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I recently stumbled on the concept of thermoluminescence, specifically when minerals or stones emit visible light after being heated. However, information about one aspect seems a bit unclear: the color of the light. For instance, I found images of chlorophane fluorite emitting a green glow. Two thermoluminescent samples of red sodalite, however, were shown glowing different colors. One was more purple and the other yellow.

I doubt the colors are totally random, so what are the rules at play here? I found one description of calcite having a strong orange glow “when doped with manganese.” Another source observes the glow shifting from blue to red depending on temperature for calcite and dolomite while aragonite is a steady blue. So it sounds like chemistry plays a role in creating the color and maybe also the level of heat applied, but I'm not getting a clear sense of how. What variables would you look at to predict the color emitted by a thermoluminescent stone?

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Essentially all of it is a matter of molecular physics, just like with all other types of luminescence. The only difference is where the energy is coming from, how it’s retained, and how it’s released.

The common case for thermoluminescence works as follows:

  1. The sample is struck with some form of electromagnetic or ionizing radiation.
  2. The energy imparted in step 1 of the process causes some of the electrons in the sample to enter an excited state. This functionally ‘stores’ the energy temporarily.
  3. Because of the specifics of the crystal structure of the sample, some of the electrons that were excited during step 2 of the process get ‘stuck’ in their excited state.
  4. Heating the sample at some later point in time provides just enough energy to release those stuck electrons.
  5. The excited electrons then rapidly drop back down to the ground state, emitting photons in the process.

If you remove steps 3 and 4, you have a concise description of how fluorescence works (real fluorescence, not phosphorescence, that’s something different and AFAIK cannot be replicated in a thermoluminescent material). Thermoluminescence can thus be though of as a kind of ‘delayed’ fluorescence.

And just like with fluorescence, the exact energy of the excited states of electrons involved exactly dictates the frequency of the electromagnetic radiation that gets emitted.

That, in turn, is a function of both the atomic properties of the specific elements that comprise the sample, the structural properties of the crystal (to some extent this can limit possible excited states), and the amount and energy of the radiation that charged the sample. Note that there is no real ‘chemistry’ involved here beyond the composition and possibly the molecular structure. If there was, you would be looking at thermally catalyzed chemoluminescence, not thermoluminescence.

However, there are other things that can matter. Three big ones that come to mind are:

  • Time spent heating the sample. Thermoluminescence is only releasing energy stored in the crystal already. Eventually all the electrons will end up back in the ground state, and you won’t get any further thermoluminescence. And in practice, the sample will stop visibly glowing long before that happens, because there’s a lower threshold of luminous intensity below which the human eye cannot detect photons. If multiple different energies of radiation charged the sample originally to different degrees, you could in theory also see the color shift over time instead of just fading.
  • Exact temperature of the sample. Above around 500 °C, black-body radiation starts to matter, because at that point it’s shifting into high enough frequencies to be visible to humans. Get the sample hot enough, and that will completely overpower any thermoluminescent effect.
  • Candoluminescence. This is a different effect from thermoluminescence whereby some materials have much stronger emissions at certain frequencies than normal black-body radiation would suggest should be the case. I am not aware of any thermoluminescent minerals that are also candoluminescent (candoluminescent materials tend to be rare-earth or transition metal ceramics, like thorium dioxide or zinc oxide), but if such a material exists, it’s possible that the candoluminescent effect could also overpower any thermoluminescent effect.
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The light emitted by a mineral due to thermoluminescence is caused by electrons shedding extra energy obtained during radiation exposure; this can be in the form of heat, light, or ionising radiation. The colour emitted depends, obviously, on the peak emission spectrum of the material under the conditions of emission.

The conditions of emission are set by two factors: the mineral in question and its exact state of excitement. Thus, a chemically identical sample that has absorbed a little energy, and is therefore not particularly excited, will glow with a different peak emission than one that has absorbed large amounts of energy and is in a high state of molecular excitement. The temperature a sample is heated to is not as important as the amount of "heat work" and thus the amount of energy the sample has absorbed.

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