Optical Anomalies in Spessartine from Wushan Granitic Pegmatite, Fujian Province, China and Their Response to Fluid Activity

Gem-quality spessartine from the Wushan granitic pegmatite in Zhangzhou, Fujian Province, China, commonly exhibits anomalous extinction under cross-polarized light, deviating from the complete extinction typical of cubic minerals. This study investigates the relationship between these optical anomalies in spessartine and late-stage pegmatitic fluid activity using polarizing microscopy, electron probe microanalysis (EPMA), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) coupled with multichannel fluorescence imaging. The results indicate that: (1) Wushan spessartine samples are dominated by Mn as the principal X-site cation, with a spessartine end-member content of 82%-95%. A negative Fe-Mn substitution correlation is observed among samples of different colours, and their lattice misfit parameter (M=0.09-0.45) suggests varying degrees of cation site occupancy distortion. (2) FTIR spectra of intergrown spessartine show distinct OH absorption bands in the 3 700-3 000 cm^-1 region, with markedly stronger signals than those of euhedral single-crystal spessartine. (3) Raman spectra reveal double-peak splitting in the 172/174 cm^-1 and 222/226 cm^-1 region, indicating differentiation in the X-site coordination environment and a consequent reduction in crystal symmetry. (4) SEM-EDS analysis detects local enrichments of F element, while multichannel fluorescence imaging shows OH distributed linearly along microfractures. Together, these lines of evidence demonstrate that the optical anomalies in the Wushan spessartine are closely associated with the infiltration of late- or post-magmatic volatile-rich fluids along fractures. The incorporation of volatile components (e.g., F, OH) into the crystal lattice leads to a deviation from the ideal cation coordination. This study elucidates the modifying effects of fluids on garnet crystal chemistry. The systematic deviation in this cation coordination environment provides direct gemmological and mineralogical evidence for late-stage fluid modification of garnet crystal chemistry, offering a new perspective for tracing fluid activity during the late evolution of granitic pegmatites, China.