Abstract Spectroscopic Characterization of Fluorite
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ABSTRACT SPECTROSCOPIC CHARACTERIZATION OF FLUORITE: RELATIONSHIPS BETWEEN TRACE ELEMENT ZONING, DEFECTS AND COLOR By Carrie Wright This thesis consists of two separate papers on color in fluorite. In the first paper, synthetic fluorites doped with various REEs (10-300 ppm) were analyzed using direct current plasma spectrometry, optical absorption spectroscopy, fluorescence spectrophotometry, and electron paramagnetic resonance spectroscopy before and after receiving 10-25 Mrad of 60Co gamma irradiation. The combined results of these techniques indicate that the irradiation-induced color of the Y-, Gd-, La- and Ce-doped samples are the result of a REE-associated fluorine vacancy that traps two electrons. Divalent samarium may be the cause of the irradiation-induced green color of the Sm- doped sample. In the second paper, fluorite crystals from Bingham, NM, Long Lake, NY, and Westmoreland, NH were similarly investigated to determine the relationship between sectorally zoned trace elements, defects, and color. The results indicate causes of color similar to those in the synthetic samples with the addition of simple F-centers. SPECTROSCOPIC CHARACTERIZATION OF FLUORITE: RELATIONSHIPS BETWEEN TRACE ELEMENT ZONING, DEFECTS AND COLOR A Thesis Submitted to the Faculty of Miami University In partial fulfillment of The requirements for the degree of Master of Science Department of Geology By Carrie Wright Miami University Oxford, OH 2002 Advisor_____________________ Dr. John Rakovan Reader______________________ Dr. Hailiang Dong TABLE OF CONTENTS Chapter 1: Introduction to the cause of color in fluorite 1 Manuscript 1-Chapter 2 29 “Spectroscopic investigation of lanthanide doped CaF2 crystals: implications for the cause of color” Manuscript 2-Chapter 3 95 “Spectroscopic characterization of fluorite from Bingham, NM, Long Lake, NY and Westmoreland, NH: relationships between trace element zoning, defects and color ii TABLE OF FIGURES Chapter 1 Figures 21 Figure 1a. Ball and stick model of fluorite in the [100] direction 21 Figure 1b. Ball and stick model of fluorite in the [110] direction 22 Figure 1c. Ball and stick model of fluorite in the [100] direction 23 Figure 2. Schematic of Frenkel and Schottky defects in fluorite 24 Figure 3. Schematic of energy levels of F centers 25 Figure 4. Schematic of types of F centers 26 Figure 5. Color center model by Staeber and Schnatterly (1971) 27 Figure 6. Image of fluorite from Bingham, NM with color zoning 28 Chapter 2 Figures 55 Figure 1a-m. Optical absorption spectra for each synthetic sample before and after irradiation 55 Figure 2a-m. Fluorescence spectra for each synthetic sample before and after irradiation 68 Figure 3a-m. EPR spectra for each synthetic sample before and after Irradiation 81 Figure 4. Color center model by Staeber and Schnatterly (1971) 94 Chapter 3 Figures 133 Figure 1. Color center model by Staeber and Schnatterly (1971) 133 Figure 2. Image of slices of naturally colorless Long Lake fluorite before and after irradiation 134 Figure 3. Image of slices of naturally colored Long Lake fluorite before and after irradiation 135 Figure 4. Fluorite crystal from the Tex Mex Mine in Bingham, NM 136 Figure 5a-c. Chondrite normalized REE patterns for each locality 137 Figure 6a-m. Optical absorption spectra for each natural sample before and after irradiation 140 iii Figure 7a-m. Fluorescence spectra for each natural sample before and after irradiation 153 Figure 8a-q. EPR spectra for each natural sample before and after irradiation 166 iv TABLES Chapter 2 Tables 53 Table 1. REE concentrations (ppm) and irradiation induced color of synthetic samples 53 Table 2. Luminescence peaks due to scattering 54 Chapter 3 Tables 131 Table 1. Sample names, localities and color before and after Irradiation 131 Table 2. REE concentrations (ppm) for natural samples 132 v ACKNOWLEDGEMENTS I would like to thank several people for helping me through the process of finishing this thesis. First and foremost, the completion of this project would not have been possible without the support, patience, encouragement and constant availability of my advisor John Rakovan. Thank you for all of your help! To Dr. Hailiang Dong, thank you for agreeing to review this thesis. There are many people in the Chemistry Department at Miami University who helped me with experiments and data analyses. Thanks to Dr. Andy Sommer, Dr. Gil Pacey and Brian Patterson, who helped me with my optical absorption experiments. Thanks go to Dr. Mike Crowder and two of his graduate students, Patrick Crawford and Nathan Wenzel for helping me with EPR experiments. In the Geology Department at Miami University, many people helped me with experiments. To John Morton, thank you for all your patience and guidance with my DCP experiments. The first stages of sample preparation could not have begun without the knowledge and support of Stephanie Bosze, and thanks go to Art Losey for his help and encouragement. A special thanks to Joseph Talnagi