Spectral Characteristic of Gem Grade Clinohumite

Clinohumite is a rare yellow to orange silicate mineral.In recent years, gem grade clinohumite has appeared on the market, but there are few related gemmological studies in China.To supplement the research gap of this gemstone, clinohumite samples of three different colour tones (yellow, orange-yellow and orange-red) were selected for conventional gemmological tests. Combined with electronic probe, laser Raman spectrometer, Fourier transform infrared spectrometer, and UV-Vis spectrometer, the chemical compositions, spectral characteristics, and mechanism of coloration were preliminarily studied.The results show that RI of clinohumite is 1.629-1.668, with a strong birefringence(DR: 0.031-0.036).Their relative density ranges from 3.14 to 3.27, and medium yellow to orange fluorescence can be observed under shortwave ultraviolet light. Some samples have obvious twin crystal patterns and rich inclusion morphology. Rutile inclusions were found in gemstone grade clinohumite.Clinohumite contains main elements Si, Mg, and Ti, as well as trace elements Ca, Fe, K, and V. Combined with UV-Vis spectra, it is speculated that the colour is closely related to Fe and Ti.Laser Raman spectra and infrared spectra are mainly related to vibrations of SiO₄ tetrahedron, MgO₆ octahedron and -OH.The strongest Raman shifts (830 cm^-1 and 862 cm^-1) and the strongest infrared spectral reflection peaks (around 925-935 cm^-1) both related to SiO₄ vibration.