Arfvedsonite

Arfvedsonite
Arfvedsonite from New Hampshire, USA
General
Category	Inosilicates Amphiboles
Chemical formula	[Na][Na2][(Fe2+)4Fe3+][(OH)2|Si8O22]
Strunz classification	VIII/F.08-100
Dana classification	66.1.3c.9
Identification
Color	Black, deep green on thin edges
Crystal habit	Fibrous, radial prismatic aggregates
Crystal system	Monoclinic – Prismatic
Twinning	Simple or lamellar parallel to [100]
Cleavage	Perfect on [110]
Fracture	Uneven
Tenacity	Brittle
Mohs scale hardness	5–6
Luster	Vitreous
Streak	Deep bluish gray, gray-green
Diaphaneity	Translucent to opaque
Specific gravity	3.3–3.5
Optical properties	Biaxial (-)
Refractive index	nα = 1.652–1.699 nβ = 1.660–1.705 nγ = 1.666–1.708
Birefringence	δ = 0.014
Pleochroism	Strong: Blue-greens, yellow-browns, gray-violets
Dispersion	r > v strong
References

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).

See also

• List of minerals
• List of minerals named after people

Arfvedsonite, Poudrette quarry, Mont Saint-Hilaire, Montérégie, Quebec

References

• Deer, W.A., R.A. Howie, and J. Zussman (1963) Rock-forming Minerals, v. 2, Chain Silicates, p. 364-374
• Mineral Galleries