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Hercynite is a spinel mineral with the formula FeAl2O4.

Hercynite
Hercynite.jpg
General
CategoryOxide minerals
Spinel group
Spinel structural group
Formula
(repeating unit)
Fe2+Al2O4
Strunz classification4.BB.05
Crystal systemIsometric
Crystal classHexoctahedral (m3m)
H-M symbol: (4/m 3 2/m)
Space groupFd3m
Identification
Formula mass173.81 g/mol
ColorBlack
Crystal habitEuhedral crystals. Also massive to granular
Cleavage[111] indistinct
FractureUneven - flat surfaces (not cleavage) fractured in an uneven pattern.
Mohs scale hardness7.5
LusterVitreous (glassy)
Streakdark green
Specific gravity3.95
Optical propertiesIsotropic
Refractive indexn=1.8
Other characteristicsnon-radioactive
References[1][2][3]

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 (Al3+), however some ferric ions (Fe3+) may be located in the structure of hercynite.[4]

ReferencesEdit

  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.