CHARACTERIZATION OF VIBRATIONAL AND ELECTRONIC FEATURES IN THE RAMAN SPECTRA OF GEM MINERALS by Renata Jasinevicius A Prepublication Manuscript Submitted to the Faculty of the DEPARTMENT OF GEOSCIENCES In Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE In the Graduate College THE UNIVERSITY OF ARIZONA 2009 ii ACKNOWLEDGMENTS This work could not have been completed without the sponsorship of the Israel Diamond Institute (IDI). A special thanks to the following contributors for making this work possible: James Shigley, Ph.D.-Gemological Institute of America (GIA) Dr. M. Bonner Denton-University of Arizona, Dept. of Chemistry Bear Williams, Stone Group Labs (photos) Tom Tashey-Professional Gem Sciences, Inc. Charlene Estrada, photography Sue Robison, RRUFF Project, University of Arizona To my advisor, Bob Downs, you have challenged me in new ways and taught me how to network. You have helped me fine tune my writing skills and master the art of thinking scientifically. You have made me a better student and opened doors for my future. Thank you. I’d like to extend my gratitude to all the people who provided me with personal support and encouragement throughout my academic career: To my future husband, Jason Lafler, I never would have made it without you. I am the luckiest lady in the world. Madison Barkley, you are a dear friend, and having you in my life made all of my graduate school experiences more memorable. To my mentor and friend, Elizabeth Gordon, your guidance has helped me become the person I am today. You are an inspirational teacher and I appreciate everything you have done for me. To my best friend, Andrea Mikenas, you always believed I could do it and never failed to remind me. To my former roomie, Lindsay Draeger, your Sunday morning polka phone calls made me smile and made me feel a little less lonely. Last, but certainly not least, I’d like to thank my family. To my Mom and Dad who have always pushed me to achieve excellence, I love you and appreciate everything you have sacrificed to help me achieve my goals. Rich, you are a great big brother. To my future parents, Val and Eunice, since the first day we met you have taken me under your wings and treated me like your daughter. I love you both and feel incredibly blessed to have you in my life. iii CONTENTS INTRODUCTION 1 Raman Spectroscopy 2 IR Spectroscopy 14 OH in Minerals 14 Luminescence Spectroscopy 15 Color Theory 16 RAMAN ANALYSIS 1. Beryl 19 2. Chrysoberyl 26 3. Corundum 30 4. Diamond 35 5. Diopside 43 6. Garnet 48 7. Olivine 74 8. Quartz 79 9. Spinel 82 10. Spodumene 87 11. Titanite 90 12. Topaz 97 13. Tourmaline 102 14. Zircon 114 15. Zoisite 125 APPENDICES Appendix A & B: Features in Raman Spectra 132 Appendix C: Unit Conversions 141 Appendix D: Cause of Color Chart 143 Appendix E: Point Symmetry Notations 147 iv Introduction Raman spectroscopy, a non-destructive technique used to interpret atomic vibrations, has become a popular tool for the rapid identification of materials. Raman spectra produce unique vibrational fingerprints useful in identifying a multitude of materials. With the advent of numerous gem treatments and a variety of methods available for mineral synthesis, Raman spectroscopy is particularly useful in identifying and characterizing gemstones. Micro-inclusions in minerals can be analyzed using Raman spectroscopy providing evidence of mineral genesis or geologic origin. More recently, fluorescence features attributed to chromophoric ions and trace elements have been observed in Raman spectra, revealing important information about crystal chemistry. Analysis and interpretation of these features may help distinguish between natural, treated, and synthesized materials. Advances in optical technologies are bringing hand-held Raman spectrometers to the forefront of materials research. As new instruments are developed, with both increases in portability and decreases in production costs, hand-held Raman units will likely be fundamental to laboratory and field-based Geoscience and gemological research in the future. Therefore, the development of databases and interpretation of spectra in anticipation of the new instrumentation is required. Raman spectra and associated interpretation of spectral features for the important gemstones are presented in this study. In this study I will present a characterization of the vibrational and electronic features present in the Raman spectra of gem minerals including: The effects of orientation - Spectral features associated with the vibrational modes of OH and H2O The causes of color in various gem minerals and the electronic spectral features associated with particular color-inducing cations Spectral features associated with luminescence of REE The effects of metamictization 1 Basic Raman Theory Raman spectroscopy is a type of vibrational spectroscopy. Intense electromagnetic radiation (typically generated by a laser) interacts with a substance and is scattered into radiation of different wavelengths associated with nuclear motion and producing a unique spectral fingerprint of atomic