Trilobite Paciphacops eye fluorescence

This is the luminescence spectrum of a single trilobite schizochroal eye. The lenses of the eye are calcite crystals grown so the the c-axis of the crystal (the direction that does not produce double refraction) is oriented along the optical axis of the eye. It has been discovered that incorporation of magnesium behind a shaped surface at the rear part of the crystal eye improves the image quality by changing the refractive index and so reducing 'spherical aberration'.

This eye - in a fossil of a Paciphacops trilobite from the Devonian - is nearly 400 million years old and I was interested to see if I could learn anything about the impurity composition of the calcite. Since the crystal fluoresces when illuminated by UV radiation and the spectrum of the emitted light carries information about the crystal composition, I carried out this experiment.

Under a low-power microscope, I oriented the 'turret' of the compound eye (the turret is about 5mm in diameter) so that I could illuminate a single eyeball (about 250 microns in diameter) with a sharply focussed 404nm laser as shown in the images. Through a yellow filter (to remove the scattered violet laser light) I collected the fluorescent light with the 400 micron fibre of my JAZ (350-1000nm) spectrometer to give the spectrum shown as the thick(er) purple line. This spectrum is on a wavelength scale accurate to around 0.3nm and is calibrated in energy flux per unit wavelength (not simply in counts from the spectrometer). This spectrum is highly reproducable and even the small spectral features (above the statistical noise) are real.

The other spectra, in red, green and grey, are the fluorescence from other pieces of calcite that I have. One of them (the red line) matches the eye spectrum very closely. This sample is shown in: www.flickr.com/photos/bob_81667/6042880574/
The other two samples, one a clear 'rhomb' shown in: www.flickr.com/photos/bob_81667/6132716912/ , and a sample from Terlingue (USA) given to me by Gérard Barmarin, have rather different spectra.

My next task is to interpret the fluorescence spectra and see what they are telling me about the composition.

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