Gemological Characteristics Research Of Tourmaline Occurred In Altai Region, Xinjiang

A variety of gem minerals is mined in Altai region of northern Xinjiang. Tourmaline is one of them. Tourmaline with its rich and varied colors has been well received by the people of the world. In order to known more about gemological properties of tourmaline,this paper selected tourmaline occurred in Altai region as the object of research, using a variety of conventional gemological instruments that tested the basic characteristics of gem.The research methods include gemological routine test, EPMA, Raman spectroscopy,infrared spectroscopy and UV-Vis spectrophotometer, to analyze color and internal structure of tourmaline occurred in Altai region.The tourmaline crystals in the Altai region of Xinjiang, have higher transparencies,with crystal forms of trigonal prism or hexagonal prism. The refractive index is1.620~1.646. Relative density is 2.93~3.11. The tourmaline crystals have strong dichroism.In the tourmalines of different colors, the main components of SiO2 and Al2O3 are very similar which content of SiO2 is 36.293%~38.284%, Al2O3 is 36.378%~39.989%. The tourmalines of different colors contain different quantity of Fe. Black tourmaline contains the highest content of Fe in the minerals of all colors. Tourmaline presents pink when the content of Mn increases.In infrared spectra, the O-H stretch-vibration absorption is within the scope of3200cm-1-3700cm-1. When the absorption appears bimodality, the sample type belongs to elbaite. In red tourmalines the peak around 850cm-1 disappears. But in the tourmalines of colors from green to dark green, 850cm-1 peak gradually appears obvious. Accompanied with the deepening in the color, the absorbed peak is more clear, which should be attributed to Fe/[SiO4] common stretching vibration. The peak becomes clearer with increasing content of iron.In Raman spectra, the absorption of 835cm-1 could be due to stretching vibrations of SiO4. The content of Fe in the mineral restricts the absorption intensity. Test results show that within 1200cm-1~1600cm-1 there is a weak and complex vibration band. Absorption only in 1320cm-1 appears a strong peak, indicating the stretch vibration of C-O.Carbonaceous material would exist in the internal gas-liquid inclusions.UV-visible spectra show that tourmalines have Mn3+ absorption which peaks near360 nm and 520 nm, which led to pink color. The absorption band at 720 nm is caused by Fe2+. Fe3+ may cause deviation of absorption at 720 nm. The content of Fe is not connected with Al-OH1-Al decomposition which could reflect the variation of electrovalence.In crystal growth characteristics of tourmaline, angular terraced incremental growth could be observed. Achieved a certain state in the process of growth, the features of the crystal faces would interconvert. The coarse and smooth degree of the crystal faces is controlled by temperature. In ribbon tourmalines different positions appear different colors,which were caused by change of ore-forming environment when tourmalines grew to designated phases.