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
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