SODALITE  , a

group of rock-forming minerals comprising the following isomorphous species: Sodalite . Na4(AICI) Alz (SiO4) 3 Hatlynite (Naz, Ca)z(NaSO4.Al)Alz(SiOs)s Noselite Na4(NaSO4•Al)Alz (SiO4)1 Lazurite Na4(NaSz•Al)AIz(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 51, 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-Ditr6 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 . Haiiynite, or hauyne (named after R . J .

Flatly), 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 haiiynite and sodalite, of lapis-lazuli (q.v.) . (L: J .