Arfvedsonite - Encyclopedia

    Class : Silicates
    Subclass : Inosilicates
    Crystal System : Monoclinic
    Chemistry : Na3Fe5Si8O22(OH)2
    Rarity : Uncommon

Arfvedsonite belongs to the family of sodium amphiboles. It forms a series with magnesio-arfvedsonite. It is a frequent amphibole of alkaline granites and especially of nepheline syenites with sodalite. It was named in honor of the Swedish chemist Johan A. Arfvedson. It is black to green-black, rarely dark blue, exceptionally orange. Its crystals are elongated prismatic, measuring up to 60 cm, sometimes tabular, often united in fibroradiated aggregates.

Main photo : Arfvedsonite from Mont-St-Hilaire, Quebec, Canada - © Modris Baum

Arfvedsonite in the World

The finest crystals have been found in the Ilimaussaq alkaline complex, near Narssaq (Greenland) and at Mont-Saint-Hilaire (Quebec, Canada), where the pegmatitic dykes contain 20 cm arfvedsonite prisms. The nepheline syenite of Lovozero (Kola, Russia) also has superb decimetric crystals. Arfvedsonite has also been discovered in large crystals at Tirodi, Madhya Pradesh (India).
Right photo : Arfvedsonite from Hurricane Mountain, New Hamshire, USA - © Scott M. Whittemore

Arfvedsonite in France

In France, arfvedsonite is found in certain Corsican alkaline pegmatites (Evisa-Calasima, Bonnifatto, etc.), as well as in certain volcanic products from Mont-Dore (Puy-de-Dôme) and Velay (Haute-Loire).

Twinning and special crystallizations

Arfvedsonite exhibits simple and lamellar twins parallel to {100}. It was also discovered at Imilchil (Morocco) epitaxial octahedral titaniferous magnetite pseudomorph after arfvedsonite (see the sheet on magnetite).

Fakes and treatments

No fake recorded for this mineral species, but it can easily be confused with aegyrine.

Hardness : 5-6
Density : 3.3 to 3.5
Fracture : Irregular
Trace : Gray

TP : Opaque to translucent
RI : 1.652 to 1.708
Birefringence : 0.014
Optical character : Biaxial -
Pleochroism : Strong
Fluorescence : None

Solubility : Insoluble

Magnetism : Paramagnetic
Radioactivity : None


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