Study on the Infrared Spectral Characteristics of H2OⅠ-type Emerald and the Controlling Factors

BACKGROUND The infrared absorption mechanism is mainly related to the Si-O lattice, channel structure water, other alkaline metal cations, and vibration of molecules. Relevant research at home and abroad focuses mainly on peak position attribution and spectral peak feature comparison. It is considered that molecular vibration is related to different types of structural water. However, there are few studies on deeper mineralization or chemical controlling factors. OBJECTIVES To unravel the controlling factors of H2OⅠ-type infrared spectral characteristics. METHODS The typical H2OⅠ-type natural emeralds were collected from 4 mining areas, including the Eastern Cordillera mountains in Colombia (EC), the Panjshir valley in Afghanistan(P), the Ural mountains in Russia(U), and the Kaduna/Plateau state in Nigeria (KP). The samples were analyzed by Fourier Transformed Infrared Spectrometer (FTIR). The typical H2OⅠ-type infrared (IR) spectral characteristics and their controlling factors were studied. The chemical composition data were obtained from the EMPA analyses. RESULTS The results show that the spectral characteristics of H2O Ⅰ-type emeralds from different mining areas share a consistent pattern. Several absorptions related to structural water, basic ions and macromolecules had stable peak positions, approximately similar relative peak intensities and peak shapes. As the analysis proved, the H2OⅠIR spectra were first directly controlled by the mixed ratio of the two types of the structure water in the channel, and further related to the substitution of Al3+, chemically controlled by the (Mg2++Fe2+) concentration in the ore fluids. When the concentration of (Mg2++Fe2+) was low, the degree of isomorphic substitution was lower, and the emerald structure water was mainly characterized by Ⅰ-type water. The related elements were characterized by high Si and Al but low Mg and Fe, corresponding to the typical infrared characteristics, indicating the relationship between chemical ion concentration and infrared spectral characteristics. CONCLUSIONS The research process showed that Infrared Spectroscopy could assist in the identification of Ⅰ-type water emerald production discrimination and the understanding of the metallogenic environment.