Sodalite

Sodalite
A sample of sodalite
General
Category	Tectosilicates without zeolitic H2O
Formula (repeating unit)	Na 8(Al 6Si 6O 24)Cl 2
Strunz classification	9.FB.10
Crystal system	Cubic
Crystal class	Hextetrahedral (43m) H-M symbol: (4 3m)
Space group	P43n
Unit cell	a = 8.876(6) Å; Z = 1
Identification
Color	Rich royal blue, green, yellow, violet, white veining common
Crystal habit	Massive; rarely as dodecahedra
Twinning	Common on {111} forming pseudohexagonal prisms
Cleavage	Poor on {110}
Fracture	Conchoidal to uneven
Tenacity	Brittle
Mohs scale hardness	5.5-6
Luster	Dull vitreous to greasy
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.27-2.33
Optical properties	Isotropic
Refractive index	n = 1.483 - 1.487
Ultraviolet fluorescence	Bright red-orange cathodoluminescence and fluorescence under LW and SW UV, with yellowish phosphorescence; may be in magentas
Fusibility	Easily to a colourless glass; sodium yellow flame
Solubility	Soluble in hydrochloric acid and nitric acid
Other characteristics	Emits hydrogen sulfide upon fracture
References
Major varieties
Hackmanite	Tenebrescent; violet-red or green fading to white

Stereo image

Left frame

Right frame

Parallel view ()

Cross-eye view ()

Small specimen of Sodalite from Brazil.

Sodalite is a rich royal blue tectosilicate mineral widely used as an gemstone. Although massive sodalite samples are opaque, crystals are usually transparent to translucent. Sodalite is a member of the sodalite group with hauyne, nosean, lazurite and tugtupite.

Discovered in 1811 in the Ilimaussaq intrusive complex in Greenland, sodalite did not become important as an ornamental stone until 1891 when vast deposits of fine material were discovered in Ontario, Canada.

A light, relatively hard yet fragile mineral, sodalite is named after its sodium content; in mineralogy it may be classed as a feldspathoid. Well known for its blue color, sodalite may also be grey, yellow, green, or pink and is often mottled with white veins or patches. The more uniformly blue material is used in jewellery, where it is fashioned into cabochons and beads. Lesser material is more often seen as facing or inlay in various applications.

Although somewhat similar to lazurite and lapis lazuli, sodalite rarely contains pyrite (a common inclusion in lapis) and its blue color is more like traditional royal blue rather than ultramarine. It is further distinguished from similar minerals by its white (rather than blue) streak. Sodalite's six directions of poor cleavage may be seen as incipient cracks running through the stone.

It is sometimes referred to as "poor man's lapis" due to its similar color and the fact that is much less expensive. Its name comes from its high sodium content. Most sodalite will fluoresce orange under ultraviolet light, and hackmanite exhibits tenebrescence.