RESEARCH OF ZONATION CHARACTERISTICS OF EMERALDS FROM DAVDAR, NORTH-WESTERN CHINA

The Davdar emeralds have been the second discovered deposit in Xinjiang province, north-western China, which were known for “Emeralds of the Silk Road”. We have used laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to study the chemical composition characters of zoning structure and the relationship between metal contents and the intensity of water type Raman band combining with Raman spectroscopy. We used these methods in an attempt to indicate the details of mineralization process for Davdar emeralds.

LA-ICP-MS analysis of samples displayed the variation of metal contents which caused the distinction of color zone. From the inner to the outer zones, samples evolved continuous increase in the chromophores such as Cr from the average value 2296ppmw to 5408ppmw, V from averaging 1355ppmw to 2548ppmw, with the decrease in the alkali ions dominated by Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺ with the mean values from total 11314ppmw to 9615ppmw. Also, the results of Raman tests revealed that the H₂O II Raman band is higher than H₂O I Raman band combining with the relatively higher concentrations of alkaline cations in the colorless core. The I _{H2O II} /I _{H2O I} ratio exceeded 1 with high-alkali cores, while in low-alkali rims the ratio was <1. The contrast of intensity between type I and type II water band can indicate the content variations of alkali metal in color zonation and identify the multiple growth zones and mineralizing environment (alkali-poor, alkali-rich).

We obtained that the green outer rim of Davdar emeralds were alkali-poor, Cr and V-rich and the colorless inner zones enriched alkali ions with the lower concentrations of the chromophores. The changing concentration of chemical compositions indicated that emerald formation environment may make the transition from alkali-rich at the initial stage into alkali-poor at the late metallogenic stage. Therefore, the study of the zonation characteristics provided useful information for crystalization growth of Davdar emeralds.