EMERALD CHARACTERIZATION BY RAMAN SPECTROSCOPY AND ITS APPLICATION FOR PROVENANCE

Raman spectroscopy is a useful non-destructive tool to identify minerals, especially gem-quality minerals. Interaction of the laser beam with the mineral provides information about lattice structure and molecular vibrational modes. Beryl (Be₃Al₂Si₆O₁₈) and its varieties are hexagonal cyclosilicates with six-membered rings of (SiO₄)^-4 tetrahedral units forming crystal-oriented channels parallel to the c axis that are occupied by alkalis, OH^-, H₂O, CO₂, CH₄, and N₂. Beryl can incorporate chromophores responsible for its different varieties, including Cr³⁺, V³⁺, Fe²⁺, Fe³⁺, and Mn²⁺ that substitute for Al³⁺ and/or Be²⁺ within the octahedral and tetrahedral sites, respectively, but they can also be incorporated within the channels. Known substitutions and incorporation of elements or molecules into channels within the lattice structure of beryl varieties can be identified by Raman spectroscopy. Therefore, a systematic crystal-oriented Raman spectroscopy study of beryl varieties from localities around the world can aid in beryl characterization, and be used to identify provenance.

Preliminary results of Raman spectra obtained parallel and perpendicular to the c axis for 46 natural emerald and other beryl varieties from 19 localities are presented. The Raman shift between 150 and 2500 cm^-1 of each crystal was characterized, and vibrations for Si-O, Be-O, Al-O, and the CO₂ Fermi doublet were identified based on previous studies. At higher frequencies (3300-3900 cm-¹) two major peaks exist at 3593 cm^-1 ± 3 cm^-1 and 3605 cm^-1 ± 3cm^-1. The first peak at 3593 cm^-1 is attributed to type-II water, whereas that at 3605 cm^-1 corresponds to type-I water. The water region in the spectra of Colombian emeralds and those from Nigeria, Australia, Brazil, China, South Africa, and Russia is similar in that they are bimodal, showing the presence of the two peaks at 3593 cm^-1 and 3605 cm^-1. In emeralds from these localities the intensity of the 3605 cm^-1 Raman shift is the highest of the two, showing the dominance of type-I water. Non-emerald beryl varieties have a bimodal peak, but this feature is more prevalent when the crystal is oriented parallel to the c axis. Preliminary data show that Raman spectroscopy is powerful to characterize beryl varieties and to determine provenance for major gem-quality emerald deposits.