Effect of Heat Treatment and Electron Beam Irradiation Tourmaline: UV-Visible, EPR, and Mid-IR Spectroscopic Analyses



The origin of the color change in 3 types of tourmaline gemstones, 2 pink samples from Afghanistan and one green sample from Nigeria, was examined by using UV-Vis, FTIR, EPR, and EDXRF spectroscopy. After heating at a temperature of 600 °C in air, the color of pink tourmaline gemstone changed to become colorless. This colorless tourmaline recovered its pink color by being irradiated with 800 kGy of electrons (e-beam). The UV-Vis absorption spectrum of the pink tourmaline with higher Mn concentration (T2, 0.24 wt%) showed characteristic absorption peaks originating from the Mn3+ color center: 2 absorption bands centered at wavelengths of 396 and 520 nm. Both absorption bands disappeared after heat treatment in air at 600 °C, and then displayed again after e-beam irradiation at 800 kGy. EPR spectra of T2 showed that the color change was related to the valence change of Mn3+ to Mn2+ ions and vice versa. The pink tourmaline of lower MnO content (T1, 0.08 wt%) also changed to being colorless with heat treatment, but the color was not recovered by e-beam irradiation. Instead, a yellow color was obtained. UV-Vis and FTIR spectra indicated that this yellow color originated from the decomposition of the hydroxyl group (-OH) into O- and Ho by the e-beam irradiation. Green tourmaline did not show any color change with the heat treatment or the e-beam irradiation.


Tourmaline, heat and electron treatment, UV-Vis, EPR, FTIR

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