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Sodalite
Mineral Facts:
Chemical
Formula: Na4Al3Si3O12Cl
Silicate and chloride of
sodium and aluminum. Natural crystals usually contain a little potassium in
place of some of the sodium and often some calcium. The content of Chlorine
is not constant.
Colors:
Blue, also white, gray, green.
Transparent to opaque, white
streak.
Hardness:
5.5 to 6
Density:
2.15
to 2.3
Cleavage:
Full dodecahedral
cleavage.
Crystallography: Isometric
Crystals rare, usually
dodecahedrons in habit, though some are
tetrahexahedral and others octahedral. Commonly massive, in embedded grains
in rock. Interpenetration twins of two dodecahedrons
are common.
Luster:.
vitreous luster. It is transparent, translucent and
sometimes opaque.
Optics:
(Refractive Index): n = 1.4827
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Composition, Structure and
Associated Minerals:
Sodalite occurs principally as a constituent of igneous rocks rich in
alkalies and as crystals on the walls of pores in some lavas. It is also
known as an alteration product of nepheline.
A comparatively rare rock-making mineral associated with nephelite,
cancrinite, etc., in nephelite-syenites, trachytes, phonolites, etc. Related
similar minerals, but even rarer in their occurrence, are hauynite, and
noselite. As a result of weathering sodalite loses Cl and Na and gains
water. Its commonest alteration products are zeolites, kaolin, and
muscovite.
Identification
and Diagnostics
Before the blowpipe, colored varieties bleach and all varieties swell and
fuse readily to a colorless blebby glass. The mineral dissolves completely
in strong acids and yields gelatinous silica, especially after heating. When
dissolved in dilute nitric acid its solution yields a chlorine precipitate
with silver nitrate. Its powder becomes brown on treatment with AgNOs, in
consequence of the production of AgCl.
The mineral is best distinguished from other similarly appearing minerals by
the production of gelatinous silica with acids and the reaction
for chlorine.
It is distinguished from
lazurite and similar minerals by its white (rather than blue) streak.
Some specimens are distinctly fluorescent and phosphorescent.
Localities
Occurs in soda-rich igneous
rocks, but is comparatively uncommon. Good crystals are found in nepheline
syenite at Ditro, in Hungary, in the lavas of Mte. Somma, Italy; in the
pegmatites of southern Norway; and at many other points where nepheline
rocks occur. In North America it is abundant in the rocks at Brome, near
Montreal; in the Crazy Mts., Montana, and at Litchfield, Maine. The material
at the last-named locality is light blue.
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