Arfvedsonite

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)...
Found on http://en.wikipedia.org/wiki/Arfvedsonite

arfvedsonite

amphibole mineral, an iron-rich sodium silicate. Lithium and magnesium replace iron in the structure to form eckermannite. Both minerals ... [1 related articles]
Found on http://www.britannica.com/eb/a-z/a/97

Arfvedsonite

Arfvedsonite is a black or greenish black mineral of the amphibole group.
Found on http://www.probertencyclopaedia.com/browse/HA.HTM
No exact match found