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After some years I got an email response from Todd Peters.
Todd wrote:
It sounds like the purple state is the low energy state where all the electrons have settled back down into their lowest energy orbitals. Almost like fluorescence, but with a very long decay rate. This makes me wonder if you could get those electrons up into yet a higher state with an intermittent (low heat/non destructive) UV laser and perhaps find a color past the whiteish.
Are you able to do any experiments with a UV/Vis spectrometer? If so, you may see some wild phenomena happening. So I did a quick search and first result: https://www.fluomin.org/uk/fiche.php?id=815 So much for my theories
The reverse hackmanite is interesting too. It sounds like heating drives the water out. Perhaps heated samples will return to normal when re-hydrated? Or, oxides are being formed and no hope for reversal. There is something in the back of my mind that is telling me that I am missing something about this... the connection to an industrial application; perhaps a photodiode sensor or something like Transitions lenses (https://www.transitions.com/en-us/). I wonder if the conductivity/resistance varies too, beyond just binary.--that would have some applications for sure. Or, if you really want to blow your mind check out "Surface Plasmon Polaritons" as it seems to be a good fit to this tenebrescence situation. The no-heating constraint is a tough one.
We have discussed the topic before. With hackmanite and also tenebrescent zircon. Hackmanite: Purple color is achieved by leaving the stone in the dark for an extended period, or exposure to SWUV. Addition of heat kills the tenebrescent property for good
Photo by Robert Weldon from an article by Eloise Gaillou and George R Rossman Color in Natural Diamonds: The Beauty of Defects Which can be downloaded on Research Gate. (Contact me if you do not have a membership)
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