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Semiprecious stones Library

Sodalite

Sodalite

Sodalite (length 7.5 cm)
General
CategoryMineral
Chemical formulaSodium aluminium silicate with chlorine (Na4Al3(SiO4)3Cl)
Identification
ColorRich royal blue, green, yellow, violet, white veining common
Crystal habitMassive; rarely dodecahedrons
Crystal systemCubic
CleavageDodecahedral (six directions), poor
FractureConchoidal to uneven - brittle
Mohs scale hardness5.5-6
LusterDull vitreous to greasy
StreakWhite
Specific gravity2.27-2.33
Refractive index1.48 - isotropic (sodium light)
PleochroismNone - isotropic
FusibilityEasily to a colourless glass; sodium yellow flame
SolubilitySoluble in hydrochloric and nitric acids
Major varieties
HackmaniteTenebrescent; violet-red or green fading to white

A sample of sodalite-carbonate pegmatite from Bolivia, with a polished rock surface.

Sodalite is a rich royal blue mineral widely enjoyed as an ornamental gemstone. Although massive sodalite samples are opaque, crystals are usually transparent to translucent. Sodalite is a member of the sodalite group and—together with hauyne, nosean, and lazurite—is a common constituent of lapis lazuli.

Discovered in 1806 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. It has since been named Princess Blue after Princess Patricia who, upon visiting Ontario some time after its discovery, chose sodalite as interior decoration for Marlborough House in England.[citation needed]

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 very similar to lazurite and lapis lazuli, sodalite is never quite comparable, being a royal blue rather than ultramarine. Sodalite also rarely contains pyrite, a common inclusion in lapis. 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.

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 colour 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 colour 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. (See also photochromism).

Hackmanite

Occurrence

Occurring typically in massive form, sodalite is found as vein fillings in plutonic igneous rocks such as nepheline syenites. It is associated with other minerals typical of undersaturated environments, namely leucite, cancrinite and natrolite.

Significant deposits of fine material are restricted to but a few locales: Bancroft, Ontario, and Mont-Saint-Hilaire, Quebec, in Canada; and Litchfield, Maine, and Magnet Cove, Arkansas, in the USA. The Ice River complex, near Golden, British Columbia, is being investigated for sodalite recovery. Smaller deposits are found in South America (Brazil and Bolivia), Portugal, Romania, Burma and Russia. Hackmanite is found principally in Mont. Saint-Hilare and Greenland, the latter locale producing a green specimen nicknamed "chameleon sodalite."

Euhedral, transparent crystals are found in northern Namibia and in the lavas of Vesuvius, Italy.

This hippopotamus ornament carved from sodalite demonstrates the mineral's poor cleavage - cracks can be seen throughout the stone.

See also

References





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