and is magnetic. It is a bio-mineral produced by and found in magnetotactic bacteria. It is a mixed valence compound, featuring both Fe<sup>2+</sup> and Fe<sup>3+</sup> centers, in 1:2 ratio.
The crystallographic unit cell is cubic, with space group Fd3m. The S anions form a cubic close-packed lattice, and the Fe cations occupy both tetrahedral and octahedral sites.
Like the related oxide magnetite (Fe<sub>3</sub>O<sub>4</sub>), iron(II,III) sulfide is ferrimagnetic, with the spin magnetic moments of the centers in the tetrahedral sites oriented in the opposite direction as those in the octahedral sites, and a net magnetization. Both metal sites have high spin quantum numbers. The electronic structure of greigite is that of a half metal.