| Hercynite | |
|---|---|
 | |
| General | |
| Category | Oxide minerals Spinel group Spinel structural group |
| Formula (repeating unit) | 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] |
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]