i Rare Earth Crystal Chemistry and Be-Si Disorder in Gadolinite from the White Cloud Pegmatite, South Platte District, Colorado, USA. By RHIANA ELIZABETH HENRY B. A. University of Colorado at Boulder, 2013 A thesis submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment of requirements for the degree of Master of Science Department of Geological Sciences 2018 ii This thesis entitled: Rare Earth Crystal Chemistry and Be-Si Disorder in Gadolinite from the White Cloud Pegmatite, South Platte District, Colorado, USA. written by Rhiana Elizabeth Henry has been approved for the Department of Geological Sciences Joseph Smyth Markus Raschke Date The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline. iii Henry, Rhiana Elizabeth (M. S., Geology, Department of Geological Sciences) Rare Earth Crystal Chemistry and Be-Si Disorder of Gadolinite from the White Cloud Pegmatite, South Platte Pegmatite District, Colorado, USA. Thesis directed by Professor Joseph Smyth Gadolinite is a monoclinic orthosilicate mineral that is part of the Gadolinite Supergroup of minerals. It occurs in beryllium and rare earth element (REE) bearing granites, pegmatites, and some metamorphic rocks (Baćík et al., 2014). The White Cloud Pegmatite is a small but rare earth element rich NYF (niobium-yttrium-fluorine) pegmatite located in the South Platte Pegmatite district of Colorado, USA. It is associated with the 1.08Gy (Smith et al., 1999) granitic Pikes Peak Batholith. The gadolinite in the White Cloud Pegmatite ranges from strong HREE (heavy rare earth element) to LREE (light rare earth element) dominant species. The gadolinite occurs in close association with other REE minerals such as thalénite, fergusonite, allanite, yttrian fluorite, bastnäsite, synchysite. We explored the crystal chemistry and structure of two petrographically distinct samples by electron microprobe chemical analysis (EMPA), single crystal X-ray diffraction (XRD), and Raman spectroscopy. One sample (Z2A) shows nearly full occupancy of Fe, and partial substitution of Si for Be in the Q tetrahedral site with slight substitution of Be into the T-site. The second sample (SWC) has up to 15% vacancy in the Fe site and up to 15% disorder between Be and Si tetrahedral sites. The Be-Se partial disorder indicates that Be may substitute for Si in natural systems and be more abundant than previously thought. The different REE distributions indicate that gadolinite can accept a broad range of rare earth element cations. iv ACKNOWLEDGEMENTS This research has been supported by the National Science Foundation grant NSF-EAR 14-16979, the Bolyard Scholarship from the Rocky Mountain Association of Geologists, and a donation from the Mile High Rock and Mineral Society. My heartfelt thanks go to my adviser and mentor, Professor Joseph Smyth, for his support through my undergraduate and graduate careers. His academic brilliance and outstanding compassion make him a treasure among all scientists. More thanks go to Professors Lang Farmer who inspired me to keep studying geochemistry and Professor Craig Jones for his unique grumpy geophysics instruction and inspiring my interest in tectonic history. I would also like to thank Professor Markus Raschke and Philip Persson for discovering field relations at the White Cloud pegmatite and initializing this project. Extra thanks go to Jason Van Fosson for preparing the thin sections and doing the preliminary investigations. Julien Allaz has my thanks for his thorough feedback and patience, guiding me on the microprobe, and being a beacon of logic to look up to. My gratitude also extends to Kristine Johnson for her guidance throughout graduate school, and to Derek Weller for his advice and support. Ann-Marie Odasz has my thanks for always believing in me, and encouraging me to look for the beauty in everything. Many extra thanks go to Bryan Barnhart for being at my side with many mugs of tea and plates of cheese as I wrote this thesis. v CONTENTS CHAPTER 1: INTRODUCTION…………………………………………………………………1 THE GADOLINITE SUPERGROUP OF MINERALS.………………………………….1 HISTORY AND OCCURRENCES OF GADOLINITE………………………………….6 CHEMISTRY AND OCCUPANCY OF STRUCTURAL SITES FOR GADOLINITE…………………………………………………………………….7 RARE EARTH ELEMENTS.……………………………………………………10 REE PEGMATITES.…………………………………………………….12 THE WHITE CLOUD PEGMATITE.…………………………………..14 GADOLINITE OF THE WHITE CLOUD PEGMATITE...…………………………….16 CHAPTER 2: SAMPLES AND METHODS….………………………………………………...17 SAMPLES AND DESCRIPTION.……………………………………………………...17 WHOLE ROCK ANALYSIS...………………………………………………………….21 X-RAY DIFFRACTION………………………………………………………………...21 ELECTRON MICROPROBE ANALYSIS.…………………………………………….22 RAMAN SPECTROSCOPY……………………………………………………………23 CHAPTER 3: RESULTS.