Gemmological and Mineralogical Characteristics of Coloured Tourmaline from Minas Gerais State,Brazil

The gemmological and mineralogical characteristics of the different coloured tourmaline samples from Minas Gerais State, Brazil, are studied by using the electron microprobe (EMPA), X-ray photoelectron spectrometer (XPS), ultraviolet-visible spectrophotometry (UV-VIS) and Fourier transform infrared (FTIR) spectroscope.The results show that the contents of Al₂O₃ and Na₂O are relatively higher and the content of FeO is relatively lower, with a certain amount of MnO and Cr₂O₃ in the samples by EMPA, which are primarily approved as Li-tourmaline by XPS.Based on the analysis of chemical compositions and UV-VIS transmission spectra, the one of the colouring factors of the samples is related to the trace elements such as Fe²⁺, Fe³⁺, Mn³⁺, Ti⁴⁺ and Cr³⁺.The IR Absorption spectrum results show that the strong stretching vibration band at 1200~1450cm^-1 is induced by B-O group for proving BO₃ radical and the double stretching bands at 3000~3750cm^-1 are respectively induced by H₂O and hydronium.Comparing with the IR Absorption spectra of the samples, it is indicated that the spectrum frequency bands with their positions are due to the isomorphism and chemical compositions in the different coloured tourmaline samples.