Mineral · Pyrite group
Pyrite
FeS2
Pyrite is iron sulfide, 'fool's gold', crystallising as brassy cubes (Navajún, Spain), pyritohedra (Peru), octahedra and the radiating 'pyrite suns' of Illinois coal mines. It is cheap and abundant and also the mineral that destroys fossil collections through pyrite decay in humid air. Brass colour, a greenish-black streak and hardness 6 (gold is soft) identify it; marcasite is the orthorhombic polymorph.
| Formula | FeS2 |
|---|---|
| IMA status | grandfathered |
| Crystal system | Cubic |
| Mohs hardness | 6–6.5 |
| Group | Pyrite group |
| Type locality | None (known since antiquity) |
| Streak | Greenish black |
| Specific gravity | 5.0 |
| Reference list | IMA List of Minerals (RRUFF) — verify formula and status against the current list |
Fakes and treatments
Navajún cubes are sometimes glued back into matrix after cleaning; 'pyrite' cubes cast in metal exist. Unstable pyrite must be kept below 45 % humidity.
Handling and safety
Decays in damp storage, producing sulphuric acid; see the Study article on pyrite decay.
Elsewhere
Cite this article
Stratum Study. "Pyrite" (Mineral). Stratum Study pack 2026.09, as of 2026-09-19. https://stratum.black/study/minerals/pyrite. Accessed 2026-09-20.
@misc{stratum-pyrite,
title = {Pyrite},
author = {{Stratum Study}},
howpublished = {\url{https://stratum.black/study/minerals/pyrite}},
year = {2026},
note = {Mineral article, Stratum Study pack 2026.09, as of 2026-09-19; sources: stratum, wikidata, commons. Accessed 2026-09-20.}
} Citations carry the pack version and the article's as-of date so they resolve to the same text later. The facts themselves belong to the sources listed below; cite those for the science.