Colour Origin of Dark Green Tourmaline from Zambia

A series of tests have been conducted on dark green tourmaline crystals from Zambia, including conventional characteristics, LA-ICP-MS, EPMA and microscopic polarized UV-Vis absorption spectra in order to fully understand their colour origin. Electron micro-probe results indicated that, mass fraction of Al₂O₃ and SiO₂ are high in the tourmaline samples, and a certain amount of FeO_T,MnO,TiO₂ and Cr₂O₃ are contained, while FeO_T is relatively higher. EPMA results showed the content of the trace element of different directions and colour zones. Dark green area contains more content of Fe. Microscopic polarized UV-Vis absorption spectra showed that all samples have strong absorption bands centered at 415 nm and 720 nm. The absorption band near 415 nm is assigned to charge transfer between Fe³⁺-Fe³⁺, as well as Mn²⁺. The absorption band near 720 nm is usually ascribed to the charge transfer between Fe²⁺-Fe³⁺. The intricate interplay between these two bands is the main cause of the colour that we observed.