1911 Encyclopædia Britannica/Sodalite

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SODALITE, a group of rock-forming minerals comprising the following isomorphous species:—

Sodalite . . Na4(AlCl)Al2(SiO4)3
Haüynite . . (Na2, Ca)2(NaSO4•Al)Al2(SiO4)3
Noselite . . Na4(NaSO4•Al)Al2(SiO4)3
Lazurite . . Na4(NaS3•Al)Al2(SiO4)3

They are thus sodium (or calcium) aluminium silicates, with chloride, sulphate or sulphide. In their orthosilicate formulae, as above written, and in their cubic crystalline form they present a certain resemblance to the members of the garnet group. Crystals usually have the form of the rhombic dodecahedron, and are often twinned with interpenetration on an octahedral plane. They are white, or often blue in colour, and have a vitreous lustre. The hardness is 5½, and the specific gravity 2.2-2.4. These minerals are characteristic constituents of igneous rocks rich in soda, and they also occur in metamorphic limestones.

The species sodalite (so named because it contains soda) occurs as well-formed, colourless crystals in the ejected limestone blocks of Monte Somma, Vesuvius, and in the sodalite-syenite of Julianehaab in south Greenland. Massive blue material is common in the elaeolite-syenites of southern Norway, Gyergyo-Ditró in Transylvania, Miyask in the Urals, Litchfield in Maine, Dungannon in Ontario, Ice river in Kootenay county, British Columbia, &c.; at the three last-named localities it is found as large masses of a bright sky-blue colour and suitable for cutting as an ornamental stone. Recently, large masses with a pink colour, which quickly fades on exposure to light, have been met with in elaeolite-pegmatite at Kishangarh in Rajputana. Haüynite, or haüyne (named after R. J. Haüy), occurs as bright blue crystals and grains in the lavas (phonolite, tephrite, &c.) of Vesuvius, Rome, the Eifel, &c. Noselite, or nosean, is found as greyish crystals in the sanidine bombs of the Eifel. Lazurite is an important constituent, together with some haüynite and sodalite, of lapis-lazuli (q.v.). (L. J. S.)