STRUCTURAL VARIATION in the PHOSPHATE OLIVINE LITHIOPHILITE- TRIPHYLITE SERIES and CHARACTERIZATION of LIGHT ELEMENT (Li, Be, AN
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STRUCTURAL VARIATION IN THE PHOSPHATE OLIVINE LITHIOPHILITE- TRIPHYLITE SERIES AND CHARACTERIZATION OF LIGHT ELEMENT (Li, Be, AND B) MINERAL STANDARDS Arthur Bill Losey Abstract This research is comprised of two projects involving the use of single crystal X- ray diffraction to for mineral structure determination. Chapter 1 involves determination of structural variation along the Mn ⇔ Fe solid solution of the lithiophilite-triphylite series, olivine structure types. Bond lengths and angle variance are examined to determine their variation along the Mn ⇔ Fe join and their relationships to other silicate and germanate olivine structures. The angle variance of the phosphate olivines was smaller in the M1 octahedron, which is in contrast to the other olivine structure phases examined in this study. In Chapter 2, diffraction data were collected on phenakite, danburite, spodumene, hambergite, lithiophilite, axinite, and prismatine, which are to be used as light element (Li, Be, and B) mineral standards. The data were used to determine the atomic positional parameters, site geometry, site occupancy, and polyhedral connectivity of each sample. STRUCTURAL VARIATION IN THE PHOSPHATE OLIVINE LITHIOPHILITE- TRIPHYLITE SERIES AND CHARACTERIZATION OF LIGHT ELEMENT (Li, Be, AND B) MINERAL STANDARDS 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 Arthur Bill Losey Miami University Oxford, Ohio 2001 Advisor Dr. John F. Rakovan Reader Dr. John M. Hughes TABLE OF CONTENTS Page Chapter 1 1 Structural Variation in the Phosphate Olivine Lithiophilite-Triphylite Series Chapter 2 27 Structure Refinement of Light Element (Li, Be and B) Mineral Standards ii TABLES Page Chapter 1 - Tables (1-4) 17 Table 1. Electron-microprobe results for the lithiophilite-triphylite 17 Series samples. Table 2. Crystal data and results of structure refinements for 18 lithiophilite-triphylite series samples. Table 3. Positional parameters and isotropic B values for atoms 19 in the lithiophilite-triphylite series samples. Table 4. Bond length (Å) and tetrahedral bond angles (°) for 20 the lithiophilite-triphylite series samples. Chapter 2 - Tables (1-8) 39 Table 1. Crystal data and structure refinement results. 39 Table 2. Atomic positional parameters and isotropic B values 40 for phenakite. Table 3. Atomic positional parameters and isotropic B values 41 for danburite. Table 4. Atomic positional parameters and isotropic B values 42 for spodumene. Table 5. Atomic positional parameters and isotropic B values 43 for hambergite. Table 6. Atomic positional parameters and isotropic B values 44 for lithiophilite. iii Table 7. Atomic positional parameters and isotropic B values 45 for axinite. Table 8. Atomic positional parameters and isotropic B values 46 for prismatine. iv TABLE OF FIGURES Page Chapter 1 - Figures (1-6) 21 Figure 1. (001) projection of the lithiophilite-triphylite series crystal 21 structure. Figure 2. Composition versus the bond lengths of the M2 site. 22 Figure 3. Composition versus the bond lengths of the M1 site. 23 Figure 4. O3 site geometry and the bond length variation that occurs 24 with Fe substitution for Mn. Figure 5. Variation in the M1 and M2 angle variance across the 25 lithiophilite-triphylite series samples.. Figure 6. (a) T angle variance versus T cation radius. (b) M1/M2 26 angle variance versus T cation radius. v Acknowledgements I would first like to thank my advisor John Rakovan for all of your guidance and support over my graduate career. Even though the apatite project was not feasible at this time, you were very supportive in suggesting alternate projects to research. John Hughes was also instrumental in the development and understanding of these single crystal X-ray diffraction studies. I would also like to thank you both for you patience, sharing your vast knowledge, and making my time at Miami University both challenging and enjoyable. I would like to add a special thanks to John Hughes for his understanding with the flow-through switch fiasco. I would also like to thank Darby Dyar and Carl Francis. Both contributed samples and chemical analyses that were used in these studies. I would also like to thank them for their comments and discussions about this research. Bill Lack of the Instrumentation Lab was also essential to my research. He kept the Cad4 in working condition and was always there on a moments notice to make repairs whenever they were necessary. I would like to thank all of the graduate students for their support and their discussions about crystallography. Even though this may not have been your area of expertise, talking with others was very helpful to my understanding of these projects. I would also like to especially thank Darin, C.B., Matt, and Shawn for their help with computing problems and for our lunchtime trips uptown. Special thanks goes to Kathleen Counter Benison. If it wasn’t for your