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Tenebrescence, also known as reversible photochromism, is the ability of minerals to change color when exposed to sunlight (or UV). The effect can be repeated indefinitely, but is destroyed by heating. Self-adjusting sunglasses, which darken on exposure to sunlight, is an example of tenebrescence used in everyday life.
Tenebrescent minerals include sodalite (hackmanite), spodumene and tugtupite.
Most of the sodalite found in the Ilimaussaq Complex is tenebrescent; pieces which do not change color upon exposure to UV are the exception. This color change can vary from a slight darkening to a dramatic deep purple change - almost black - when exposed to SW UV. In most specimens the color can easily be faded back by exposure to a bright light (UV free) in a matter of seconds.
A rarer number will also exhibit this color change under LW UV, or even ordinary sunlight - although the color change is much more dramatic using SW UV.
The pages in this section show miscellaneous “observations” over the past few years of various types of tenebrescent sodalite from Greenland.
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