What is Tenebrescence? (All about Hackmanite - Hint: it's not fluorescence)
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.)
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
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.
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:
"Think of the electrons as basketballs. If the ball is lying on the basketball court, this is like having one arrangement of electrons and the mineral will have one color. We pick up the basketball and throw it through the hoop, this is like energizing the electrons with the ultraviolet light. After going through the hoop the ball falls back to the court floor similar to the electron falling back to a lower energy level and the mineral emits light of some color – it fluoresces. Hackmanite fluoresces, but many electrons get stuck in a new, high-energy position in atoms and this is what causes the mineral to have a different color when the lights are turned on. In our basketball analogy, imagine what would happen if we tied the bottom of the basketball net together. We could throw the ball through the hoop, but it would get stuck in the basket. If ultraviolet light causes electrons to get stuck at the higher energy level, then a new mineral color is produced. But when we turn the room lights on, the new color fades. White light also energizes electrons, just not as much as ultraviolet light. The white light has enough energy to unstick the electrons from their new home. It is the equivalent of beating on the bottom of the basket with net tied together. When the basketball/electron is in the basket/higher position in the atom, the mineral has one color. When the basketball/electron is on the court/lower position, the mineral has a different color (Nassau, 1978). When the basketball/electron is falling it fluoresces. This whole color change phenomenon is called tenebrescence or photochroism."
From FB FLM Group Howie G. explains it this way:
What we fluorescent mineral collectors call ‘hackmanite’ is essentially just sulfide-rich sodalite. The ideal crystal structure of sodalite is that of a cage made up of linked aluminate and silicate tetrahedra. Four Na+ ions line each cage, with a Cl- anion sitting in the middle. The disulfide anion in ‘hackmanite’, S2-, is unstable in its free-state, but becomes stabilized in the sodalite crystal lattice. In sodalite (and tugtupite) a disulfide ion substituting for an absent Cl- can absorb UV light, initiating fluorescence or phosphorescence via the usual mechanisms. (In tugtupite, where BeO4 tetrahedra replace some aluminate or silicate tetrahedra, the cage is smaller, squeezing the disulfide ion, lowering the energy emission, and shifting the visible light output to the red end of the spectrum.)
The tenebrescence of sodalite is best understood by appreciating the processes of fluorescence and of the F-center. An F-center (from the German Farbzentrum; Farbe means color, and zentrum means center) is a type of crystallographic defect in which an anionic vacancy in a crystal captures available electrons in order to maintain charge neutrality. Electrons in this vacancy tend to absorb light in the visible spectrum resulting in a usually colorless material becoming colored. The greater the number of F-centers, the more intense is the color of the compound. In ‘hackmanite’ there are neighboring aluminosilicate cages, one with a vacancy at the site of an absent Cl-, and another with a disulfide ion substituting for a Cl-. The disulfide ion does not contribute to the optical properties of sodalite under ordinary conditions. However, an UV-excited disulfide ion is a ready source of electrons, which flow to and become trapped in the F-center of the adjacent cage. When then illuminated in visible light, the F-center absorbs green, yellow, and some blue light and returns red and some blue light to the observer, resulting in the perceived purple ‘tenebrescent’ color. With subsequent application of the 'bleaching' energy source, the electrons trapped at the F-center are excited, return to the conductance band, and eventually return to their ground state, with the hackmanite returning to its original daylight color.
Hackmanite does not seem to lose any of its "color changing" capabilities over time no matter how often it is "charged", and subsequently bleached. I have pieces in my collection collected in 2001 which are still going strong. Of course this is only an observation; someday I hope to build a timer allowing me to repeatedly tenebresce and fade a specimen for 6 months, and compare the results to an unexposed control example of the same specimen. For now, there have been no reports of loss of tenebrescence, and no indication from any of the literature published. It should be noted that I have "heard" occasional comments that some specimens seem to lose their natural color over time (most notably Greenland blue sodalite) – but this too is unproven.
There are several singular characteristics of sodalite (hackmanite) from different locales. It has been reported that sodalite from Kola, Bancroft, MSH, and Greenland all fluoresce the same color; they peak between 625nm and 635nm under longwave (365nm) UV light, fluorescing yellow with an orange tint. But the tenebrescence (and apparently the fluorescence under SW) is quite variable between these locales:
Hackmanite from Bancroft, Canada is white and darkens modestly to pink under shortwave UV (varying degrees of color change – rarely, some can get quite dark). The color change is quick but, in most specimens, is not as dramatic as observed in specimens from other locales. It fades readily in sunlight or bright artificial light. Hackmanite from Mont St. Hilaire usually has a deeper color change but none of the Canadian hackmanite locales seem to produce vividly fluorescent specimens (when compared to Greenland hackmanite).
