Tenebrescence is a remarkable property of certain minerals to temporarily change their color.
Very few minerals exhibit tenebrescence, also known as reversible photochromism, but at least two that do—the ‘hackmanite’ variety of sodalite, and tugtupite—occur at Ilímaussaq. In fact, certain Ilímaussaq sodalite exhibits some of the most pronounced tenebrescence known.
Such behavior is exploited in synthetic materials for the manufacture of self-adjusting sunglasses, which darken on exposure to sunlight.
In addition to having brilliant orange fluorescence under UV, tenebrescent varieties of sodalite from Ilímaussaq exhibit two distinct color states in daylight: a pale magenta to deep purple state, and a pale green or yellow to colorless state.
When such samples are first broken, and exposed to white light, they will appear violet. Within a few seconds (to minutes) exposure to white light however, they change or "bleach" to a pale yellow or green, or may even become colorless.
Subsequent exposure to shortwave ultraviolet will cause these samples to revert back to their magenta state. The violet coloration persists for seconds to minutes in daylight, or apparently indefinitely if left in darkness.
The effect varies among specimens and even within a specimen, from grain to grain. Collecting at Ilímaussaq is often punctuated by flashes of fleeting color: momentarily violet rocks quickly changing to pale green.
The best examples display strong color change, such as from a very deep violet to a very light color, and back again, with only 10 seconds or so exposure to the appropriate light source. These are typically white or colorless when fully "bleached" [Note: photo above is an example of this material].
Apparently this photochromic effect can be repeated indefinitely. Heating destroys it however.
Reversible photochromism in ‘hackmanite’ sodalite is related to trace sulfur, substituting for some chlorine.
Electronically, the mechanism involves the sulfur
forming an S22- hole center precursor, which through
UV excitation becomes an S2- hole color center,
absorbing light at ~400 nm, and an e-, which is trapped in a Cl site
vacancy forming an 'F' or Farbe (color) center, absorbing at ~528
nm. The net effect of these two absorptions gives us the violet or magenta
color. Subsequent exposure to white light empties the electron traps, and the violet color
A specimen of off-white or light pink tugtupite will become pink or even bright pink. The intensified color fades if the specimen is placed in darkness for an extended period.
Some specimens display this effect quite strongly, and it has been marketed as an appealing attribute of tugtupite jewelry.