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

Apichate MANEEWONG; Kanwalee PANGZA; Nongnuch JANGSAWANG; Tasanee CHAROENNAM

Authors

Apichate MANEEWONG Gems Irradiation Center, Thailand Institute of Nuclear Technology (Public Organization), Nakhon Nayok 26120
Kanwalee PANGZA Gems Irradiation Center, Thailand Institute of Nuclear Technology (Public Organization), Nakhon Nayok 26120
Nongnuch JANGSAWANG Gems Irradiation Center, Thailand Institute of Nuclear Technology (Public Organization), Nakhon Nayok 26120
Tasanee CHAROENNAM Gems Irradiation Center, Thailand Institute of Nuclear Technology (Public Organization), Nakhon Nayok 26120

Keywords:

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

Abstract

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 H^o by the e-beam irradiation. Green tourmaline did not show any color change with the heat treatment or the e-beam irradiation.

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