Hercynite - Encyclopedia
Class : Oxides and hydroxides
Subclass : Oxides
Crystal system : Cubic
Chemistry : FeAl2O4
Rarity : Quite common
Hercynite is an oxide from the aluminous spinel group. It is a so-called “normal” spinel, the bivalent ions being all in tetrahedral sites. It forms three continuous series : one with the chromiferous term, chromite, another with the zinciferous term, gahnite, and a third with the magnesium term, spinel in the strict sense. Hercynite is a spinel that is found in iron-bearing aluminous rocks having undergone intense metamorphism, in certain basic and ultrabasic rocks, and in placers, due to its great resistance to erosion. It owes its name to its locality of discovery, the Latin name of the Bohemian Forest (De Silva Hercynia), place where the mineral was discovered. Hercynite occurs in opaque octahedral crystals or in masses, very dark green to black.
Main photo : Hercynite from In Den Dellen, Mendig, Germany © Volker Betz
Hercynite in the World
Twinning
The spinel twin is common on {111}.
Fakes and treatments
No fakes recorded for this mineral species.
Hardness : 7.5
Density : 4.39
Fracture : Conchoidal
Streak : Grey-green
TP : Translucent to opaque
RI : 1,800 to 1,830
Birefringence : 0
Optical character : None
Pleochroism : None
Fluorescence : None
Solubility : Sulfuric acid
Magnetism : ParamagneticRadioactivity : None