……………………………………………………………………….24 WHOLE ROCK ANALYSIS…………………………………………………………...24 X-RAY DIFFRACTION.……………………………………………………………….26 vi ELECTRON MICROPROBE ANALYSIS…………………………………………….31 RAMAN SPECTROSCOPY...…………………………………………………………37 CHAPTER 4: DISCUSSION…………………………………………………………………...38 CHEMICAL CONSTITUENTS.……………………………………………………….38 RARE EARTH ELEMENT DISTRIBUTION…………………………………………38 OCCUPANCY OF A AND M CATION SITES.……….………………………………44 PRESENCE OF HYDROXYL AT THE ϕ-SITE.……………………………………....47 BE-SI DISORDER...…….………………………………………………………………49 TETRAHEDRAL RINGS AND STRUCTURAL VARIABILITY…....……………….50 LACK OF METAMICTIZATION………………………………………………………54 BEST CHEMICAL FORMULA………………………………………………………...55 COMPARISON TO ALLANITE AND THALÉNITE………………………………….60 PHASES OF CRYSTALLIZATION……………………………………………………61 CHAPTER 5: CONCLUSIONS…………………………………………………………………63 THE SEARCH FOR BE…………………………………………………………………63 REE AND VARIABLE CHEMICAL AND STRUCTURAL PARAMETERS………...63 REFERENCES.………………………………………………………………………………….65 APPENDIX………………………………………………………………………………………70 vii LIST OF TABLES 1. Gadolinite supergroup mineral classification diagram……………………………………2 2. a. Summarized whole rock elemental data for the White Cloud Pegmatite………..........25 b. Summarized whole rock elemental data for the White Cloud Pegmatite continued….26 3. X-ray diffraction data and refinement parameters of gadolinite samples SWC and Z2A.27 4. a. Selected nearest neighbor distances of gadolinite samples SWC and Z2A…………...28 b. Selected nearest neighbor distances of gadolinite samples SWC and Z2A continued..29 5. Atomic position, displacement parameters, and electrostatic energy of sites for gadolinite SWC……………………………………………………………………………………...30 6. Atomic position, displacement parameters, and electrostatic energy of sites for gadolinite Z2A………………………………………………………………………………………30 7. Refined structural occupancy of cation sites of gadolinite samples SWC and Z2A…….31 8. Average elemental compositions from the electron microprobe for thin sections of gadolinite samples SWC and Z2A, and grain mount of SWC …………………….........35 9. Summary of selected average atoms per formula unit from the electron microprobe for thin sections of gadolinite samples SWC and Z2A, and grain mount of SWC …………36 10. Total cation charge with EMPA data…………………………………………………….49 11. Total cation charge with XRD data, with EMPA data for REE+Y in the A-site...……...49 12. Si-Si long and short distances in the tetrahedral rings for gadolinite SWC and Z2A…...52 13. Best chemical formulas for gadolinite samples SWC and Z2A, calculated from EMPA data……………………………………………………………………………………….59 viii LIST OF FIGURES 1. The structure of gadolinite supergroup minerals.…………………………………………4 2. The structure of gadolinite supergroup minerals, displaying alternating layers.………….5 3. TO4QO4 tetrahedra in alternating rings…………………………………………………....5 4. Abundance of elements in Earth’s upper continental crust as a function of atomic number……………………………...................................................................................11 5. Basic geologic map of the Pikes Peak Batholith and South Platte Pegmatite District.….15 6. Photomicrograph of thin section WhC3A………………………………………………..18 7. Photomicrograph of thin section WhC4A………………………………………………..18 8. Photomicrograph of sample SWC in thin section………………………………………..19 9. Photomicrograph of sample Z2A, bottom left gadolinite, in thin section……………….20 10. Photomicrograph of sample Z2A, top right gadolinite, in thin section………………….20 11. BEI of gadolinite sample SWC in thin section…………………………………………..32 12. BEI of gadolinite sample SWC in thin section…………………………………………..32 13. BEI of gadolinite sample Z2A in thin section ……...…………………………………...33 14. BEI of gadolinite sample Z2A in thin section…………………………………………...33 15. BEI of gadolinite grain mount SWC……...……………………………………………...34 16. Raman spectra of thin sections and grains of gadolinite samples SWC and Z2A……….37 17. a. Chondrite normalized REE distribution in gadolinite sample SWC via EMP data…...41 b. Chondrite normalized REE distribution in gadolinite sample Z2A via EMP data……41 18. Sum Y+HREE vs. Sum LREE…………………………………………………………...42 19. Whole rock chondrite normalized REE distribution.…………………………………….43 20. Y/(HREE+Y) vs. Y ……………………………………………………………………...44 21. Fe vs. total LREE………………....……………………………………………………...45 ix 22. Trace amounts of Ca vs. Fe …………………………………….......................................45 23. Trace amounts of Th vs Fe……………………………………………………………….46 24. M-site cation (Fe) vs. A-site cations (REE, Y, Th, Ca) ……………................................47 25. b lattice parameter vs. Si-Si Short tetrahedral ring distance with XRD data…………….52 26. c lattice parameter vs. Si-Si