Characterisation Of Different Tourmaline Species Using Raman Spectroscopy

Tourmalines represent one of the most chemically and structurally complex mineral supergroups, with their compositional variety making them remarkable recorders of geological processes and extremely important materials in gemmological studies. This thesis emphasises the characterisation of various tourmaline species—specifically, dravite, schorl, and elbaite—using Raman spectroscopy to establish quantitative connections between the Raman spectrum parameters and chemical composition. Raman spectroscopy, due to its non-destructive properties and high spatial resolution, was used as the principal analytical method. Comprehensive spectrum investigations were conducted on several tourmaline samples to examine the correlation of distinct Raman bands—particularly in the low-frequency range (200–450 cm⁻¹) linked to YO₆ and ZO₆ octahedra—with cation occupancy at the Y-sites. Additional examination of the OH stretching region (3400–3770 cm⁻¹) elucidated influences of the local structural context on the hydrogen bonding. Complementary analytical methods were used to corroborate and enhance Raman-based findings. SEM-EDS (Scanning Electron Microscopy-Energy Dispersive X-Ray Spectroscopy) and XRF (X-Ray Fluorescence) measurements were employed to ascertain elemental compositions and validate Y- and Z-sites occupancies. UV-VIS-NIR absorption spectroscopy was used on selected gem-quality Paraíba-type tourmalines to correlate optical absorption patterns with chromophore concentrations. LIBS (Laser-Induced Breakdown Spectroscopy) facilitated quantitative analysis of light elements, especially lithium, yielding more accurate data on the composition of Li-tourmalines. The enhancement and testing of results were achieved using portable Raman spectroscopy to establish the possible application as well for in situ analysis, µXANES (X-Ray Absorption Near Edge Structure) for assessing Fe oxidation states, and ab initio calculations of vibrational modes to support peak assignment and mode interpretation. These methods validate the accuracy and applicability of Raman spectroscopy as the primary diagnostic instrument for tourmaline characterisation. Overall, this thesis presents a comprehensive multi-technique framework centred on Raman spectroscopy for the identification and compositional differentiation of tourmaline species. The results underscore the ability of Raman spectroscopy to deliver quantitative chemical information, hence affirming its significance in geological and gemmological applications via a non-destructive analytical method.