Sodalite
Sodalite is a rich royal blue
tectosilicate mineral widely used as an
ornamental gemstone. Although massive
sodalite samples are opaque, crystals
are usually transparent to translucent.
Sodalite is a member of the sodalite
group with hauyne, nosean, lazurite and
tugtupite.
Discovered in 1811 in the Ilimaussaq
intrusive complex in Greenland, sodalite
did not become important as an
ornamental stone until 1891 when vast
deposits of fine material were
discovered in Ontario, Canada.
Properties
A light, relatively hard yet
fragile mineral, sodalite is named after
its sodium content; in mineralogy it may
be classed as a feldspathoid. Well known
for its blue color, sodalite may also be
grey, yellow, green, or pink and is
often mottled with white veins or
patches. The more uniformly blue
material is used in jewellery, where it
is fashioned into cabochons and beads.
Lesser material is more often seen as
facing or inlay in various applications.
Although somewhat similar to lazurite
and lapis lazuli, sodalite rarely
contains pyrite (a common inclusion in
lapis) and its blue color is more like
traditional royal blue rather than
ultramarine. It is further distinguished
from similar minerals by its white
(rather than blue) streak. Sodalite's
six directions of poor cleavage may be
seen as incipient cracks running through
the stone.
It is sometimes referred to as "poor
man's lapis" due to its similar color
and the fact that is much less
expensive. Its name comes from its high
sodium content. Most sodalite will
fluoresce under ultraviolet light and
hackmanite exhibits tenebrescence.
Hackmanite
Hackmanite
is an important variety of sodalite
exhibiting tenebrescence. When
hackmanite from Mont Saint-Hilaire
(Quebec) or Ilímaussaq (Greenland) is
freshly quarried, it is generally pale
to deep violet but the color fades
quickly to greyish or greenish white.
Conversely, hackmanite from Afghanistan
and the Myanmar Republic (Burma) starts
off creamy white but develops a violet
to pink-red color in sunlight. If left
in a dark environment for some time, the
violet will fade again. Tenebrescence is
accelerated by the use of longwave or,
particularly, shortwave ultraviolet
light. Much sodalite will also fluoresce
a patchy orange under UV light.
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