Raman Spectroscopy Characterization of Emerald and Other Beryl Varieties Worldwide

ECU Author/Contributor (non-ECU co-authors, if there are any, appear on document)

Daniel Gray (Creator)

Institution

East Carolina University (ECU )

Web Site: http://www.ecu.edu/lib/

Abstract: Emeralds and other beryl varieties were characterized using micro-Raman spectroscopy from 150 cm-1 to 3900 cm-1. In total, 46 emerald and other beryl varieties were characterized, including morganite, heliodor, goshenite, aquamarine, green beryl, and bixbite, representing a wide range of geologic settings from 17 geographic locations worldwide, including Argentina, Australia, Austria, Brazil, China, Colombia, Egypt, Ireland, Madagascar, Namibia, Nigeria, Pakistan, Russia, South Africa, Ukraine, the United States (California, Maine, North Carolina, Utah), and Zambia. The first Raman spectroscopy characterization is presented for emeralds from Australia, Egypt, and North Carolina, heliodor from Ukraine, morganite from Brazil and California, pale-yellow beryl from Maine, aquamarine and green beryl from Argentina, goshenite from California, and aquamarine from Ireland, Namibia, Pakistan, and Russia. Raman characterization includes presence or absence of peaks, Raman shift, intensity, shape, Full Width Half Maximum, and assignment to specific bonds. The characterization includes identification of 8088 peaks present in the fingerprint region (150-1500 cm-1) representative of Si-O, Be-O, and Al-O bonds, the water region (3500-3900 cm-1), and other peaks representative of carbon dioxide and hydrocarbons (~2335 cm-1 and ~2914 cm-1, respectively) analyzed with crystals oriented with their c axis parallel and perpendicular to the laser beam. In the water region, two Raman peaks representative of two types of water molecules are discerned in all studied beryl, except for bixbite (red anhydrous beryl). Type-I water, defined as not being associated with alkalis, is present at ~3605 ± 3 cm-1, whereas type-II water, defined as being associated with alkalis, is present at ~3593 ± 3 cm-1. Emeralds from Colombia hosted in carbonaceous black shales (nonschist type) have a dominant type-I water peak in both orientations, similar to most non-emerald beryl varieties, whereas many non-Colombian emeralds from schist-type deposits are characterized by a dominant type-II water peak. Among emeralds from other locations, those from South Africa, China, and Russia have a strongly bimodal water peak in both orientations with a dominant type-I water peak. In most non-emerald varieties of beryl, specifically aquamarine from Pakistan, Argentina, Ireland, Russia, and Namibia, heliodor from Brazil and Ukraine, goshenite from California, and yellow beryl from Maine, the intensity of the type-I water peak is higher in both orientations. In contrast, goshenite and morganite from a pegmatite in San Diego, California, have a strongly bimodal water peak with a slightly higher intensity in both orientations of the type-II water peak. An evaluation of chemical compositions of emeralds reported in the literature and the dominant water peak present in the Raman spectra of emeralds from the same localities obtained in this study confirm that variations in the concentration of alkali ions (Na, Li, Cs) influence the prevailing type of water peak and the intensity of each of the water peaks present. Orientation effects were identified for peaks representing various molecules, including CO2 at ~1240 cm-1 and ~1380 cm-1 and Si-O at 1010 and 1060 cm-1. Striking characteristics of the Raman spectra of emeralds from Colombia compared with those from other localities and non-emerald beryl varieties are the higher intensity of the CO2 peak at ~2325 cm-1 and of CH4 at 2800 -- 3000 cm-1, and the dominant type-I water peak representing low alkali content in these emeralds. The new Raman spectroscopy data of emeralds and other beryl varieties worldwide provide several new elements to fingerprint crystals that can be used in the market and to explain the origin of the minerals.

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Language: English

Date: 2020

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