Arfvedsonite is a sodium amphibole mineral with composition: Na3(Fe,Mg)4FeSi8O22(OH)2: sodium, iron, magnesium, inosilicate, hydroxide. It crystallizes in the monoclinic prismatic crystal system and typically occurs as greenish black to blueish grey fibrous to radiating or stellate prisms. It has a specific gravity of 3.44 to 3.45 and a Mohs hardness of 5.5 to 6. Refractive indices are n?=1.652 - 1.699, n?=1.660 - 1.705, and n?=1.666 - 1.708.
It is rather rare and occurs in alkalic igneous mineral occurring in nepheline syenite intrusions and agpaitic pegmatites and agpaitic (peralkaline) granites as the Golden Horn batholith in Okanogan county, Washington (type locality for zektzerite). Occurrences include Mont Saint-Hilaire, Quebec, Canada; the Ilimaussaq Intrusion 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 1923 and named for the Swedish chemist Johan August Arfwedson (1792-1841).