Electronic and vibrational spectra of tourmaline – The impact of electron beam irradiation and heat treatment

Abstract

Irradiation and heat treatment were performed on tourmalines of various colors from Antandrokomby, Madagascar. The samples were irradiated with 10MeV electrons to fluencies of 2 ×1017cm−2 for 1h and were heated at 550°C for 3h in air. Their electronic and vibrational spectra were investigated by UV–vis, mid-infrared, and WD-XRF spectroscopy for comparison to pristine samples. Changes in the Mn3+ ions after irradiation resulted in darker pink tourmalines, which had absorption peaks at 390 and 520nm. These samples became colorless after subsequent heat treatment. After irradiation, colorless, light blue and yellow tourmalines displayed a new absorption band at 365nm. Alteration of the stretching absorption bands and wavenumber after irradiation could be explained by the following reactions:OH−+e− beam irradiation→O−+H°,Mn2++e− beam irradiation→Mn3++e− andFe2++e− beam irradiation→Fe3++e−.Stretching vibration of the BO3 structure occurred at 1330cm−1, while the SiO vibration absorption bands were assigned to around 1100cm−1. Colorless, green, and yellow tourmalines showed high-intensity peaks around 3608 and 3505cm−1 after irradiation. Pink and dark green tourmalines showed low-intensity peaks at 3605 and 3585cm−1, respectively. The combination modes of stretching and bending in the range of 4600–4300cm−1 were split after irradiation and heat treatment, and different color changes occurred after irradiation.