Tenebrescence Experiments, Theories, Questions, and even some Facts

Recently I posted a question about Afghan sodalite (hackmanite) after reviewing the MinRec article about Sar-E-Sang. Someone reminded me that there was a large discussion on this topic last year. I dug out that discussion and copied it here to the blog. This is one reason we created Nature's Rainbows - to capture these excellent discussions. Perhaps someday someone will need this information and write a paper putting all those tenebrescent questions to bed. Impossible to find on FB unless you know what you're looking for.


Below is a merged copy of several threads on this topic from the FLM Facebook Group

(Initials have replaced the full name of the posters - you know who you are)

(I deleted the several attempts at humor and off-topic posts)

MC: Hackmanite - there has been a lot of discussion around this topic lately. In reviewing the 2014 Sar-E-Sang issue of the MinRec I noticed that RW: wrote that after placing a completely bleached piece of hackmanite in a dark, light sealed box the tenebrescent color will return in a few months due to thermal effects (at just room temperature). Most of the article was centered around Afghan hackmanite even though they are actually pretty poor "performers" (some take hours/days to darken or fade). But they do have the purty crystals that non-fluorescent collectors love I have tested this theory with Greenland hackmanite and have never been able to reproduce it. A piece of GL hackmanite (or tugtupite for that matter) placed in a light-tight box never deepens in color. As you might imagine, I have quite a few GL specimens lying around - many of them sealed away, some for years (I just opened a box that was filled with hackmanite and tugtupite cabochons; it hadn't been opened for perhaps 5 years or more - they were all as pale as a baby's butt). So - my question is: Has anyone experimented with Afghan hackmanite in a similar manner? Does only Afghan hackmanite do this?

ED: My experience with both locals is the same as yours. I have never noticed thermal effects. However, the top grade Afghan hackmanite is so sensitive to UV light that its "Natural" color changes depending on the type of light source. I have a box that I open once in a while and they are currently a purple/rose at about a mid state.

MC: Here's a case in point. I tumbled up a bunch of small chips off the original find of green sodalite back in 2001-3. The pieces were wonderfully gemmy. I put them in a box and stored them away, never to again see the light of day until just now. I just found them (amazing in its own right), pulled them out in a dark room, setup the camera, and took a quick pic. This is as they looked coming out of the box:

Green Sodalite

MC: Then I hit them with 30 secs of SW UV:

MC: FWIW - here's the fluorescent pics, SW

MC: and LW:

RW: Some sodalites darken under sunlight and some don't. Ones that don't may have had a lovely pink colour when fractured out of country rock and faded quickly in sunlight. That fading after fracturing is an indication that it will darken under strong UV which then fades in sunlight. So how can sodalite either bleach or darken in sunlight depending on the specimen?

MC: "So how can sodalite either bleach or darken in sunlight depending on the specimen?" - it has been my experience that certain Greenland sodalites will darken nicely under diffused sunlight, but when presented with full, noontime sun the visible light overwhelms the UV and prevents the tenebrescence from setting in. This is best observed by viewing the specimen in the shade, or on a cloudy day. UV gets through the clouds (and I guess bounces around in the shade) such that it will darken the sodalite. Quite often in Greenland we would bust open a boulder and see only white rock, toss the specimens aside (in a shady area) and turn around to see they've turned pink. This is how "Red Sodalite" got its name Any sodalite that does this will tenebresce under LW UV too.

MC: Almost every rock (not mineral) in the Ilimaussaq Complex that you crack will exhibit the brief pink color - it's fun to watch. But not a reliable predictor of tenebrescence. Sodalite is a basic rock-building mineral in the complex; they all have some. But it seems only the purer sodalites retain the tenebrescing ability (at least that's my field experience).

ED: Another observation I can add is that the Afghan hackmantites exhibit world class phosphorescence which may somehow be related to their high uv sensitivity.

MC: I actually had the same thought Eric - wonder if the phosphorescence has anything to do with the differences between AFghan and GL hackmanite. GL pieces are very rarely phosphorescent, and if they are, usually only in very small areas. JR: We've been discussing fluorescence triggered by the ultraviolet rays in sunshine. Has anyone heard of the same rays causing Tenebrescence? This hackmanite originated in Afghanistan. It was not illuminated by any ultraviolet light. First picture was taken as it came out of the bag, second after a brief exposure to sunshine. This appears to fly into the face of conventional wisdom about tenebrescence being reversed by daylight. Both were taken at 1/2 second, asa 100 f11 from about 15 inches.

MC: Yes to both questions. That's the problem with this kind of testing - it takes years. I did this several years ago, but just went back to the cabinet where I placed the "subjects" under test. They were wrapped tightly in aluminum foil and then in a closed cabinet. I had left one specimen in the cabinet (unwrapped from the foil). It was still as faded as ever, even after several years in the dark. Beyond that, as you might imagine, I have a pretty good inventory of hackmanite. It is mostly stored in light-tight containers and I never see any darkening when I go to retrieve new specimens. I can say pretty confidently that Greenland hackmanite requires UV to tenebresce, and white light to fade. I cannot say with confidence what very long-term test results would be. One interesting consideration is that when you first crack open a GL sodalite, it exhibits intense tenebrescence, then fades quite rapidly. But if viewed in the dark this fading is not so rapid; I wonder if it would ever fade in total darkness? (Inside the rock, before cracking, it seemingly didn't fade - and stayed that way for millions of years - but that's a Schrodinger's cat issue I guess).

