Mineralogical Characteristics of Gem-quality Lepidolite Rock in Xinjiang

The gemmological characteristics of gem-quality lepidolite rock in Xinjian are studied, which is called lilac purple jade in Chinese jewelry market. Five samples were investigated by conventional gemmological methods and advanced testing technologies, including X-ray diffraction spectrometer, Raman spectrometer, infrared spectrometer and laser induced breakdown spectroscopy. The testing results show that the refractive index of the sample is about 1.55 (point measurement), and the specific gravity ranges from 2.82 to 2.87. The colour differs slightly from dark purple to light purple, with rare glassy luster. Under gem microscope, a kind of brown inclusion is observed in samples. which distributes like infection.Mineral components were investigated by X-ray diffraction. The main mineral of the sample is mica, which occupies approximately 90%. Minor mineral is albite, which is accounting for nearly 10%, and there is trace amount of quartz (certainly less than 1%).The Raman spectra were characterised by a strong band at 3 628 cm^-1 and a relatively weak band at 3 496 cm^-1 in high wavenumber region (3 000~3 800 cm^-1), which are assigned to OH-stretching modes. The groups of 600-800 cm^-1 peak vary in wavenumber and intensity because of the replacement of Si by Al in the tetrahedral. Peaks at 712, 636 and 754 cm^-1 are recognized as vibrations of Si-O-Si, Si-0-Al and Al-0-Al. The Raman spectra observed are very similar with the standard spectrum of lepidolite, indicating that the purple jade is mostly consisting of lepidolite.The corresponding infrared spectra showed strong overlapping Si-O-Si modes around 1 010 cm^-1 plus less intense bands around 808,751,541 and 481 cm^-1. Two overlapping OH-stretching modes can be observed around 3 450-3 650 cm^-1,in agreement with a broad band in the infrared spectrum around 3 454 cm^-1 and a complex band around 3 625 cm^-1 which are in good agreement with the Raman spectra assigned to OH.LIBS results demonstrated that samples were mainly composed of H, Li, Na, K, Cs, Ca, Mg, Al, Si, O,etc. and some trace elements, such as Be, Ti, Fe, Mn, Cu, Cr, Hg, Pd,which accords with conventional chemical composition of lepidolite.