Multi-Analytical Spectroscopic Characterization and Color Mechanism of Mint Garnet
Jie Luo, Su–wei Yue, Li-wen Yuan, Kong Gao, Tong-yue PangBackground:
“Mint garnet” is a trade name used in the gem market for a light to yellowish- green variety of grossular characterized by moderate saturation and relatively high lightness. Its fresh, bright green appearance distinguishes it from more intensely colored green grossular, such as tsavorite. Although increasingly recognized in the trade, its mineralogical characteristics and the mechanisms underlying its color remain insufficiently constrained by systematic multi-analytical studies.
Methods:
Six mint garnet samples were investigated using conventional gemological testing, Fourier- transform infrared spectroscopy (FTIR), Raman spectroscopy, ultraviolet-visible absorption spectroscopy (UV-Vis), X-ray Photoelectron Spectroscopy (XPS), photoluminescence spectroscopy (PL), Electron Probe Microanalysis (EPMA), and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). These methods were applied to characterize the mineralogical identity, major- and trace-element compositions, valence states of chromophoric elements, and spectroscopic responses of the samples.
Results:
All samples are grossular-rich garnets in the ugrandite series, with minor andradite components. EPMA and LA-ICP-MS results indicate relatively high Fe and low Cr and V contents. The UV-Vis spectra show two major absorption bands centered at ~430 nm and ~600 nm. XPS provides qualitative near-surface evidence for Al³⁺, Cr³⁺, V³⁺, and mixed-valence Fe. PL spectra display Cr³⁺- related red emissions at 697- 716 nm, with intensities broadly correlated with green saturation.
Discussion:
The mint-green color is mainly controlled by combined Fe- and Cr-related absorptions. The ~430 nm band is interpreted as contributing to the yellow-green baseline color, whereas the ~600 nm band is associated with increased green saturation and more negative a* values. V appears to play only a subordinate role.
Conclusion:
The studied samples are grossular-rich ugrandite garnets with no detectable hydrous or secondary phases. Their mint-green appearance results primarily from Fe-related baseline absorption and Cr-related enhancement of green saturation. This study combines gemological, chemical, and spectroscopic evidence to clarify the origin of the color in mint garnet.