Hercynite

Hercynite
General
Category	Oxide minerals Spinel group Spinel structural group
Formula (repeating unit)	Fe^2+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]

Hercynite is a spinel mineral with the formula FeAl₂O₄.

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

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. Handbook of Mineralogy
3. 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.