Color enhancement of tourmaline by electron beam irradiation

Abstract

Tourmaline is a semi-precious stone that is quite popular in gem market. Red-purple tourmaline, so-called “Rubellite”, is particularly famous. Consequently, pink tourmalines have recently been enhanced by gamma radiation to intensify color as rubellite. In this study, an electron beam technique was applied for experimental treatment. Moreover, gamma ray irradiation was also applied to treat the same sample collection for comparison. Color enhancement was designed for both radiations at three levels of doses, i.e., 400, 800, and 1,200 kilogreys. Pink and green tourmalines from Nigeria were collected for the experiment. Each sample was cut into two tablets for experimental irradiations using e-beam and gamma ray. Based on chemical compositions, these samples were identified as elbaite. The UV-Vis absorption spectra (300-850 nm) of pink tourmalines showed the maximum absorption band in blue-green range around 515 nm, which was caused by Mn³⁺ yielding pink color. The minor absorption band was also observed in purple range (about 395 nm) causing yellow which related to Mn²⁺. As the results, electron beam irradiation produced more intense pink which absorption bands at 395 and 515 nm were intensified after irradiation. Therefore, e-beam irradiation provides more efficiency for intensification of pink color in tourmaline. Moreover, higher Mn/Fe ratio in sample yielded more intense pink after irradiation. Two color centers, O- center (hole color center) and H⁰ center (electron color center), appeared in pink tourmalines after irradiation . For green tourmalines, yellow shade appeared slightly after both irradiation techniques. The irradiated green tourmalines after both types of radiation presented only O- center signal in EPR spectra. Therefore, it can be assumed that the intense pink color after irradiation is probably related to H⁰ center (electron color center) with involvement of Mn impurity. On the other hand, yellow tonality produced after irradiation is caused by O- center (hole color center).