Hercynite (Image width: 5 mm) Locality: In den Dellen quarries, Niedermendig, Mendig, Laach lake volcanic complex, Eifel, Rhineland-Palatinate, Germany — Photo: Fred Kruijen · CC BY-SA 3.0 nl
Hercynite
Iron aluminate — the iron spinel of high-grade metamorphic and mafic igneous rocks.
Intro adapted from Wikipedia (CC BY-SA).
Hercynite is an IMA-recognized mineral species in the oxides & hydroxide class, with the chemical formula Fe²⁺Al₂O₄.
It crystallizes in the cubic crystal system (space group Fd3m). It is a member of the Spinel structural group.
Key properties: Mohs hardness 7.5–8; specific gravity 4.4; Dark grayish green to dark green streak; Vitreous luster; refractive index 1.8; optically isotropic.
IMA symbol Hc, grandfathered status — known before the IMA’s founding, named in 1839.
Found in high-grade metamorphosed ferruginous argillaceous sediments and in some mafic and ultramafic igneous rocks. Also in placers.
It is found associated with Magnetite, corundum, ilmenite, sillimanite, andalusite.
The name comes from the Latin, Silva Hercynia, for the Bohemian Forest, Czech Republic, where the mineral was first found.