vibrations (Smith and Dent, 2005). An intense light source is necessary in Raman spectroscopy because Raman scattering is a very weak process; only one out of every 106-108 photons will Raman scatter (Smith and Dent, 2005). In general, gem minerals at room temperature are in the ground vibrational state (lowest energy vibrational level). When light interacts with a crystal the incident radiation can be scattered in several ways that include: 1) Rayleigh scattering, in which the scattered photon retains the energy of the incident beam (no energy change, elastic scattering) (Fig. I1) and 2) Raman scattering, in which the scattered photon experiences a change in energy (inelastic). There are two types of Raman scattering: 1) Stokes and 2) anti-Stokes. Stokes scattering occurs when atoms in the crystal (at the ground vibrational state) absorb the energy from the incident photon and are ultimately promoted to a higher energy vibrational state; the incident photon loses energy relative to its original state and the wavelength of the scattered light is shifted towards the red end of the electromagnetic spectrum (Fig. I1). During anti- Stokes scattering, energy is transferred from the already excited atoms to the incident photon and subsequently, the atoms associated with this specific vibration are demoted to the ground vibrational state; the scattered light is higher in energy than the Rayleigh line and therefore, the wavelength of the scattered light is shifted towards the blue end of the spectrum (Fig. I1) (Smith and Dent, 2005). Anti- Stokes scattering occurs less frequently than Stokes scattering because it requires that the atoms already be in a higher energy vibrational state when the laser interacts with it (Smith and Dent, 2005). There are characteristic temperatures at which certain vibrations are activated in different sets of bonded atoms. For example, strongly bonded atoms like Si-O are not vibrationally active at room temperature; they are considered high energy vibrations. Fig. I1 Energy diagram showing transitions in various types of spectra (modified from Smith and Dent, 2005). IR Rayleigh Stokes Anti-Stokes Fluorescence Raman Raman Electronic Vibrational Relaxation & States Internal Conversion Virtual States Absorption (Emission) (Excitation) Fluorescence Vibrational States Ground State Whether or not a vibrational mode is Raman active depends on the polarization of the vibrating bonded atoms. When the incident beam interacts with the crystal, the atoms begin to oscillate at the same frequency of the incident radiation. As an atom oscillates, its electrons are pulled in various directions, depending on their distribution in the electron cloud, resulting in deformation of the cloud. As the electrons move, so does the atom’s nucleus producing a separation of charges in the atom called a dipole 2 (the atom becomes polarized). Changes in the polarization of bonded atoms produce Raman active vibrational modes (Ferraro et al., 2003). In minerals, chemistry and crystal structure dictate the types of vibrations that can occur. The way bonded atoms in a crystal can bend, stretch, or rotate, i.e. their degrees of freedom of movement, depends on the crystal symmetry (Smith and Dent, 2005). Complex correlation matrices involving specific site symmetries can predict Raman active modes in minerals (Ferraro et al., 2003). The details of this process are beyond the scope of this study. However, for a more detailed discussion of Raman selection rules, character tables, factor group analysis, and crystal field theory see the following sources: “Infrared and Raman Selection Rules for Molecular and Lattice Vibrations: The Correlation Method,” W.G. Fateley and F.R. Dollish, 1972 “Molecular Vibrations: The Theory of Infrared and Raman Vibrational Spectra,” E.B. Wilson, J.C. Decius, and P.C. Cross, 1955 “Chemical Applications of Group Theory,” F.A. Cotton, 1971 “Symmetry in Bonding and Spectra: An Introduction,” B.E. Douglas and C.A. Hollings Note: Raman mode analysis tables in this study were generated using the Raman mode prediction tool found on the Bilbao Crystallographic Server available online at http://www.cryst.ehu.es/. References Bilbao Crystallographic Server II: Representations of crystallographic point groups and space groups”. Acta Cryst. (2006), A62, 115-128. Ferraro, J.R., Nakamoto, K. & Brown, C.W. (2003) Introductory Raman Spectroscopy, 2nd edn, Academic press, San Diego, CA. Smith, E. & Dent, G. (2005) Modern Raman spectroscopy: a practical approach, John Wiley & Sons, Chichester, West Sussex, England. 3 Experimental Procedures and the RRUFF Database All Raman spectra and X-ray diffraction data utilized in this study have been taken from the RRUFF database (found at www.rruff.info). This database, directed by Dr. Robert T. Downs at the University of Arizona, provides a large set of chemical and spectral data of minerals. All information is available to the public including the downloadable software, CrystalSleuth, used in this study to view, stack, and process Raman spectra (Laetsch and Downs, 2006).