The Study On Gemological Characteristics Of Crust Of Jadeite Jade

As the old words saying, "Gold can be judged by price, but jade cannot be." Jadeite jade play an important part in Chinese traditional culture. Because of its rich colors, moist texture, and good toughness, jadeite jade has becoming the "king of jade" undoubtedly. The shell formation of jadeite jade is mainly the result of weathering. The study of crust characteristics will be helpful for determining the internal quality of jadeite jade. Previous research has came to some fruition, but the formation principium is still not thorough enough.The author collected samples with different color of the crust of jadeite stones from Pingzhou street, Foshan City, Guangdong Province. Using these samples to do some experiments including gemological characteristics test, the microscope observation, electron microprobe test, X-ray powder diffraction test, infrared spectrum test and UV-visible spectroscopy test to analyze the mineral composition and chemical composition, structural characteristics and spectral characteristics under secondary effects. Trying to discuss the reasons for the shell of different coloration and the changes of the structure.The result of electron microprobe tests show that the main mineral of samples is jadeite, the secondary minerals are achromaite and glaucophane. The composition of mineral particles of the crust does not change, and interior minerals and minerals from the crust are all jadeite. The probable secondary minerals filled in the gaps of the particles are mainly limonite, hematite and chlorite, which is coloring by element of Fe2+. Although the crust of the emerald green is weathered into maroon and other colors, the shell of jadeite still contains 0.1-0.2% of Cr2O3, and the interior are more pure jadeite minerals.The result of polarizing microscope observation shows that the features from the shell and internal are relevant. Mineral particles in the crust and interior of most samples are jadeite. The structure of the crust is loose and the gap between particles is larger, which can be easily filled with dark cryptocrystalline-microcrystalline mineral. Mineral particles in the black shell of some samples are manifested as alteration phenomenon. There are microcracks and metasomatic relict textures in the oil green jadeite jade. The dark aphanitic minerals may cause color by filling or metasomatism.The result of X-ray powder diffraction test shows there are jadeite and chlorite mineral in the black crust; the result of IR spectra shows that jadeite jade of the black and yellow crust displaying a wide and gentle peaks at 3500cm-1, which indicates secondary action may contribute to color; the result of UV-visible spectrum shows the color emerald green is mainly caused by Cr3+. Apple green jadeite jade has both iron and chromium absorption peaks, while emerald green jadeite jade only has the significant absorption of chromium. Since ferric iron contribute little to emerald green color, combining electronic probe data, it can be inferred that apple green color is mainly caused by Cr3+, while emerald green color is cause by Cr3+ and Fe2+.