Fibroferrite crystal aggregates from Le Cetine mine (Siena province, Tuscany, Italy). Photo by Massimiliano Marcelli — Photo: Etrusko25 · CC BY-SA 3.0
Fibroferrite
Fibroferrite is an IMA-recognized mineral species in the sulfate class, with the chemical formula Fe³⁺(SO₄)(OH)·5H₂O.
It crystallizes in the hexagonal crystal system (space group R-3).
Key properties: Mohs hardness 2.5; specific gravity 1.84–2.09; Pearly, silky in aggregates luster; optically biaxial.
IMA symbol Ffr, grandfathered status — known before the IMA’s founding, named in 1833.
An uncommon dehydration product of melanterite in oxidized portions of rocks and sediments containing iron sulfide, most stable in arid regions; rarely a volcanic sublimate.
It is found associated with Melanterite, amarantite, hohmannite, krausite, coquimbite, parabutlerite, butlerite, copiapite, epsomite, gypsum, pyrite.
The name refers to the fibrous habit and essential ferric iron.