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Arfvedsonite
Chemical Formula	[Na][Na2][(Fe2+)4Fe3+][(OH)2|Si8O22]
Species	Silicates
Crystal System	Monoclinic
Mohs Scale	5-6
Specific Gravity	3.3–3.5
Color	Black, deep green on thin edges
Streak	Deep bluish gray, gray-green
Luster	Vitreous
Refractive Index	n = 1.652 - 1.699 n = 1.660 - 1.705 n = 1.666 - 1.708
Diaphaneity	Translucent, Opaque
Cleavage	PerfectPerfect on
Fracture	Irregular/Uneven
Crystal Habit：Fibrous, radial prismatic aggregates
Geological Setting：Alkalic granites and other alkalic rocks

Arfvedsonite is a sodium amphibole mineral with composition: [Na][Na2][(Fe2+)4Fe3+][(OH)2|Si8O22]. It crystallizes in the monoclinic prismatic crystal system and typically occurs as greenish black to bluish grey fibrous to radiating or stellate prisms.
It is a rather rare mineral occurring in nepheline syenite intrusions and agpaitic (peralkaline) pegmatites and granites as the Golden Horn batholith in Okanogan County, Washington (type locality for zektzerite). Occurrences include Mont Saint-Hilaire, Quebec, Canada; the Ilímaussaq complex in Southern Greenland; and in pegmatites of the Kola Peninsula, Russia. Its mineral association includes nepheline, albite, aegirine, riebeckite, katophorite and quartz.
Arfvedsonite was discovered in 1823 and named for the Swedish chemist Johan August Arfwedson (1792–1841).