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.
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.
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
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.
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
photochromic effect can be repeated indefinitely, although any heating of the mineral destroys tenebrescence
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.
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,
Gems & Gemology, Winter 1985, Gary Bowersox :"A
Status Report on Gemstones From Afganistan"
& Gemology, Winter 1989, Gem News, "Update
Physics & Chemistry of Color, Kurt Nassau, 1983
Intorduction to Rock Forming Minerals, Deer, Howie &
of Hackmanite", w/ localitites
a Brochure supplied by SoCalNevada, on Hackmanite from the
Kola Peninsula, Russia