Arfvedsonite is a sodium with composition:. 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 (peralkaline) pegmatites as the Golden Horn batholith (type locality for zektzerite). Occurrences include Mont Saint-Hilaire; the Ilímaussaq complex; and in pegmatites of the Kola Peninsula. Its mineral association includes nepheline , aegirine and quartz
Arfvedsonite was discovered in 1823 and named for the Swedish chemist Johan August Arfwedson