Gemological and Spectral Characteristics of Andradite Garnets with Usambara Effect from Yuanjiang in Yunnan Province

Yuanjiang County is one of the most important gem-producing areas in China. The authors of this study discovered and collected gem-quality andradite Garnsts in the epidote amphibolite from the periphery of the ruby deposit in Shaku Village, Yuanjiang County. After careful observation of the collected andradite, it was found that these andradite samples appear green when the thickness is less than 2 mm and reddish-brown when the thickness is greater than 2 mm, exhibiting the typical Usambara effect. To investigate the gemological and spectroscopic characteristics of Yuanjiang andradite, this study conducted basic gemological tests, microscopic observation, electron probe microanalysis (EPMA), ultraviolet–visible (UV-Vis) absorption spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and laser Raman spectroscopy on the collected samples. Tests show that Yuanjiang andradite has a lower specific gravity than typical andradite, which is due to the presence of epidote inclusions inside. EPMA results indicate that the samples contain a certain amount of Cr element. The crystal chemical formula of the samples calculated from the EPMA results is (Ca2.89–2.93, Mn0.01–0.02, Fe0.15–0.10)(Fe1.69–1.95, Al0.00–0.23, Cr0.00–0.23, Si0.05–0.08)(SiO4)3. UV-Vis tests show that the samples have transmission windows in both the green- and red-light regions, with Fe3+ and Cr3+ acting as the main chromogenic ions, among which Cr3+ is crucial for the occurrence of the Usambara effect. The FTIR and Raman test results are basically the same as previous research results, but some peak positions related to metal cations differ from the theoretical values, which may be caused by the presence of a certain amount of Cr3+ in the samples.