Chemical Composition and Spectra Characteristics of Hydrothermal Synthetic Sapphire
LÜ Yang1, PEI Jing-cheng1*, ZHANG Yu-yang2
1. Gemmological Institute，China University of Geosciences (Wuhan)，Wuhan 430074，China
2. School of Earth Sciences，China University of Geosciences (Wuhan)，Wuhan 430074，China
Abstract：Sapphires has high economic value as one of the five precious stones. Among them, “royal blue” and “bonnet blue” are the most expensive. Using the hydrothermal method can synthesize “royal blue” sapphires, and the synthesized crystal is large and can have a pure interior by cutting and grinding, so it is difficult to identify it only by appearance and conventional methods. The researchers select seven blue hydrothermal synthetic sapphires to analyze their chemical composition and spectra features by LA-ICP-MS, Raman spectrometer, Infrared spectrometer, UV-Vis absorption spectrometer and Three-dimensional Fluorescence spectrometers. These samples are compared to natural and flame-fusion synthetic sapphires with a similar appearance. The analysis shows that the chemical compositions of hydrothermal synthetic sapphires are simple, but natural sapphire usually contains rich trace elements. In Raman spectra, three kinds of sapphires samples all show typical corundum vibration modes with A1g and Eg. In the lattice vibration region of Infrared spectra, there is no obvious difference, which fits the result of Raman spectra. However, in the 3 000～4 000 cm-1 region of Infrared spectra, hydrothermal synthetic sapphires show the hydroxide radical vibrations generated by hydrated mineral inclusion, but it isn’t displayed in natural and flame-fusion synthetic sapphires. UV-Vis spectra show that Fe2+-Ti4+ ion pairs produce colors in these three kinds of samples, but hydrothermal and flame-fusion synthetic sapphires lack the 450nm absorption peak of the natural sapphire. 3D fluorescence spectra show that both hydrothermal and flame-fusion synthetic sapphires have blue fluorescence induced by Ti4+ ion-related charge transfer under the excitation of a 240nm light source, which is lacking in natural sapphire. Nevertheless, there is no such fluorescence in the natural sapphire. To conclude, chemical composition, Infrared spectra, UV-Vis absorption spectra, and 3D Fluorescence spectra can provide important evidence for identifying hydrothermal synthetic sapphires.
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