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IR and UV-Vis Spectroscopy of Gem Emeralds, a Tool to Differentiate Natural, Synthetic And/Or Treated Stones?

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IR and UV-Vis Spectroscopy of Gem Emeralds, a Tool to Differentiate Natural, Synthetic And/Or Treated Stones? FACULTEIT WETENSCHAPPEN Vakgroep Geologie en Bodemkunde IR and UV-Vis spectroscopy of gem emeralds, a tool to differentiate natural, synthetic and/or treated stones? Mathieu Van Meerbeeck Academiejaar 2009–2010 Scriptie voorgelegd tot het behalen van de graad van Master in de Geologie Promotor: Prof. Dr. K. De Corte Co-promotor: Prof. Dr. P. Van den haute Leescommissie: Prof. Dr. P. De Paepe, Prof. Dr. P. Vandenabeele PREFACE AND ACKNOWLEDGEMENTS I’d like to thank my promotor Prof. Dr. Katrien De Corte and HRD Antwerp for giving me the opportunity to perform this unique investigation. Her support, enthusiasm, patience and criticism during the progress of this research have really inspired me. I’m also thankful for the scientific, technical and gem-related assistance of her colleagues, Jef Van Royen and Anita Colders. Nathalie Crepin also gets some of the credit, by providing me with important articles. It has also been nice working with the research staff in Lier, with special attention to the head of research, Mr. Yves Kerremans, and lab assistant Wendy Lembrechts, who initiated me with the instrumentation. The recognised experience of professor Peter Van den Haute, my co-promotor, truly helped me create a fully scientifically based work. My appreciation goes to Prof. Dr. P. De Paepe and Prof. Dr. P. Vandenabeele as well, who put time and effort in the critical analysis of this thesis. I also show my gratitude to my colleague student and friend Tim Verstraeten, who performed a similar study on sapphires. He was a great co-worker during data acquirement and our discussions lead me to several new insights. Further, I’d like to thank everybody who improved this thesis with some necessary language corrections. And of course I should not forget my parents, who gave me the chance to study and who supported me during the progress of this thesis. Cover picture: http://www.modernjeweler.com/web/online/colored-gemstone-gem-profiles/emerald/1$451. ABSTRACT As emerald is one of the most valuable precious stones, research on its characteristics and origins is of great importance in the gem industry. Until now, non-destructive methods to determine all provenances are however not sufficient. Therefore, we test the added value of two non-destructive spectroscopic methods to (1) distinguish synthetic from natural emeralds, (2) discriminate between the different provenances of the natural or the type of the synthetic emeralds, (3) determine the presence of filling substances (treatments). Spectroscopy in the visible and ultraviolet region (UV-VIS) and in the infrared (IR) region, using the Fourier Transform principle (FT-IR) were performed on a set of 133 cut emeralds. By making classifications based on our own spectral observations, we were able to assign 78 % of the emeralds to the right origin. We separated all synthetic from natural emeralds and recognised all natural Colombian emeralds, synthetic flux and synthetic hydrothermal emeralds. Zambian emeralds have also been distinguished, with only 4 on 29 inconsistencies. By applying UV-VIS spectroscopy, four major groups of emeralds could be distinguished: (1) flux synthetic emeralds, (2) Colombian and chlorine bearing hydrothermal synthetic emeralds, (3) copper bearing hydrothermal synthetic emeralds and (4) all other natural emeralds. Further IR spectroscopic analysis allowed a further determination in specific origins. Treatments are recognised by absorptions in the IR region in the 3100-2800 cm-1 range. Unfortunately, further identification is not possible because of very similar absorption features by many types of filling substances an mixtures. TABLE OF CONTENTS 1. INTRODUCTION ........................................................................................................................................................... 3 2. CHARACTERISTICS, GENESIS AND LOCALITIES OF EMERALDS....................................................................................... 4 2. 1. SITUATION: HISTORY AND IMPORTANCE OF EMERALDS ............................................................................................................ 4 2. 2. CHARACTERISTICS OF EMERALD .......................................................................................................................................... 4 2.2.1. Crystal chemistry of beryl .................................................................................................................................. 4 2.2.2. Colouration ........................................................................................................................................................ 5 2.2.3. Channel ions ...................................................................................................................................................... 6 2.2.4. Inclusions ........................................................................................................................................................... 7 2. 3. TREATMENT OF EMERALDS ................................................................................................................................................ 8 2. 4. DEPOSITS OF EMERALDS.................................................................................................................................................... 9 2.4.1. Origin conditions ................................................................................................................................................ 9 2.4.2. Important deposits .......................................................................................................................................... 11 2. 5. SYNTHETIC EMERALDS AND IMITATIONS ............................................................................................................................. 15 2.5.1. Synthetic emeralds .......................................................................................................................................... 15 2.5.2. Imitations ......................................................................................................................................................... 16 3. METHODOLOGY ........................................................................................................................................................ 17 3. 1. RESEARCH STRATEGY AND PROGRESS ................................................................................................................................. 17 3. 2. SPECTROSCOPY AS RESEARCH TECHNIQUE ........................................................................................................................... 17 3.2.2. Infrared spectroscopy ...................................................................................................................................... 18 3.2.3. UV-VIS Spectroscopy ........................................................................................................................................ 23 4. RESULTS AND DISCUSSION ........................................................................................................................................ 26 4. 1. DESCRIPTION OF A TYPICAL EMERALD SPECTRUM ................................................................................................................. 26 4.1.1. The UV-VIS region ............................................................................................................................................ 27 4.1.2. The IR region .................................................................................................................................................... 29 4. 2. OBSERVATIONS AND INTERPRETATIONS .............................................................................................................................. 31 4.2.1. Step 1: blind classification ............................................................................................................................... 31 4.2.2. Step 2: classification based on reference samples ........................................................................................... 37 4.2.3. Step 3: Comparison with the dataset of HRD Antwerp .................................................................................... 41 4. 3. SPECTROSCOPIC FEATURES OF NATURAL EMERALDS .............................................................................................................. 42 4.3.1. Colombia .......................................................................................................................................................... 43 1 4.3.2. Brazil ................................................................................................................................................................ 46 4.3.3. Zambia ............................................................................................................................................................. 47 4.3.4. India ................................................................................................................................................................. 49 4.3.5. Madagascar ..................................................................................................................................................... 50 4.3.6. Sandawana (Zimbabwe) .................................................................................................................................. 50 4.3.7. South-Africa ....................................................................................................................................................
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