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Category Oxide minerals
Spinel group
Spinel structural group
(repeating unit)
Strunz classification 4.BB.05
Crystal system Isometric
Crystal class Hexoctahedral (m3m)
H-M symbol: (4/m 3 2/m)
Space group Fd3m
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 [1][2][3]

Hercynite is a spinel mineral with the formula FeAl2O4.

It occurs in high-grade metamorphosed iron rich argillaceous sediments as well as in mafic and ultramafic igneous rocks. Due to its hardness it also is found in placers.[1]

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.[1][2]

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, however some Fe3+ may be located in the structure of hercynite.[4]


  1. ^ a b c Handbook of Mineralogy
  2. ^ a b Webmineral
  3. ^ Mindat
  4. ^ Jastrzębska, Ilona; Szczerba J.; Stoch P.; Błachowski A.; Ruebenbauer K.; Prorok R.; Snieżek E. (2015). "Crystal structure and Mössbauer study of FeAl2O4". Department of Ceramics and Refractories. Nukleonika-Journal of Nuclear Research. Institute of Nuclear Chemistry and Technology (60(1)): 47–49. doi:10.1515/nuka-2015-0012.