Gemological Characteristics and Discoloration Mechanism of Color-Chang Pyrope-Spessartine Garnet

Garnet group belongs to nesosilicate minerals with A(3)B(2)[SiO4](3) as its standard chemical formula. Rarely, few garnets generate two distinct transmission windows due to the isotropic substitution of transition elements, such as Cr and V. This phenomenon leads to color-change as a variation of external light source, which produces purple-green and yellow-red colors under daylight and incandescent light, respectively. As the color-change effect is found in pyrope, spessartine, and pyrope-spessartine, we select the gem grade pyrope-spessartine in this study. Electron probe (EPMA), Fourier transform infrared (FT-IR) spectrometer, ultraviolet-visible (UV-Vis) spectrophotometer, and photoluminescence (PL) spectrometer were conducted on samples to understand the color-change mechanism. EPMA results showed that the samples could be defined as pyrope-spessartine, with few amounts of grossular and almandine. UV-Vis spectra showed that the tailing absorption band around 420 nm (belongs to d-d orbital transition of Mn2+, (6)A(1g) -> 4Eg) overlay with a wide absorption band of 571 nm (belongs to d-d orbital transition of Cr3+, (4)A(2g) -> T-4(2g)), forming two transmission windows in the blue-green and red regions. The samples excited by daylight and incandescent light produced cyan and orange colors, respectively, due to the alternative transmission between the above two regions leading to the color-change effect. UV-Vis and PL spectra results showed that the isotropic isomorphism mainly occurred on Mn2+, Fe2+, and Cr3+ when substituting in cubic voids and octahedral structures. And the strongest luminescence center of Cr3+ is located near 686 nm, which is also the reason why samples exhibit weak red UV fluorescence at long wavelengths. However, this fluorescence property is inhibited or replaced by Fe2+ for the strong fluorescence quenching effect. The color-change effect in samples was attributed solely to Cr3+.