Spectral Characteristic and Provenance Discrimination of Cu- and Mn-Bearing Purple Tourmaline from Mozambique

Mozambique stands as one of the world's most significant tourmaline producing regions, particularly renowned for its large, vivid-coloured, and high-clarity crystals, notably Paraíba-type and purple to purplish-blue tourmalines. This study investigated 7 faceted pink-purple and blue-purple tourmaline samples from Mozambique using conventional gemmological tests, Fourier-transform infrared (FTIR) spectroscopy, laser confocal micro-Raman spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and electron probe microanalysis (EPMA). The results indicate that the samples exhibit refractive indices ranging from 1.620 to 1.640, birefringence from 0.018 to 0.020, and relative densities from 3.01 to 3.04, with all samples displaying strong dichroism. Hydroxyl (OH) vibration peaks were detected in both infrared and Raman spectra, indicating the hydrothermal origin of these lithium tourmalines. UV-Vis absorption spectra reveal that this type of tourmaline primarily exhibits absorption characteristics attributable to Mn and Cu ions: absorption peaks at 490 nm and 540 nm are assigned to the ⁵E_g→⁵T_2g splitting of Mn³⁺, whereas a broad absorption band at 690 nm is attributed to the ²E_g→²T_2g splitting of Cu²⁺. Compositional analysis further demonstrates that the colour of the samples is controlled by the Mn/Cu mass fraction ratio: a larger ratio corresponds to a more pronounced purple hue, while a smaller ratio results in a more distinct blue hue. Crystal-chemical classification indicates that 6 samples (A1-A6) are classified as Elbaite (a lithium tourmaline) of alkali group 2, while sample A7 is assigned to the Rossmanite subgroup of vacancy group 2.SPSS discriminant analysis demonstrates that a model based on oxide mass fractions effectively distinguishes tourmaline samples from Brazil, Mozambique, and Nigeria, outperforming a trace-element model and exhibiting a certain degree of stability.