Outstanding crystals and crystal aggregates of the very rare mineral ferroselite (the selenium analog of marcasite) from a little known locality in Argentina characterized by its selenides: El Quemado, General Lamadrid Department, La Rioja, Argentina. — Photo: David Hospital · CC BY-SA 4.0
Ferroselite
The orthorhombic form of iron diselenide, precipitated under anoxic reducing conditions — a selenium source in Cretaceous shales.
Intro adapted from Wikipedia (CC BY-SA).
Ferroselite is an IMA-recognized mineral species in the sulfide class, with the chemical formula FeSe₂.
It crystallizes in the orthorhombic crystal system (space group Pnnm). It is a member of the Marcasite structural group.
Key properties: Mohs hardness 6–6.5; specific gravity 7.2; Black streak; Metallic luster.
IMA symbol Fse, grandfathered status — known before the IMA’s founding, named in 1955.
Found in red-bed deposits of the Colorado Plateau type, peripheral to pyrite and uranium concentrations (Utah, USA); in U–V ores in sandstone, with coalified wood (Colorado, USA).
It is found associated with Uraninite, pyrite, marcasite, chalcopyrite, sphalerite, selenium, clausthalite, cadmoselite, bornite, cobaltomenite, laumontite, barite.
The name refers to iron, FERrum, and SELenium in the composition.