HERCYNITE

    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 from the Laacher See volcanic complex, Germany © Stephan Wolfsried
Hercynite from Kaluganga valley, Ratnapura, Sri Lanka © Carles Curto Milà
Hercynite from the Laacher See volcanic complex, Germany © Michael Förch
Twinned hercynite from the Laacher See volcanic complex, Germany © Stephan Wolfsried

Hercynite in the World

It is a relatively common spinel, abundant especially in the emery deposits of Peekskill (New York) and Whittles (Virginia). Vangraindrano (Madagascar) probably provided the largest spinel crystals. Beautiful crystals have been discovered in Erode, near Coimbatore in India. Significant hercynite placers exist in Tasmania at Moorina (Australia). Hercynite is also common in the Swiss metamorphic gabbros of Le Prese, associated with corundum and sillimanite.

Hercynite in France

In France, hercynite is present in small octahedrons in the peridotites of the Lherz pond (Ariège) as well as in those of Puy-en-Velay (Haute-Loire). It is also found in numerous alluviums and in Corsican serpentinites (Calvi, Corte, Canari).

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

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