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Formula: Na4(Si3Al3)O12Cl
Tectosilicate (framework silicate), sodalite group of the
feldspathoid group
Varieties
Hackmanite is a variety of sodalite which is tenebrescent due to sulphur
substituting for some Cl.
Crystal System: Isometric
Specific gravity: 2.27 to 2.33 measured, 2.31 calculated
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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