Sodalite

minerals

cancrinite

eudialyte

arfvedsonite

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Formula: Na4(Si3Al3)O12Cl
Tectosilicate (framework silicate), feldspathoid

Varieties

Hackmanite is a variety of sodalite which is tenebrescent due to sulphur substituting for some Cl.

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
Common impurities: Fe,Mn,K,Ca,H2O,S
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 (DHZ 4 p297). 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.
Associated minerals include nepheline, cancrinite, titanium-bearing andradite, aegirine, microcline, sanidine, albite, calcite, fluorite, ankerite and baryte (HOM ).

Localities

At the type locality, the Igaliku Complex, Kujalleq, Greenland, sodalite variety hackmanite is associated with feldspar, arfvedsonite and eudialyte (DHZ 4 p298). 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 (MM 55.380 - 459).

Alteration

albite and NaCl (aqueous) to sodalite and silica
6Na(AlSi3O8) + 2NaCl → 2Na4(Si3Al3)O12Cl + 12SiO2
(http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.613.9474&rep=rep1&type=pdf)

nepheline and NaCl from fluid to sodalite
6NaAlSiO4 + NaCl ⇌ 2Na4(Si3Al3)O12Cl

At the Igaliko Complex, South Greenland, sodalite is formed in this way (MM 55.459-463).

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