The Study On Gemmological And Mineralogical Characteristics Of Omphacite Jade

As the king of jade, jadeite has becoming the most collectible, ornamental and culturalvariety of jadeite for its colorful color, delicate texture, hard hardness, good toughness, scarceorigin, favored by the people. In recent years, as a rare and expensive jade material, omphacitejade has been in pursuit of the market. It was black or forest green in reflected light, and darkgreen in transmission light. It was so sedate and constraint that becoming a hot varieties of jade.However, due to the arisen time is short, the gemological and mineralogical study on omphacitejade is slightly weak.The author collected samples with different qualities of omphacite jade to do thegemological characteristics test, UV-visible spectral test, thin slice observation, electronmicroprobe test, X-ray powder diffraction, infrared spectrum test to analyze the basic geologicalcharacteristics, mineral composition, structural characteristics, chemical composition andspectral characteristics.The observation under polarizing microscope shows that omphacite is fine-grained, itstexture is micro-crystalloblastic texture. Jadeite is coarse-grained, its texture is prismaticcrystalloblastic texture. Omphacite can be metasomatic by jadeite. Omphacite jade with singlemineral composition, which with a same size of grain, nematoblastic texture and undevelopedfracture is the best. Conversely, which with porphyroblastic texture and developed fracture is theworst.Because of the content of Fe in omphacite is more than in jadeite, the specific gravity ofomphacite jade is heavier(~3.34) and the refraction index is higher(~1.676) than jadeite jade. TheUV-visible spectral test shows that437nm absorption line caused by Fe3+do not exist, Fe ofomphacite exist as Fe2+.The major constituent mineral of omphacite jade is omphacite and jadeite, and thesecondary mineral is albite. Besides that, it also including celsian, titanite, zircon, pyrite, galenaand so on. The electron microprobe analysis shows that the main components of omphacite areCaO, Al2O3, MgO, Na2O and SiO2, a small amount of FeO(T) and trace amounts of TiO2, MnO,Cr2O3, K2O. The content of Ca, Mg and Fe is more than jadeite. In the pyroxene triangulardiagram, mainly project of the best omphacite jade is on the omphacite area.The X-ray powder diffraction lines is consistent well with the omphacite standard date, thesharp lines prove the well crystallized. The date shows that the structure type of omphacite isP2(or P2/n), it reveals an ordered metallogenic environment of low temperature. The result of infrared spectrum test is line of omphacite, the spectral line is slightly differentwith jadeite. Because of the effect of Ca2+replacing Na+and Ca2+, Mg2+, Fe2+, Fe3+replacingNa+, Al3+, the shape of the spectral line changed and the peak position shifted.