Mechanisms of Thermal Color Change in Brown Elbaite–Fluorelbaite Tourmaline: Insights from Trace Elements and Spectral Signatures

This study investigates the mechanism behind the heat-induced color change (brown to yellowish green) in Mn- and Fe-rich elbaite tourmaline under reducing atmosphere at 500 °C. A combination of analytical techniques including gemological characterization, electron microprobe analysis (EMPA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, and ultraviolet–visible (UV-Vis) spectroscopy was employed. Chemical analysis confirmed the samples as intermediate members of the elbaite–fluorelbaite series, with an average formula of X(Na0.66 □0.26 Ca0.08) Σ1.00Y(Li1.29Al1.10Mn0.31 Fe2+0.15Ti0.01Zn0.01) Σ2.87 ZAl6T[Si6O18] (BO3)3V(OH)3.00W(OH0.51F0.49) Σ1.00, enriched in Mn (17,346–20,669 μg/g) and Fe (8396–10,750 μg/g). Heat treatment enhanced transparency and induced strong pleochroism (yellowish green parallel c-axis, brown perpendicular c-axis). UV-Vis spectroscopy identified the brown color origin in the parallel c-axis direction: absorption bands at 730 nm (Fe2+ d–d transition, 5T2g → 5Eg), 540 nm (Fe2+→Fe3+ intervalence charge transfer, IVCT), and 415 nm (Fe2+→Ti4+ IVCT + possible Mn2+ contribution). Post-treatment, the 540 nm band vanished, creating a green transmission window and causing the color shift parallel the c-axis. The spectra perpendicular to the c-axis remained largely unchanged. The disappearance of the 540 nm band, attributed to the reduction of Fe3+ to Fe2+ eliminating the Fe2+–Fe3+ pair interaction required for IVCT, is the primary color change mechanism. The parallel c-axis section of the samples shows brown and yellow-green dichroism after heat treatment. A decrease in the IR intensity at 4170 cm−1 indicates a reduced Fe3+ concentration. The weakening or disappearance of the 4721 cm−1 absorption band of the infrared spectrum and the near-infrared 976 nm absorption band of the ultraviolet–visible spectrum provides diagnostic indicators for identifying heat treatment in similar brown elbaite–fluorelbaite.