Ardennite - Encyclopedia

    Class : Silicates
    Subclass : Sorosilicates
    Crystal System : Orthorhombic
    Chemistry : Mn4(Al,Mg)6(SiO4)2(Si3O10)[(As,V)O4](OH)6
    Rarity : Rare

Ardennite is a complex silicate of manganese and aluminum which also includes arsenate and vanadate groups, it is found in hydrothermal veins and pegmatites as well as in the oxidation zone of manganese deposits. It owes its name to its locality of discovery : Ottré, near Salmchâteau in the Belgian Ardennes. The crystals, prismatic to acicular lamellae are rare and often united in rosettes, in fibroradiated groups, measuring up to 4 cm, or in crystalline aggregates. Its color is usually orange-yellow to orange-brown, sometimes black-brown to black, with an adamantine luster.

Main photo : Ardennite of Vielsalm, Belgium - © Jan Vanheuverzwijn

Ardennite in the World

Ardennite forms delicate rosettes of crystals in the hydrothermal quartz vein cutting through the Ottré shales, near Salmchâteau (Belgium). But it is above all a mineral of oxidation zones in a manganiferous and aluminous context, in particular shales rich in Mn and Al, and sedimentary deposits of manganese (Jhabua, India), where it rubs shoulders with other manganiferous minerals (spessartite, pyrolusite, piemontite, etc...). Ardennite is known in particular in 7 mm crystals at Arrow Junction (New Zealand), on the Greek island of Andros and in Val d'Ala (Piedmont, Italy).

Right picture : Ardennite of Salmchâteau, Belgium - © Andrew Hodgson

Ardennite in France

In France, ardennite is reported at Bonneval-sur-Arc, in Savoie.


No known twins for this mineral species.

Fakes and treatments

No fake identified for this mineral species.

Hardness : 6-7
Density : 3.7
Fracture : Irregular to sub-conchoidal
Trace : White

TP : Opaque to transparent
RI : 1.740 to 1.780
Birefringence : 0.015 to 0.020
Optical character : Biaxial +
Pleochroism : Strong
Fluorescence : None

Solubility : -

Magnetism : None
Radioactivity : None