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Sodalite Sodalite, a chlorine-bearing sodium aluminosilicate, occurs is a myriad of forms and associations at Ilímaussaq. In grains and solid masses to many cm across it may be colorless, white, gray, pale yellow, green, blue-green, pale violet or pinkish in daylight. Sodalite is a major rock-forming mineral in several local rock types and occurs in very large quantities. However, not all sodalite fluoresces and not all that does fluoresces brightly. The best fluorescent sodalite from Ilímaussaq glows brilliant yellow-orange under longwave and midwave ultraviolet; distinctly less bright under shortwave.
In some specimens, sodalite phenocrysts, 1-5 cm across, occur in a dark matrix of non-fluorescing lujavrite. Specimens of this material present a stunning spotted appearance under ultraviolet (above). Sodalite also occurs associated with a range of other fluorescent minerals, such as tugtupite, making for spectacular specimens. In other specimens, sodalite occurs as glassy single-crystal anhedra 10 cm or more in size, pale yellow in daylight. The fluorescent response of these specimens is generally brilliant, and they often contain various bluish-green fluorescing phases as well.
The sodalite fluorescence here presents a translucent, ethereal appearance (similar—but not in color—to ‘hyalite’ opal). A less common form of Ilímaussaq sodalite is the tenebrescent (reversibly photochromic) ‘hackmanite’ variety. This is generally green or pale green in daylight, and shows strong yellow-orange fluorescence. However, after as little as a few seconds exposure to shortwave ultraviolet, the daylight color of this sodalite changes to pale violet; in some cases quite deeply. If kept in the dark this color persists. When exposed again to daylight or
longwave ultraviolet, however, it fades and returns to its “normal” daylight
color (greenish). Large masses of such material with grains of sodalite up to
5 cm across have recently been found.
The most intensely tenebrescent sodalite found is transparent to white in daylight and turns deep amethyst purple after exposure to shortwave ultraviolet. This color change is so strong that when viewing a specimen under shortwave ultraviolet, the apparent fluorescent response changes; from an initial yellow-orange to deep rust or reddish-orange color, in seconds. See photo above and tenebrescence for details. Ilímaussaq sodalite provides some of the best and strongest fluorescent and tenebrescent responses known for the species.
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