Afghanistan hackmanite darkens to a deep purple color under SW, is often quite phosphorescent, and is often found in crystal form – a creamy white to light purple in natural color. Once tenebrescence sets in, these pieces usually do not fade quickly (or completely) upon exposure to sunlight or artificial light, and can retain coloration for weeks/months. The fluorescence of these pieces is normally quite subdued – orange most often under LW, and sometimes grayish/white under SW.
Hackmanite from the Ilímaussaq Complex in Greenland demonstrates the widest variety of responses. The LW fluorescence of all the material from Greenland is striking – a vibrant yellow/orange. The SW fluorescence can also be quite bright (contrary to material from Canada), but depending on the piece it may darken after a few minutes of exposure.
An added bonus in the specimens from Greenland is that many of these pieces are found with other fluorescent minerals, resulting in unique multi-color specimens under UV. There are several varieties of sodalite/hackmanite found within the complex:
Blue Sodalite (natural color) – a vibrant blue naturally and upon exposure to SW it darkens in seconds to a deep purple (grape jelly). Occasionally a piece will be found that will turn light purple under ordinary sunlight, or longwave UV. This color fades almost instantly upon exposure to bright visible light.
White, Yellow and Green Sodalite (natural color) – all turn purple (from light purple to a deep grape jelly purple) upon exposure to SW UV. Tenebrescence from exposure to LW (or sunlight) is rarely observed. Bleaching occurs almost immediately upon exposure to sunlight or bright artificial light.
Red Sodalite is white when completely faded, but the UV in bright sunlight causes it to change color to a pinkish hue. On a cloudy day (UV still present, but visible light not as intense) the color deepens to a nice red (same results under LW UV). Under SW UV the color change deepens to a very dark purple (near black). It can take months for a specimen to fade (when kept in the dark), thus these pieces always seem to have a reddish tint. Even a bright halogen light often requires longer times to fade these specimens completely. Specimens will often have areas of phosphorescence. (When fully charged and placed in a lightproof container for six months, test specimens only slightly faded to a dark reddish purple.)
Generally I have found that purer pieces of Greenlandic sodalite (hackmanite) are often the most tenebrescent, (unlike hackmanite from Afganistan, the purest crystals of which often only tenebresce mildly). The Greenland specimens (described above) are gemmy translucent examples of colored sodalites, and are often associated with other fluorescent minerals such as tugtupite, ussingite, chkalovite, and polylithionite. Other Greenlandic sodalite (from Ilímaussaq) is usually vividly fluorescent under both SW and LW, but not notably tenebrescent – and very often these pieces are also not "gem quality" varieties, usually gray and white examples mixed with other minerals. One remarkable non-tenebrescent example is an extremely fluorescent light-green sodalite (natural color) - fluorescent a bright yellow/orange under any wavelength.
Any discussion of tenebrescent sodalite/hackmanite should include a mention of tugtupite. Tugtupite was discovered in 1957, and named for the locality where the mineral was originally found: Tugtup agtakôrfia in the Ilímaussaq Complex, Greenland. Initially it was called beryllium sodalite due to its similarity to sodalite. It is a very rare mineral, and has since been found in only two other places in the world, simultaneously on the Kola Peninsula (Russia) and later at Mont. St. Hilaire (Canada) – but only Greenland produces specimens of significant quantity, size, and color.
Tugtupite, like hackmanite, is tenebrescent. Tugtupite varies in natural color from white, pale pink to a saturated red (and an exceptionally rare light blue variety). Tugtupite, like hackmanite, will deepen in color when exposed to a UV light source. It is very reactive to sunlight. Usually all that is required is exposure to the sun to darken the color. The color will fade when exposed to ambient light (or a bright UV-free light source), but very slowly. If kept in the dark it will fade over a period of days/weeks, but is easily recharged under sunlight. Field explorations extensively use the "rock cracking" technique (sidebar above) to see if the inside of a rock turns red and is lasting, as tugtupite darkens in sunlight instead of fading like most hackmanite.
Most varieties of tugtupite fluoresce a bright cherry red under SW UV, and a bright salmon orange under LW. Some very interesting examples fluoresce a bright peach color under SW, or even white. Many specimens are also intensely phosphorescent.
Tugtupite is used as a gemstone (usually in cabochon form – but only if it has escaped my grasp!). The finest gem-grade tugtupite retains its dark red coloring for weeks, even in ambient light, and only fades to a light red color. But – keep in mind – just like sodalite, tugtupite will fade in the dark. The finest pieces will retain some red coloration but require exposure to UV to return to their deep red color.