A Preliminary Study on the Gemmological Characteristic and Formation of "Longxi Yu" in Wenchuan, Sichuan Province

The "Longxi Yu" in Sichuan Province is considered to be the possible source of the ancient jade materials of the famous "Sanxingdui site" and "Jinsha site" in China.In this paper, the spectral characteristics and chemical compositions of typical "Longxi Yu" produced in Wenchuan, Sichuan Province were tested by conventional gemmological tests, Fourier transform infrared spectrometer, Raman spectrometer, X-ray fluorescence spectrometer, etc.The following conclusions are obtained: (1) The refractive index of the samples is 1.61(point measurement) and the relative density is 2.95-2.99, which is consistent with nephrite. Two kinds of "Longxi Yu" with high and low quality can be identified as type A and type B. The type A samples are with yellowish green colour, grease luster, pure in quality with fine texture, and the content of tremolite can reach 98%. The type B samples are gray-white mixed with gray-black in colour and mainly composed of tremolite (70%-90%) with a small amount of carbonate and other minor minerals. The type B samples are mainly coarse texture, silk-glass luster, fibrous interleaving and leaf-oriented structure, with obvious bedding structure. Both types of samples are classified as nephrite.(2)The infrared spectra and Raman spectra of "Longxi Yu" samples are consistent with standard tremolite spectrum. And trace minerals such as molybdenite and molybdenite are first reported. The infrared spectra of type B samples show a spectrum peak of 1 446 cm^-1, which is attributed to carbonate. (3)The chemical content of Na₂O of "Longxi Yu" is similar to that of nephrite formed by contact metasomatism between intermediate-felsic rocks and magnesia marble / dolomitic limestone, in which there is obvious hydrothermal participation in the forming process of "Longxi Yu". However, the contents of CaO and loss on ignition (LOI) of the type B samples are obviously higher than that of general nephrite, which can be determined as replacement forming nephrite(R-type nephrite). Meanwhile, type A samples have a simple mineral component and fine structure, which is similar to precipitation forming nephrite (P-type nephrite).According to the comparison of chemical composition characteristics between "Longxi Yu" and nephrite from other producing areas in China, it is suggested that the distinctive combination of high contents of MnO and P₂O₅ may be one of the important origin typomorphic characteristics of "Longxi Yu". The above results can provide an important reference for the traceability study of ancient jade.