Hackmanite

Na8Al6Si6O24Cl2

An illustration of tenebrescence

In 1896, a vibrant pink variety of sodalite was disovered in Greenland by L.C. Boergstroem. The pink color of this unusual sodalite faded to colorless when exposed to light. The sodalite will return to its original pink color when it is placed in the dark for an extendended period of time, or when exposed to short wave ultraviolet light. This transformation can be repeated endlessly. Tenebrescence is defined by minerals that are able to make this color transformation; minerals that display the ability to change color in this fashion are termed tenebrescent. Tenebrescence is the property that some minerals and phosphors show of darkening in response to radiation of one wavelength and then reversibly bleaching on exposure to a different wavelength. Very few minerals exhibit this phenomenon, also known as reversible photochromism, a word that applies to sunglasses which change color density on exposure to sunlight.

SODALITE that shows this behavior has been given the variety name Hackmanite. The pink color in this mineral is unstable because it fades very quickly when exposed to light. There are other examples of minerals that lose or gain color when exposed to light:

TUGTUPITE , some light colored varieties of tugtupite, especially pale pink material, will intensify in color as a result of exposure to shortwave UV—or even strong sunlight (but not artificial light) .

SPODUMENE , a darkening of color to pink or purple can be acheived with exposure to high-energy radiation.

CHAMELEON DIAMONDS are olive colored diamonds that temporarily change color after having been stored in darkness or when gently heated. Chameleon diamonds display hues and tones from light to dark olive (stable color phase) through light to medium yellow (unstable color phase). After one to two days in darkness, exposure to light changes the color of a chameleon diamond from the unstable yellow color back to the stable olive . This is observed as an infinitely repeatable process.

AMETHYSTS from Globe, Arizona, and some SHERRY-COLORED TOPAZ are reported to loose their color in the sun. This loss of color is irreversible .

WHITE BARITE from the Gaskin Mine in Pope Cunty Illinoios, will change to blue, and yellow barite to grey-green when exposed to ultraviolet light.



The pink color of hackmanite may be restored in two different ways. One is by leaving the specimen in the dark for a few hours to several weeks, or, exposure to short-or long wave ultraviolet will also restore color. Short wave ultraviolet is the most efficient for this purpose. The speed with which this is accomplished and the depth of the color acheived varies from specimen to specimen.

In some specimens, long exposure to ultraviolet light is required to produce a faint degree of pink color. In other specimens, exposure to shortwave ultraviolet will almost instantly produce a pink color. In the latter specimens, additional exposure to ultraviolet light for several minutes to a few hours will produce a deep pink to raspberry-red color in which a weak blue component is evident. This can be seen in some specimens from Mont Saint-Hilaire and Khibina. If the specimen is now put in the dark, the deep red color will exhibit phosphorescence also known as "after glow". Visible light (wavelengths between 480-720 nanometers) will quickly reverse the process and render the specimen colorless once again.

This photochromic effect can be repeated indefinitely, although any heating of the mineral destroys tenebrescence forever.

Research indicates that F-Centers are the cause, at least partially, for the tenebrescence in hackmanite. The term F-Centers is derived from the German word Farbe, meaning color. An F-center is a defect in an ionic lattice which occurs when an anion leaves as a neutral species, leaving a cavity and a negative charge behind. This negative charge is then shared by the neighboring positive charges in the lattice. F-Centers are responsible for coloring a variety of minerals, including fluorite and barite.(Nassau, 1983) In hackmanite, it is proposed that some of the negatively charged chlorine atoms are missing. A negative electric charge is required at such vacancies to provide charge balance, and any free electrons in the vicinity become drawn to such vacancies and are trapped there. Such a trapped electron is the typical basis of an F-Center. It appears that this center in hackmanite absorbs green, yellow, and orange light and varying amounts of blue. When the hackmanite is seen in white light, red and some blue are returned to the eye, giving the hackmanite colors.

A mineral may produce a certain color that depends on different, but fixed arrangements of electrons (Nassau, 1983). Hackmanite absorbs the energy from the ultraviolet radiation and many electrons get stuck in a new, high-energy position in atoms (F-centers); this is what causes the mineral to have a different color when the lights are turned on. But when we turn the room lights on, the new color fades. White light (the visible spectrum) also energizes electrons, just not as much as ultraviolet light. The white light has the necessary energy to unstick the electrons from the F-Centers, thus returning the mineral to colorless.



Why does total darkness bring back the pink color? Where does the energy come from that traps electrons in the F-Centers to make the hackmanite appear pink? If you think you can answer this question, by all means,

LET ME KNOW!!!

References:

Gems & Gemology , Winter 1985, Gary Bowersox :"A Status Report on Gemstones From Afganistan"

Gems & Gemology , Winter 1989, Gem News, "Update on Hackmanite"

The Physics & Chemistry of Color , Kurt Nassau, 1983

An Intorduction to Rock Forming Minerals , Deer, Howie & Zussman 1966

Mindat: "Classification of Hackmanite", w/ localitites