Sodalite Group – Sodalite & Lazurite Composition, Crystallization & Structure

Sodalite
Composition, Na₄.(AlCI)Al₂(SiO₄)₉. Isometric. Crystals rare, usually dodecahedrons. Commonly massive, in embedded grains. Dodecahedral cleavage. H. = 5.5-6. G. = 2.15-2.3. Vitreous luster. Color usually blue, also white, gray, green. Transparent to opaque. Fusible at 3.5-4, to a colorless glass, giving a strong yellow flame (sodium). Soluble in hydrochloric acid and gives gelatinous silica upon evaporation. Nitric acid solution with silver nitrate gives white precipitate of silver chloride. A comparatively rare roek-making mineral associated with nephelite, eancrinite, ete. In nephelite-syenites, traehytes, phonolites, ete. Found in transparent crystals in the lavas of Vesuvius. The massive blue variety is found at Litchfield, Maine; in Ontario and Quebec near Kicking Horse Pass, British Columbia. Similar minerals, but rarer in their occurrence, are haüynite, (Na₂.Ca)₂(Na₂SO₄.Al)AI₂.(SiO₄)₃, and noselite, Na₄(NaSO₄.Al)Al₂(Si04)₃

Lazurite - Lapis-Lazuli
Cornposition, Na₄(NaS₃.AI)AI₂(SiO4)3, with small amounts of the sodalite and haüynite molecules in isornorphous replacement. Isometric. Crystals rare, usually dodecahedral. Commonly massive, compact. H. = 5-5.5. G. = 2.4-2.45. Vitreous luster. Color deep azure-blue, greenish blue. Translucent. Fusible at 3.5, giving strong yellow flame (sodium). Soluble in hydrocworic acid with slight evolution of hydrogen sulphide gas, and gives gelatinous silica upon evaporation. A rare mineral, occurring usually in crystalline limestones as a product of contact metamorphism. Lapislazuli is usually a mixture of lazurite with small amounts of calcite, pyroxene, etc. It commonly contains small disseminated particles of pyrite. It is used as an ornamental stone, for carvings, etc. The best quality of lapis-lazuli comes from northeastern Afghanistan. Also found at Lake Baikal, Siberia, and in Chile.

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