Mineral Type - Sodalite

Sodalite Group. Also classed as a feldspathoid


Ideally sodalite has Na4Cl in the two cages in the unit cell. Unlike haüyne and nosean, sodalite is virtually CO2-free (Bellatreccia et al., 2009).
In some sodalites the aluminosilicate cages appear to contain only Na and Cl according to EDS spectra.
Unlike haüyne and lazurite, Ca and K are seldom in the cages. The grey sodalites from Afghanistan contain minor K.
The white variety hackmanite is reported to contain S, but EDS studies show if it is there, it often occurs below detection limits (~1%) (R.W.M. Woodside, unpublished data).
The photoluminescence effect in sodalite may be connected with admixture of sulfur (presence of S enlarges the unit cell). Sodalite is also tenebrescent, likely due to formation of F-centers (Zahoransky et al., 2016).

Name

The name reflects its sodium content.

Physical Properties

Lustre: Vitreous, Greasy
Transparency: Transparent, Translucent
Colour: All Colours. Colourless, white, light yellow, green, light to very dark blue, gray, pink to violet
Streak: White
Hardness: 5½ - 6 on Mohs scale
Tenacity: Brittle
Cleavage: Poor/Indistinct. Poor on {110}
Fracture: Sub-Conchoidal
Density: 2.73 g/cm3 (Measured) 2.31 g/cm3 (Calculated)

Chemical Properties

Formula: Na8(Al6Si6O24)Cl2
Some K substitutes for Na in the grey white Afghan sodalites. Hackmanite was originally defined as containing S. The brilliant yellow fluorescence arises from a disulfide ion with a negative charge replacing the Cl.
Elements listed: Al, Cl, Na, O, Si
Common Impurities: Fe,Mn,K,Ca,H2O,S

Crystallography

Crystal System: Isometric
Class (H-M): 4 3m – Hextetrahedral
Space Group: P4 3n
Morphology: Crystals rare, dodecahedron.
Twinning: Common on {111} forming pseudohexagonal prisms. (This may not be twinning, but merely elongated growth in the (111) direction). Twinning can occur in these elongated xls. These have rentrants in the terminations and 6 terminal faces instead of 3 for an elongated dodecahedron.

Synonyms

Alomite, Canadian Blue Stone, Canadian Lapis, and Glaucolite

Varieties

Germanate-sodalite, and Molybdosodalite