Mineral · Marcasite group
Marcasite
FeS2
Marcasite is the orthorhombic polymorph of pyrite, forming cockscomb and spearhead twins and the nodules of the English Chalk and the Vinton, Iowa, Cretaceous. It is paler and more unstable than pyrite and decays even faster in humid air; many 'pyrite' fossil replacements are actually marcasite. Jewellery 'marcasite' is in fact cut pyrite.
| Formula | FeS2 |
|---|---|
| IMA status | grandfathered |
| Crystal system | Orthorhombic |
| Mohs hardness | 6–6.5 |
| Group | Marcasite group |
| Type locality | None |
| Streak | Greyish black |
| Specific gravity | 4.9 |
| Reference list | IMA List of Minerals (RRUFF) — verify formula and status against the current list |
Elsewhere
Cite this article
Stratum Strata. "Marcasite" (Mineral). Stratum Strata pack 2026.09, as of 2026-09-21. https://stratum.black/strata/minerals/marcasite. Accessed 2026-09-21.
@misc{stratum-marcasite,
title = {Marcasite},
author = {{Stratum Strata}},
howpublished = {\url{https://stratum.black/strata/minerals/marcasite}},
year = {2026},
note = {Mineral article, Stratum Strata pack 2026.09, as of 2026-09-21; sources: stratum, wikidata, commons. Accessed 2026-09-21.}
} 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.