Hercynite: Properties and Occurrences

Hercynite is a spinel mineral with the formula FeAl2O4. It is an isometric-hexoctahedral black mineral containing aluminum, iron, and oxygen.It forms dark blue-green, yellow, brown crystals with vitreous luster. It is named after Silva Hercynia, the Latin name for the Bohemian Forest, where the mineral was discovered.

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.

General Information

  • Category: Oxide minerals (Spinel group)
  • Formula: FeAl2O4
  • Crystal system: Isometric
  • Crystal class: Hexoctahedral (m3m)

Fig: Hercynite is a spinel mineral

Properties

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

  • Formula mass: 173.81 g/mol
  • Color: Black
  • Crystal habit: Euhedral crystals. Also massive to granular
  • Mohs scale hardness: 7.5
  • Luster: Vitreous (glassy)
  • Streak: dark green
  • Specific gravity: 3.95
  • Optical properties: Isotropic

Occurrence: In high-grade metamorphosed ferruginous argillaceous sediments and in some mafic and ultramafic igneous rocks. Also in placers.

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.

Association: Magnetite, corundum, ilmenite, sillimanite, andalusite.

 

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