41 Undergraduate Research III (Posters)

Friday, 29 March 2019
1:00 PM - 5:00 PM
Carolina Ballroom

EMERALD CHARACTERIZATION BY RAMAN SPECTROSCOPY AND ITS APPLICATION FOR PROVENANCE

Paper number: 41-1
Program: Theme Sessions
Time: 1:00 PM – 5:00 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 51, No. 3

doi: 10.1130/abs/2019SE-325949

© Copyright 2019 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.