Conclusive comparison of gamma irradiation and heat treatment for color enhancement of Rubellite from Mozambique

Abstract

Rubellite, one of the most attractive members of the colorful family of tourmalines, has its colors ranging from fine red hues to pinkish and violetish nuances. Some of them are very light while others are so dark that the real hue can only be identified when the stones are held against the light. Fine jewelry-grade rubellites are relatively rare; therefore, color enhancement are normally required on natural samples. In present study, pink tourmaline samples from Mozambique were enhanced using two methodologies, i.e., gamma irradiation and heat treatment. The samples were characterized before and after the color enhancements. Conclusive remarks were drawn upon the modification of their color appearance, optical absorption, and oxidation states of the chromophores. After preparation, the samples were divided into two groups: The first group were irradiated with 400, 500 and 600 kGy of gamma while the second group were heat treated at 350, 450 and 550 °C. After irradiation, the slightly pink tourmaline samples turned saturated pink. The color of heat-treated samples, however, were faded from initially very slightly pink to colorless with increasing temperatures. It was noticed that both gamma irradiation and heat treatment at current conditions did not affect any inclusions in the specimens. FTIR spectra of untreated samples displayed several bands at 3300-3700 cm−1 for OH-stretching, 5200 cm−1 for H2O and 4200–4400 cm−1 for Fe-OH. After both treatments, all bands were decreased, especially, for the OH band. UV–vis-NIR spectra of the untreated samples displayed the absorption band at 520 nm related to Mn3+. Upon treatments, this band was increased with peak height depended on the treatment conditions. Mn K-edge x-ray absorption spectra of the samples exhibited the shift of Mn oxidation to a higher number after both treatments. © 2019 Elsevier B.V.