RW: Mark, I suspect Tugtupite uses the same mechanisms as sodalite for its photochromism. I don't know what the most effective wavelength is for bleaching Tugtupite, but it will be lower than the blue or UV light that darkens it.. Before you put the tugtupite on the shelf the repairable color centres were in equilibrium with the darkening and fading components of the illumination. That equilibrium gave a darker colour than the eqilibrium on the shelf. THat proves that tugtupite will fade in the dark. Your further observation of which I was unaware actually proves that your tugtupite will darken in the dark!!! The fact that it darkens on warming means that the repairable colour centres are thermally created and without light that is the only mechanism that could darken them in the dark. It is also a good warning to keep the temperature constant with these fading and darkening tests.Thanks!

MC: In my experience tugtupite does not darken in the dark. That is one mineral that I have a lot of. And it is stored in many places. Some of these places are in closed jewelry boxes. When I open them I must charge them with a light to see the tugtupite in many cases (gem tug retains the red color forever, just darkens when exposed to UV; non gem tug fades to a pure white, and stays white while in the dark - I am certain of that). As far as fading in the dark, I cannot address that with certainty. I just looked at some gem tug that I hadn't opened in years - still very red, but how much they faded I cannot tell as I did not record their color when I put them away. All this discussion makes me want to conduct some more testing, but 6 months or a year is a long time for this old man to maintain an his attention span...

RW: Mark, I was puzzled by your observation that afghan material bleached more slowly than Greenland material. I had not noticed this, but had noticed that some things darkened faster than others. At the risk of just making ad hoc assumptions, these rosy colour centres can be trapped leaving the specimen pernanently, pink, lavender or purple. By trapped I mean there is no easy place for the electron to come from or go to so this colour centre is not easily repairable. Possibly all the sodalite cages surrounding the trapped electron cage contain only Chlorines. Sulfur in an adjacent cage to a vacant cage seems a ready source for electrons to fill the vacant cage and make a repairable colour centre. The fast acting Greenland material is yellow green, different than the slower pale pink to lavender afghan material. Conventional wisdom has that it is electrons from S2 that make the repairable colour centres. The colorants may indicate different sources for electrons, some more mobile and faster acting than others.

MI: Ok, I have a report on testing with a yellow filter. I jammed half of an Afghani Hackmanite, already purple, into the end of a bathroom tissue tube and covered the other end with the lens of a pair of UV-blocking, yellow safety goggles and let it steep in bright sunshine (along with numerous other hand-covered attempts. Result: There is no doubt that it has faded far more than ever. I wonder how much more it would fade given stronger yellow light? Photo #1, the yellow-filtered, sun-faded face. Photo #2, half of the large crystal in the lower right has been re-exposed to SW UV and has turned dark purple. The other half is still faded from photo #1. That's the degree of fade, more than ever before. Photo #3, though out of focus (sorry, quick, handheld flash shots) shows the obverse of the piece and the typical color it displays.

MC: Rob - there is no particular white light color better than others. Yel/grn changes fast, green, white, and gray change equally fast. (GL sodalite)

MC: OK - I set up a test. This photo shows 8 piece of tugtupite that had been stored in the dark for years (I have no idea how many, they were inside a solid cardboard box in a drawer). This pic shows their color without any exposure to UV, only a white LED

MC: I then removed 4 of the pieces, wrapped them in aluminum foil and returned them to the box. I recorded the photography settings (iso, white balance, etc, and the date taken). I then exposed the remaining four to UV for 10 minutes, took a 2nd pic and then wrapped them up in foil and put them in the box. The box was then stored in a dark cabinet. Here is the pic of the tenebresced tugtupite:

MC: The dollar bill is just a constant. Remind me in a few months and I'll try to duplicate the pics and we'll see what changed.

MC: Michael - do your goggle block LW? Were all edges tightly sealed so that zero light can leak in. UV has a way of getting in everywhere and reflecting.

MI: Yes, they are the goggles I sell on my site as an accessory with my lights. I tested a pair before I bought inventory and they do indeed block UV very effectively. No, the sealing was only adequate, certainly not UV proof. I'll rig it up better to be well-sealed (the goggles aren't expecting to lie flat) as best as I can and try again. But the yellow filter clearly made a difference.

MI: Also, for the photos, I should have exposed the crystal to LW from my lights, instead of SW. It changes to a very dark purple that way. I will do that on the next round, easier to see the degree of fade.

MC: You're saying that LW causes a darker color than SW? I had never noticed that - very different from Greenland material.

RW: Thanks Mark!!! I'll be sure to remind you. From what you say I would expect the 4 you wrapped unexcited in foil and put away will show no change as they are already in equilibrium with the dark. The excited 4 will fade back to their equilibrium in the dark colour and in a couple of months will look like they did before exciting them. I suspect the permanently dark tugtupites just have a lot of trapped colour centres like the permanent purple sodalites. These tugtupites in equilibrium with no illumination will have a colour from both the trapped colour cetres and repairable ones.that are thermally excited. Can these dark adapted Tugtupites be bleached?

MI: Much darker, much faster. I'd shoot it but I've already wrapped it in foil for test #2.

MC: "Can these dark adapted Tugtupites be bleached? " - I'm pretty sure not, but obviously didn't test them. We'll try in a few months (remind me - I assure you in three months I