What is Tenebrescence? (All about Hackmanite - Hint: it's not fluorescence)


Tenebrescent sodalite (hackmanite) - Greenland

In the early 1800's geologists noted that some specimens of sodalite from Greenland exhibited a bright pink color on freshly broken surfaces. This color rapidly faded ("bleached") upon exposure to bright light, reverting back to its original (natural?) color. Exposure to UV light (in those days perhaps the source of UV was a cloudy day) apparently partially restored the purple/pink color in certain specimens, and this process could be repeated over and over It was found to be fully reversible.

We use this phenomenon of freshly broken surfaces extensively when collecting in Greenland. All one has to do is crack a rock open and observe the fleeting deep purple coloration (the reports say pink – I say deep purple) and you can be relatively sure you have a piece of sodalite. But – this is not a certain indication of tenebrescence. It seems that the majority of sodalite (from Ilímaussaq) will exhibit this purple coloration when initially broken but it does not necessarily mean that the specimen is tenebrescent under UV (field observation).

Minerals which are capable of this reversible color change by exposure to UV (or other energy sources), without any change in their essential composition, are said to be tenebrescent (from Latin – tenebrae, meaning shadows or darkness). Another term sometimes applied is photochromic – a material that undergoes a color change in the presence of photonic energy (such as glasses containing silver salts which automatically darken in sunlight). Other examples of tenebrescence in everyday life include light filters, coatings for windows/blinds, and even jewelry. Sodalite which exhibits this tenebrescence behavior has been given the variety name hackmanite.

(Blue sodalite shown switching between the "bleached" state and its tenebrescent state.)

Blue sodalite - hackmanite, Greenland

Hackmanite has been reported from a couple of localities – the most notable include: The Ilímaussaq Complex – South Greenland, Kola Peninsula – Russia, Mount St. Hilaire and Bancroft – Canada, Afghanistan and Myanmar (Burma). Only Greenland has acquired a reputation for producing large, brightly fluorescent, gem quality, multi-color hackmanite specimens. The depth and intensity of the color change varies widely, not only from locale to locale, but from specimen to specimen.

The tenebrescent color of a piece of hackmanite can range from a light pink to a deep "grape jelly" purple. The time it takes the tenebrescence to fully develop is also quite variable, as well as the wavelength to which each specimen will react. Shortwave UV (254nm) generally is the most effective wavelength in causing tenebrescence, but some specimens will also react nicely to longwave (350nm) or even obscured sunlight (cloudy days). Any bright UV-free light source will "bleach" the color and return the mineral to its previous color (sunlight, even though it has a UV component, will also bleach most hackmanite – probably due to the intensity of the other wavelengths). I have found a green laser to be most effective in bleaching the tenebrescent color, and have even been able to write my name on a tenebresced specimen with the laser dot.

The color of various hackmanite specimens (in their faded or bleached state) ranges from white, to pink, blue, green and even red. Some sodalites have very long lasting tenebrescence, most notably specimens from Afghanistan and a single area in Greenland (to date). The deep purple color can last for months in total darkness and often specimens only fade to a dark pink color (seemingly their natural color). Even exposure to a bright UV-free bleaching light source is often not very effective on this hackmanite.

Red Sodalite - SW with chkalovite, uranyl green, unknowns - phosphorescent

Red sodalite from Greenland

Most hackmanite is also quite fluorescent – usually strongest under LW. Most often the color under LW is a bright yellow/orange. Specimens

have also been observed with a creamy white fluorescence (most notably those from Afghanistan). Under SW the response is quite variable, partially due (I believe) to the tenebrescence of the specimen. Most often the fluorescent response under SW is initially the same orange glow seen under LW, but perhaps less bright. As the tenebrescence sets in, the fluorescence shifts to a "rusty" orange, or - if this tenebrescent color change is strong - shifts to a deep purple. The fluorescence may dim considerably and the specimen may seem to be just barely fluorescent. Some specimens (most notably those from Canada) do not seem to fluoresce brightly under SW – often only a dim glow, sometimes reddish. Some specimens also exhibit phosphorescence under SW; those from Greenland will exhibit a neon purple glow under SW in the phosphorescent areas, and of course continue glowing once the light is removed. Afghanistan hackmanite often has remarkable phosphorescence.

Tenebrescence

The length of UV exposure required to affect this color change is also quite variable. Some specimens of hackmanite from Afghanistan require lengthy exposure to UV to produce significant darkening, while most hackmanite from Greenland will start to darken immediately. These times can vary from a few seconds to minutes (as in Afghan hackmanite).

(Left - shows a piece of red sodalite from Greenland non-tenebrescent, LW tenebrescent, SW tenebrescent)

It has been written that once these pieces have been "tenebresced" and placed in the dark they will retain their color indefinitely. This has not been my experience with most hackmanite - In over eight years of collecting this material every specimen of Greenland hackmanite I have tested (as well as others) has bleached when placed in a dark container except for one type found only in one small area within the Ilímaussaq Complex. The fading sometimes can take days, weeks, or even months, but eventually the vast majority of specimens will fade. It has also been reported that specimens would recover their coloration (perhaps only partially) when placed in the dark "for a time" (up to 5 weeks in one report). I have never seen a piece of hackmanite tenebresce without energy input (regain its purple coloration after being left in the dark). My inventory of this material is kept in large lightproof metal storage containers, and each time I select a piece I must "charge" it with a UV light to see the coloration. I have tested many varieties of hackmanite from Greenland, Afghanistan, Bancroft, and MSH.

Wavelengths between 480 and 720nm (visible light except for violet and blue) will readily reverse the tenebrescence and return most specimens to their original color. A bright halogen light (with UV filter) is very effective in reversing this process. Depending on the brightness of the light, the color will usually fade in seconds. Most pieces present a quick fade when exposed to direct sunlight – fading in seconds, so quickly it can be missed if you look away. Heat will also reverse the color change (fade the specimen) and, if hot enough, destroy the tenebrescence (I have heated Greenland hackmanite to 500F for 10 minutes. The color faded completely and the specimen would no longer tenebresce – but it did not affect the fluorescence.)

"Most researchers appear to agree that F-Centers are the cause, or at least part of the cause, of reversible color in hackmanite. The term F-Centers comes from the German word Farbe, which means color. F-Centers are responsible for coloring a variety of minerals, including fluorite and barite. 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. It is likely that sulfur, as double negatively charged disulfide units, S22, is the source of the electrons. When ultraviolet is directed at the sodalite, it is absorbed at disulfide units. These each lose an electron and thus become S21- units. The free electrons wander to the chlorine vacancies where they are trapped coloring the mineral." (Manual Robbins - Fluorescence – Gems and Minerals Under Ultraviolet Light)

Julian Gray (PhD - Curator, Tellus Museum GA) explained fluorescence and tenebrescent bleaching using a simple basketball analogy: