Crystalloluminescence
Emission of light during crystallization.
What is crystalloluminescence?
There are a lot of different types of luminescence, where light is emitted by some process, e.g., chemiluminescence where light is given out by a chemical reaction. Crystalloluminescence (XTL) is light given out during crystallization.
Why have I not heard of this before? Is it important?
Remarkably the earliest reports of XTL go back as far as 1787! In olden times chemists were much more likely to be working in dark conditions, and so phenomena involving emission of light may have been easier to see. Brandowski was the first to report XTL of NaCl in 1894. The importance of XTL is that each emission of light represents the birth of a new crystal, and therefore can tell us something about the dynamics of the nucleation process. The animation below illustrates the flashing emissions for NaCl.
How strong is the crystalloluminescence?
The luminescence is very weak. In order to see it you need to get your eyes well accustomed to the dark. The image below shows XTL of NaCl taken with 15 s exposure using a compact digital camera [Canon S90, f/2, ISO 3200]. To create a rapid crystallization, concentrated HCl (24 wt. %) is mixed rapidly with slightly undersaturated (98%) NaCl solution.
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