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:
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 will have no idea what you are talking about, won't be able to find the box I put away, and will appear to be a blithering idiot (the box is behind me in my bookshelf cabinet). ;-)
MI: No time now to post the photos now but the results of yellow filter test #2 are spectacular. Lots of fade. So yellow light with no UV component does it!
RW: Thanks Michael. Good to hear! Have you got a tugtupite to check in yellow light? I have no idea what wavelength is most effective at bleaching tugtupite.
MC: Tugtupite does not bleach very quickly - lemme know if you find a way....
RW: Me too! Mark, I just noticed your post where you did things right to look for fading and darkening in the dark ("yes to both questions"). This is puzzling. At first I didn't believe the old literature reports of darkening in the dark. There's a wonderful one in the Lovozero book. I'll follow up on this. .
RW: I'm not sure the pink/purple that fades after fracturing was ever in the rock. I think those fading colours were produced by the fracturing and quickly heal as the fading shows. But how to test this. It's a little like wondering whether the light is on or off when the fridge door is closed. At least in that case the bulb would be warm if it was on, but here???
MC: Hey guys - I did some tests - tenebrescence and heat..... Here are the results: Three months ago there was a discussion about tugtupite tenebrescing when stored in complete darkness. I took several specimens and stored them in aluminum foil, inside a closed box and inside a closed cabinet. The pics below show the result. The first pic shows their color immediately when removed from the box in a very dim room. The second pic shows their tenebrescence once exposed to 30 seconds of UV. It is clear that tugtupite does not deepen in color when stored in the dark, and - in fact - fades very slowly. The most telling example is the piece in the upper part of the right grouping of 4; it faded almost completely in storage. The two pieces in the left group were stored without any UV applied before storage, while the four pieces on the right were stored in their fully tenebrescent state. The "gemmiest" tugtupite retain their color the best (expected).
DB: Mark, where were these stored? Inside where the temperature is stable or outside where it cycled from hot to cold or was simply really hot
MC: Stored inside my house with the AC on. Average high/low = 77f low, 81f high, 45% humidity avg
MI: Well, you have to climb into the refrigerator, of course! I have Tugtupite from Howie but I wasn't aware that it was tenebrescent. I thought only the associated Hackmanite in the same pieces was tenebrescent. But I'll be checking. I will try the yellow light test on GL material as well.
MI: Here are the #2 yellow filter test photos. #1, the exotic test rig itself, representing the years of careful engineering and grant money that were poured into it. #2, the fade on the face exposed to yellow sunlight for over 1 hour, #3, half of the large crystal on the bottom, center has been re-exposed to strong LW UV for just a few seconds to demonstrate the surprising amount of bleaching. Huzzah!
MC: So the conclusion is that filtered sunlight will fade Afghan soda. How can we tell if the yellow filter has any effect? But one hour??? GL sodalite fades in one second of bright sunlight. I'm impressed with the LW tenebrescence and want to see a comparison of SW and LW induced tenebrescence - I have some Afghan specimens coming so I will try to test on them...
MI: I had forced that piece of material to sit in the sunlight for hours when I first got it in a desperate attempt to get it to fade and it totally refused. Only with the yellow filter, allowing all the yellow light possible from the Sun, while excluding all UV, did the trick. Today I let it sit for well over an hour just because I wanted to be sure of a change. That fading may have happened in the first few minutes or less, don't know.
MC: But how do you know that a yellow filter is necessary? Would just a simple piece of OP3 suffice? (OP3 blocks 400nm and below)
CS: no, you are seeing a slightly quicker rate of LR UV knocking out electrons and creating the color, than excited states decaying to groundstates, which leaves your specimens "blanched ". i have noticed this in Hackmanite associated with Winchite, and other gemmy pakistani hackmanites.
MI: From what Rob was saying yesterday about yellow light being the most effective at bleaching. Green laser had no effect. Full sunlight only results in the state of equilibrium between that particular piece's concurrent bleaching/darkening abilities. OP3 sounds interesting but what I had were UV blocking yellow goggles, quite fortuitously, too.
MC: I just received a few pieces of Afghan hackmanite. They are all very LW tenebrescent (change color under LW easily and deeply). With this thread in mind, I immediately took them outside in the bright sunshine and placed them under a piece of OP3 (<400nm blocking). They all faded in minutes. Conclusion - any UV free broadband bright light will fade them. (my pieces are both blue and purple tenebrescent)
MI: But I would expect some range within sunlight to be more effective than another at bleaching, guessing that more towards the red is more effective and towards blue less so because...just because. I have SLR UV filters in my old Minolta box o' tricks. Is that comparable to OP3?
