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.

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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.