Ok, from our geology group, I rounded up several crystal fragments of benitoite, all on the order of 0.5 to 1.0 gram. From these, we separated out pieces showing small veins of white natrolite. From what remained, we selected the four most transparent pieces. We tried our hand at color-grading, so that we thought the four pieces ranged from colorless, to light blue, to medium blue, to dark blue. The blue color of these is an indigo blue like a pair of blue jeans.
After color grading, we ran a "white light" transmission spectrum of each. The spectra of the four samples below have been normalized so that each has the same intensity at 700 nm.
Looks like our color grading worked ok. The more blue, the higher the peak in the 400-500 nm range, relative to the background in the 500-700 nm range.
Then we set each sample on top of a 250 nm LED, and tried to record a spectrum. I say "tried to record" because when you are recording zero counts, it is pretty difficult to know how to set the vertical scale of the graph. The bottom line is that we observed no fluorescence at all using the 250 nm LED.
Then we set each sample on top of a 290 nm LED. Bright, bright blue fluorescence! Beautiful. Here are the illumination spectra we recorded using 290 nm excitation. These intensities are normalized to the same collection time, so the light blue piece exhibits the lowest fluorescence intensity.
The fluorescence looks to be composed from two broad, overlapping peaks. Notice how the peak spacing seems a little larger for the colorless sample, compared to the other samples.
Then we set each sample on top of a 350 nm LED. In a dark room, we could see a little red fluorescence from the colorless sample, and no fluorescence from the other samples. It was very difficult to capture this fluorescence. Here is the illumination spectrum of the colorless sample that we recorded using 350 nm excitation.
We had to put the spectrometer's fiber optic right on top of the stone and so we couldn't filter out the UV light entering the fiber. The integration time was so long that the CCD suffered "bloom," where too much signal in one range of channels floods adjacent channels. The bloom spread from 350 nm all the way through to about 500 nm. Then at 500 nm you can see the tail of the LED light signal descending with increasing wavelength. But then up pops the red fluorescence peak with maximum around 650 nm.
First thing we can notice about these fluorescence examples is that they are pretty broad structures, spectrally speaking. I think this is often the case with most fluorescence... the narrow fluorescence exhibited by ruby is maybe the exception rather than the rule.
The next thing we can notice is that blue fluorescence only occurs for UV light in the 290 nm range. In a recent post, I showed the spectrum produced by a short-wave Hg-based lamp. Looking at that spectrum, about 96% of the intensity in that lamp is located around 250 nm, and then there is 1% or less located around 290 nm. So when people use a SWUV lamp to make benitoite fluoresce blue, 99% of that light does nothing, and only 1% or less is actually responsible for the blue fluorescence. So just imagine how bright benitoite looks when exposed to a bright 290 nm source. It was pretty nifty, I must say.
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