Hercynite is a spinel mineral with the formula FeAl2O4.

Hercynite
General
CategoryOxide minerals
Spinel group
Spinel structural group
Formula
(repeating unit)
Fe2+Al2O4
IMA symbolHc[1]
Strunz classification4.BB.05
Crystal systemIsometric
Crystal classHexoctahedral (m3m)
H-M symbol: (4/m 3 2/m)
Space groupFd3m (no. 227)
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[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]

References

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  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ a b c Handbook of Mineralogy
  3. ^ a b Webmineral
  4. ^ Mindat
  5. ^ 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. 60 (1). Institute of Nuclear Chemistry and Technology: 47–49. doi:10.1515/nuka-2015-0012.
  6. ^ Agca, Can; Neuefeind, Jörg C.; McMurray, Jake W.; Weber, Richard; Navrotsky, Alexandra (2020-06-07). "Melting temperature measurement of refractory oxide ceramics as a function of oxygen fugacity using containerless methods". Journal of the American Ceramic Society. 103 (9): 4867–4875. doi:10.1111/jace.17216. ISSN 0002-7820. OSTI 1648884. S2CID 219429990.