MC: I would expect a photographic UV filter to be the same as OP3 but to be certain you would have to check the graphs. I just brought the rocks in from outside (after exposing them to sunlight again and letting them get dark) and used my halogen light to fade them. Quick conclusion - quick fade, not much different from Greenland sodalite in the speed. Not sure if this is true for all varieties of Afghan sodalite (I know some don't fade at all!) - but these pieces act much like Greenland, but they are not as dark/not as dramatic a change.
MI: See, my halogen, with a glass cover, won't fade my Afghan stuff at all. Possibly too much UV leak through the open sides of the glass bulb protector. Going to go try the same tube experiment as the day before with the camera daylight filter and see what happens.
MC: Check your UV filter on the halogen. Try the photo filter. Put the halogen almost touching the sodalite. Don't worry about the leaking light (I I don't).
MI: Had to dig through an interesting box of stuff. My lenses all have 1A filters but tested, they do not block LW at all. I used my UVEX clear goggles, which do block LW. With two to three minutes of exposure, I can see no perceptible bleaching. I re-exposed half a crystal to LW and covered half of it with electricians tape. Then I used the same yellow goggles from before. Got it to bleach but the change is very slow. Not sure if 10 minutes under clear would do the same thing but I'm losing the sun now. It seems, at least with the piece I have, that yellow light does do better.
MI: Yes, the clear UVEX goggles did create some bleaching but not as much as the yellow. The yellow test from two days ago did have the piece in the sun for a good hour and it was bleached more than is is now. Verrrrrrrrrrrrrry slow to change!
MC: One of my new Afghan hackmanites - fades rather quickly using halogen. Anybody know what the massive matrix usually is? (I'm thinking massive richterite)
RW: The non fluorescing brown is richterite. I've forgotten what the green SW mineral is. I'll try to remember. MC: Interesting way to kill time on a Sat night.... On another thread we got into a discussion about tenebrescence and if heat can cause a mineral to darken without any type of light. I decided to conduct a very unscientific test, but got some results so I'll post them here. Several pics: the first shows the 6 minerals I picked to be tested. These minerals had been stored in a drawer and have not seen any UV light for several years. 4 pieces of tugtupite, two pieces of sodalite - all from Greenland. The first pic shows the minerals, on a ceramic tile, at the start, only LED light applied to take the photo. The 2nd pic shows the same minerals after 10 minutes of constant exposure to a high power heat gun (on low setting). The 3rd pic shows them after another 10 minutes of exposure, but on high setting. This heat gun is used to shrink tubing and is very hot; it can make a mineral glow with heat. After heating you can see that there was no change in the hue/tint of the minerals (minor lighting change from the reflection off the heat gun as I moved it around between pics). No change to the tenebrescence. I then turned on my SW UV lights and let the tenebrescence set in. The 4th pic shows the tenebrescence after exposure to SW UV - mostly normal except for the red sodalite. It did not darken as much as I would have expected (no way to test this beforehand; the specimens would not have been virgin if I did that). The 5th pic shows the fluorescence under fullwave (SW+MW+LW). This was the big surprise; nothing seemed affected except the red sodalite. Although its ability to tenebresce wasn't affected, the heat *destroyed* the fluorescence (normally a bright rusty orange). The 6th pic was just SW - as expected. Conclusion: Heat does not cause a color change. The heat I applied under this test did not affect the tenebrescence but it did destroy the fluorescence of one item. I believe that more heat would have destroyed all of the fluorescence, as well as the tenebrescence; but I do not have the ability to do that right now (I'll try my self-cleaning oven one day). Unscientific for sure, but interesting.....
MC: On the plus side, I've learned how to make non-fluorescent hackmanite
DB: Mark, you neglected to tell us whether you paused between photos to let the rocks cool. You also didn't tell us the temperatures (get an IR thermometer. Really cool for the kitchen and screwing around). Anyway, remember the experiment I mentioned with liquid nitrogen? We also made the minerals hot. They were soaked in the oven at 390 degrees (200C) for at least an hour. The hypothesis of the experiment was that the rocks would fluoresce better at cold temps than at hot temps. Indeed, this was ALMOST universally true. The sodalite, agate, calcite, fluorite and petrified wood tested were all dimmer and the amazonite and other feldspars were basically dark. Frankline caclite didn't really care, though it was probably a little dimmer. Corundum, on the other hand, got brighter when heated. Here is a graphic summarizing the results:
DB: So the question is, did you test the sodalite hot? If so, did the fluorescence recover when it returned to room temp?
MC: No - I did not pause to let the rocks cool for the heat pics. But the last photo was taken after a good hour and everything had cooled off. Even now, a few hours later - the fluorescence is broke on that one rock. Alas, it no longer fluoresces.
MC: But this wasn't a test for fluorescence, but tenebrescence. The destroyed fluorescence was just a side note....
DB: Hmmmm. My rocks regained their fluorescence. The question is, how hot were your rocks?
MC: Very hot - (unscientific as I said). Not shocked by this - I have heated many pieces to a point of non-fluorescence.