Sodalite

tectosilicate

primary

Formula: Na4(Si3Al3)O12Cl
Tectosilicate (framework silicate), feldspathoid
Specific gravity: 2.1 to 2.3
Hardness: 5 - 6
Streak: White
Colour: Blue, grey, white, colourless
Solubility: Moderately soluble in hydrochloric acid; slightly soluble in sulphuric acid
Environments:

Plutonic igneous environments
Pegmatites
Metamorphic environments

Sodalite is a relatively rare primary rock-forming mineral, and sometimes a secondary mineral, commonly found in nepheline syenite. It never occurs together with quartz. It is also a product of volcanic eruptions and it is sometimes found in contact metamorphosed rocks.
It may be found in andesite, basalt, diorite, limestone, dolostone, gabbro, syenite and trachyte.
Sodalite is associated with nepheline, cancrinite and other feldspathoids.
Sodalite variety hackmanite from Greenland is associated with feldspar, arfvedsonite and eudialite.

Alteration

albite and NaCl (aqueous) to sodalite and silica
6Na(AlSi3O8) + 2NaCl → 2Na4(Si3Al3)O12Cl + 12SiO2

nepheline and NaCl from the fluid to sodalite
6NaAlSiO4 + NaCl ⇌ 2Na4(Si3Al3)O12Cl
At the Igaliko Complex, South Greenland, sodalite is formed by replacement of nepheline, leading to a volume change which in turn causes a network of fractures. Deep blue fluorescent fluorite forms in these fractures, because the reaction of nepheline changing to sodalite reduces the salinity of the fluid, hence reducing the solubility of fluorite, so it precipitates.

Common impurities: Fe,Mn,K,Ca,H2O,S

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