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Thick section of a tourmaline crystal cut
perpendicular to the optic axis. This section is thus not pleochroic.
The spectrum is shown below. The small absorption peaks in the visible part of the spectrum are produced by Mn3+ absorptions. These transitions give the pink color to the mineral. The intense absorption in the NIR at around 1300 nm, giving no contribution to the color, is probably due to Fe2+. |
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Highly pleochroic section of green
tourmaline. Spectrum is mainly made up of 3 peaks: 700 and 1100 nm being
due to crystal field transitions within Fe2+ ions and 420-460
nm to charge transfer band. Fe2+ in tourmaline structure can
replace either Al3+ or Mg2+. The symmetry of these
sites being different, they give rise to transitions at different
frequencies. The triplet peak around 1420 nm is produced by the vibration overtones in hydroxyl radicals. |
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