Study on the Chemical Composition and Spectroscopy Characteristics of Emeralds from Bahia Mining Area, Brazil

In this paper, the basic gemmological, compositional, infrared spectral and UV visible spectral characteristics of emeralds from Bahia mine, Brazil, are studied, aiming to explore the chemical composition and spectrum characteristics identification of emeralds from Bahia mine, to provide assistance for the origin identification of emeralds and the identification of emerald from Brazil, and to supplement domestic research of emerald from Brazil. The results show that emeralds from Bahia are typical Cr^- rich emeralds with a Cr/V ratio greater than 6.9 and a high Fe content. The alkali metal elements in the samples are mainly Li, Na, K and Cs, and the total content is high, with an average value of 12 052 ppm. The infrared spectroscopic test results of emeralds from Bahia show that their type Ⅱ water-related absorption peaks are strong, and the absorption peak of about 5 270 cm^-1 is more characteristic, indicating a higher alkali metal content. The characteristic absorptions of the UV-Vis spectrum of emeralds from Bahia are as follows: the absorption band produced by the combined action of Cr³⁺ and V³⁺ are at about 426 nm and 607 nm, the Cr³⁺ related absorption peaks are at 637 nm and 683 nm, and the Fe²⁺ related absorption band is at about 835 nm. The results of the combined composition test show that when the Cr content is low, the center of the absorption band of about 607 nm is shifted in the UV-visible spectrum, the sharp absorption peak of 637 nm is missing, and the intensity of the absorption peaks at about 662 nm and 682 nm increaseds.