Chemical Composition and Spectroscopic Characteristic of Tourmaline with Usambara Effect

As a rare optical effect, Usambara effect has been well recorded in tourmaline, but knowledge on the chemical and spectroscopic characteristics of tourmaline with this effect is still to be improved. In this paper, a batch of tourmaline showing Usambara effect is studied using conventional gemmological testing instruments, laser ablation inductively coupled plasma mass spectrometer, infrared spectroscopy, micro-UV-Visible spectrometer and fluorescence spectrometer.The experimental results show that the colour of these tourmaline samples with the Usambara effect is dark green. When multiple samples are overlapped and transmitted by a light source, the overall colour of the visible transmission area changes to red, which is consistent with the Usambara effect. The results of laser ablation inductively coupled plasma mass spectrometry indicate that these samples belong to magnesium tourmaline with high Cr content as trace element. The infrared spectra show a strong absorption peak of magnesium tourmaline at 3 569 cm^-1. The ultraviolet-visible absorption spectra show that the colour of the Usambara effect tourmaline samples is closely related to the spin-permitted electron d-d transition and the weak spin-forbidden absorption of Cr. The results of fluorescence spectra show that there are three luminescent centers in the visible range caused by Cr³⁺ multiple forbidden transitions, which produce strong red fluorescence.