Iron disulfide is found in nature in two different crystal forms, as the minerals iron pyrite and marcasite. The mineral pyrite is mostly used for the production of iron and sulfur. Iron disulfide also is used to produce sulfuric acid, although the latter is commercially made by other processes that are more economical.
The natural pyrite consists of yellow cubic crystals; density 5.02g/cm3; hardness 6.3 Mohs; melts at 1,171°C; soluble in dilute acids.
The mineral marcasite constitutes yellow rhombic crystals; density 4.87g/cm3; transforms to more stable pyrite form when heated at 480°C; insoluble in dilute acids. Both forms dissolve in concentrated nitric acid and are insoluble in water (4.9 mg/L at 20°C).
Both forms of iron disulfide are very stable at ordinary temperatures and also inert towards most chemicals. Heating at elevated temperatures gives iron(III) oxide and sulfur dioxide. This process for producing sulfur dioxide is also applied to manufacture sulfuric acid:
4FeS2 + 11O2 → 2Fe2O3 + 8SO2
When the disulfide is heated at 600°C in a vacuum, it decomposes to iron(II) sulfide and sulfur:
FeS2 → FeS + S
black cub powder(s) [LID94] [STR93]
Iron disulfide is obtained from its naturally occurring minerals, pyrite and marcasite. In the laboratory it may be prepared along with iron(II) sulfide by passing dry hydrogen sulfide through a suspension of hydrated iron(III) oxide or iron(III) hydroxide in alkaline medium. The unstable product formed decomposes to FeS2 and FeS.