help and encouragement during my undergraduate education, I would not have made it this far in the first place. vi Finally, thanks to everyone who turned a blind eye to my best friends T.J. and Otter. Their presence always brightened my day and made those tough and stressful days a little easier. vii CHAPTER 1 STRUCTURAL VARIATION IN THE PHOSPHATE OLIVINE LITHIOPHILITE- TRIPHYLITE SERIES Submitted to Canadian Mineralogist 1 ABSTRACT The crystal structures of five natural lithiophilite-triphylite series [Li(Mn,Fe)PO4] samples were refined to determine structural variation along the Mn ⇔ Fe solid solution and to elucidate variations in the Pnma olivine atomic arrangement. The refinements converged to R ≤ 0.017. Bonds at the O3 site are fundamental in understanding the response of the atomic arrangement as Fe concentration increases. The M2-O3a bond shortens by more than 0.06 Å and the M2-O3b bond shortens by ~ 0.02 Å over the solid solution series. This shift of the O3 oxygen toward the two coordinating M2 sites causes the M1-O3 bond to increase by approximately 0.03 Å, thus causing significant structural changes in the M1 site. Much previous work has focused on polyhedral distortions in the olivine structure. The angle variance for the M1, M2, and T polyhedra were calculated for each phosphate sample and published silicate and germanate olivine structures. In each case, the angle variance of the phosphate olivines was smaller in the M1 octahedron, which is in contrast to the other olivine structure phases examined in this study. However, examination of numerous olivine structures demonstrate that if the size difference in the radius of the M1 and M2 site cation is ≥ 0.17 Å the distortion is greater in the octahedral site that is occupied by the larger cation. Keywords: lithiophilite, triphylite, olivine structure, crystal structure. SOMMAIRE 2 Mots-clés: INTRODUCTION Minerals of the lithiophilite-triphylite series [Li(Mn,Fe)PO4] occur in evolved granitic pegmatites that are enriched in both Li and P. These phases are isostructural with olivine (Fig. 1). The octahedral cations in the lithiophilite-triphylite series are completely ordered between the M1 and M2 sites. Li only occupies the M1 site, whereas the M2 site is occupied by divalent Mn, Fe, and in some cases Mg. This complete ordering of cations is in contrast to the majority of olivine-structure phases in which there is extensive disordering of octahedral cations. Natrophilite (NaMnPO4) is also completely ordered, and is the only ordered olivine structure where the M1 cation is much larger than the divalent M2 cation (Moore 1972). In other ordered olivines such as monticellite (MgCaSiO4) and glaucochroite (MnCaSiO4), the divalent cations in the M1 site are smaller than the cations in the M2 site (Lager & Meagher 1978). Olivine is one of the most important upper mantle silicate phases. Various olivine group minerals occur in rocks with very diverse geochemical histories (Brown 1980), attesting to the importance of understanding the olivine atomic arrangement. Understanding cation ordering in the olivine structure has given insights into cation partitioning between melts and coexisting mineral phases (Brown 1980). Little structure work has been performed on members of the lithiophilite-triphylite series, particularly with respect to structural variations that occur with the Mn ⇔ Fe solid 3 solution. In many of the previous single crystal X-ray studies of these phases, synthetic samples were used (Yakubovich et al. 1977; Streltsov et. al. 1993; Geller & Durand 1960). Structure refinements using natural samples were performed by Finger & Rapp (1969). There have also been studies on the relationship between composition and unit cell parameters (Lumpkin & Ribbe 1983; Fransolet et al. 1984) in the lithiophilite- triphylite series. Fransolet et al. (1984) attempted to determine the Mn/(Mn+Fe) ratio from powder diffraction experiments. However, those methods were not found to accurately predict the unit cell parameters or Mn/(Mn+Fe) divalent cation ratios of our samples. Lumpkin & Ribbe (1983) and Fransolet et al. (1984) did note a structural variation with composition. In this study, single crystal X-ray diffraction experiments were performed on natural lithiophilite-triphylite samples with Mn/(Mn+Fe) ratios of 94, 73, 50, 21, and 11 (referred to as Lth94, etc.). The atomic arrangement of each sample was refined to elucidate the structural changes with composition in this series. Structural information for the Fe end member, Lth0, used for comparisons in this paper was taken from Streltsov et al. (1993). EXPERIMENTAL DETAILS Microprobe analysis 4 Compositional analyses of the lithiophilite-triphylite samples (Lth94, Lth50, Lth21, and Lth11) were performed on a Cameca MBX electron microprobe using WDS in the Department of Earth and Planetary Sciences at Harvard University (Table 1). Beam conditions were: 15 KeV, 23 nA, 16 x 16 µm raster. Sandia BA85 was used for matrix corrections assuming 9.5 wt.% Li2O. K-alpha lines were used for all elements.