at the OSU Nuclear Reactor Laboratory for irradiating my many batches of samples, and another to Craig Hemann in the OSU EPR lab for helping me with my last EPR experiments. Thanks to all the graduate students in the department who were understanding and encouraging especially during the rough patches of this journey, including Stephanie Bosze, Art Losey, Tatia Taylor, Allison Crowley, Darin Snyder, Nicki Richmond, Jen Wingate, and many others. Finally, thanks to my family and Glen for their constant support and understanding, and especially to my Dad and Grandma Murrell, who constantly asked me “Is it done yet?” and helped me push towards the finish! vi CHAPTER 1. INTRODUCTION TO COLOR IN FLUORITE Introduction Mineralogists, physicists, and chemists have studied the colors of fluorite for almost a century. The reasons are diverse, including gemological concerns about color, the desire to understand the nature of defects within crystals (Berman, 1957), and the use of fluorite as lasers (Dantes, et al. 1996) to name a few. A great deal of spectroscopic data has been collected on both natural fluorite and synthetic fluorite doped with various impurities (Smakula, 1950; Scouler and Smakula, 1960; Staebler and Schnatterly, 1971; Anderson and Sabisky, 1971; Gaft et al., 2001 and many others). A few theories on the cause of specific colors have been accepted, such as the F center and purple color. More complex color centers involving impurities and structural defects are difficult to characterize. This study examines the color of fluorite samples from Bingham, NM, Long Lake, NY, and Westmoreland, NH, which have not been previously investigated. Pure fluorite with few or no defects detectable by present technology is invariably colorless. Add some impurities and/or structural defects and the color possibilities of fluorite are extremely varied in hue and intensity. The complex relationships between impurities and structural defects have led to great difficulty in pinpointing the exact cause of color in many fluorites, both synthetic and natural (Bill and Calas, 1978). The goals of this study include the determination of the cause of color in natural fluorites from three locations that have not been thoroughly examined (Bingham, NM; Long Lake, NY; Westmoreland, NH), and investigate the relationship between sectoral 1 zoning of REEs, defects and color. This chapter is an overview of the causes of color in minerals, and a review of previous studies of color and structural defects in fluorite. Color in Minerals There are many different causes of color in minerals, and most involve the interaction of light with electrons in the mineral structure. Nassau (1978) provides a good overview of the various causes that will be discussed only briefly here in the context of fluorite. Crystal field theory explains many of the accepted causes of color in fluorite. This formalism is predominantly associated with ionic crystals containing ions with unpaired electrons, like transition elements (with partially filled d orbitals), actinides, and lanthanides (with partially filled f shells). Of these, the lanthanides are by far the most prevalent in fluorite, in which they substitute for Ca2+ within the crystal structure. In such ions, the unpaired electrons may interact with visible light, absorbing certain wavelengths, and producing color. The energy levels (ground and excited states) at which the unpaired electrons can exist depend on the valence state of the ion, the symmetry of the ion in the crystal, the strength of the crystal field, and the strength of the bonding. Many authors have suggested, as well as presented evidence for, that lanthanides in certain oxidation states can play a role in the color of fluorite by crystal field transitions on them. Unpaired electrons do not exist exclusively on ions in minerals. They can also exist on structural defects such as vacancies. In alkali halides, extensive research has resulted in the confident characterization of various types of structural defects, most of 2 which have been found in fluorite, and some have been implicated as causes of color in fluorite. These will be discussed in more detail below. Several authors have also found evidence of defect complexes, some involving both impurities and structural defects, which are linked to color in fluorite. Crystal field theory may not be adequate to describe some of these, as they may involve molecular orbitals. This requires molecular orbital theory (MOT), which can describe situations where electrons are not located on single ions or structural defects, but in orbitals with multiple centers. MOT is similar to crystal field theory in that they both describe a set of possible energy levels for the electron(s), as well as the probability that a transition between levels will occur. A third mechanism for the cause of color in fluorite is the optical effect of scattering of light. It has been suggested that this occurs in fluorite, which has large (micron-sized) aggregates of defects such as calcium colloids (Bill and Calas, 1978; Braithwaite et al, 1973).