Hercynite

Hercynite is a spinel mineral with the formula FeAl₂O₄.

Hercynite
General
Category	Oxide minerals Spinel group Spinel structural group
Formula	Fe2+Al2O4
IMA symbol	Hc[1]
Strunz classification	4.BB.05
Crystal system	Isometric
Crystal class	Hexoctahedral (m3m) H-M symbol: (4/m 3 2/m)
Space group	Fd3m (no. 227)
Identification
Formula mass	173.81 g/mol
Color	Black
Crystal habit	Euhedral crystals. Also massive to granular
Cleavage	[111] indistinct
Fracture	Uneven – flat surfaces (not cleavage) fractured in an uneven pattern.
Mohs scale hardness	7.5
Luster	Vitreous (glassy)
Streak	dark green
Specific gravity	3.95
Optical properties	Isotropic
Refractive index	n = 1.8
Other characteristics	non-radioactive
References	[2][3][4]

It occurs in high-grade metamorphosed iron-rich argillaceous (clay-containing) sediments as well as in mafic and ultramafic igneous rocks. Due to its hardness it also is found in placers.^[2]

It was first described in 1847 and its name originates from the Latin name for the Harz, Silva Hercynia, where the species was first found.^[2][3]

Hercynite is a spinel of regular symmetry and normal cation distribution, but some disorder occurs in its structure. It consists of ferrous (Fe²⁺) ions and aluminium ions (Al³⁺); however some ferric ions (Fe³⁺) may be located in the structure of hercynite.^[5]

Melting point of this mineral is inbetween 1,692–1,767 °C (3,078–3,213 °F).^[6]