1- RARE-EARTH ELEMENTS IN PYROMORPHITE-GROUP MINERALS A thesis submitted for the award of the degree of Doctor of Philosophy of the University of London by Helen Pauline Newby University of London Reactor Centre Department of Mechanical Engineering Imperial College of Science and Technology October 1981 2- ABSTRACT A survey of rare-earth element (REE) concentrations in pyro- morphite-group minerals, PbsCPOi^AsO^jVO^) 3CJI, was carried out using instrumental neutron activation analysis. 22 specimens from various locations, comprising 18 members of the pyromorphite-mimetite series, PbsCPOi+jAsO^) 3CJI, and 4 vanadinites were examined. REE were detected in all but two specimens (both mimetites). The lowest concentrations were found in vanadinites, in which only La and Ce were detected, and in some near end-member mimetites, and the highest in phosphatian mimetite and arsenious pyromorphite. The minerals tend to be enriched in Ce-group REE with the Ce-group/Yt-group ratio being highest in mimetite and lowest in pyromorphite. Comparison of REE concentrations in pyromorphite from the Carrock Fell Area, Cumberland with REE concentrations in associated, contemporary psilomelane, (Ba,H20)2M115O1 Q, and plumbogummite, PbAfl^PO^) 2 (OH) 5H20, suggests that crystallo-chemical factors are important in determining REE concentration profiles in minerals pre- cipitated from hydrothermal solution. In doping experiments using synthetic pyromorphite, which were designed to assess the influence of various factors on the partition of REE, enrichment of Ce-group REE pyromorphite occurred in acid solution in the absence of complexing ions. In neutral and acid solution, sulphate ions were effective in keeping a higher proportion of RE)£ ions in solution, but produced fractionation of Ce-group and Yt-group REE only in acid solution. In alkaline conditions, carbonate ions were able to hold back RE ions in solution only if the concentration of NaC£ in solution was low, and produced no fractionation of Ce-group and Yt-group elements. 3 TABLE OF CONTENTS Page ABSTRACT 2 TABLE OF CONTENTS 3 LIST OF TABLES 5 LIST OF DIAGRAMS 7 ACKNOWLEDGEMENTS 8 CHAPTER 1 INTRODUCTION 9 1.1 Reasons for the analysis of pyromorphite-group minerals by means of neutron-activation analysis 9 1.2 Survey of rare-earth element concentrations in pyromorphite-group minerals 10 1.3 Investigation of the factors influencing rare- earth element concentrations in pyromorphite- group minerals 11 CHAPTER 3 LITERATURE REVIEW 15 2.1 Chemistry and crystal structure of minerals belonging to the pyromorphite series 15 2.2 Previous studies of rare-earth elements in pyromorphite-group minerals 2.3 Factors which may determine rare-earth element concentrations in minerals precipitated from hydrothermal solutions 2.4 Brief description of the geology and mineralogy of the Carrock Fell area 24 2.5 Laboratory synthesis of pyromorphite 29 CHAPTER 3 EXPERIMENTAL 31 3.1 Analysis of rare-earth elements in natural pyromorphite-group minerals 31 3.2 Analysis of rare-earth elements in rocks and minerals from the Carrock Fell area 35 3.3 Doping experiments using laboratory-synthesized pyromorphite 36 4 Page CHAPTER 4 RESULTS 45 4.1 Survey of rare-earth element concentrations in pyromorphite-group minerals 45 4.2 Study of pyromorphite and accompanying minerals from the Carrock Fell area 46 4.3 Results of the doping experiments 47 CHAPTER 5 DISCUSSION 81 5.1 Analysis of minerals and rocks from the Carrock Fell area 81 5.2 Results of doping experiments 83. 5.3 mineralRare-earts ho f elementhe pyromorphite-grout concentrationsp in natural 87 5.4 Summary 92 CHAPTER 6 SUGGESTIONS FOR FURTHER RESEARCH 94 APPENDIX 1 CHEMICAL ANALYSIS OF PYROMORPHITE-GROUP MINERALS 97 APPENDIX 2 NON-STOICHIOMETRY IN SYNTHETIC PYROMORPHITE 101 REFERENCES 105 5 LIST OF TABLES Page Table No. Title 1 Unit cell dimensions of pyromorphite-group minerals 19 2 y-rays used for REE analysis 1 : Ge(Li) detector 33 3 Detection limits for the determination of REE in pyromorphite, vanadinite and mimetite 34 4 y-rays used for REE analysis 2 : Ge detector 37 5 Pyromorphite-group minerals used in general survey of REE distribution 41 6 Mineral specimens from the Carrock Fell area 42 7 Rock specimens from the Carrock Fell area 43 8 Conditions under which precipitates of synthetic pyromorphite were left to age 44 9 REE concentrations (ppm) in pyromorphite, mimetite and vanadinite 49 10 Chondrite-normalized REE abundances in pyromorphite, mimetite and vanadinite 51 11 REE concentrations in pyromorphite and accompanying minerals from the Carrock Fell area 58 12 Chondrite-normalized REE abundances in pyromorphite and accompanying minerals from the Carrock Fell area 59 13 Distribution coefficients for the partition of REE between pyromorphite and associated psilomelane and plumbogummite 65 14 REE concentrations (ppm) in rocks from the Carrock Fell area 68 15 Chondrite-normalized REE abundances in rocks from the Carrock Fell area 69 16 Normalized REE concentrations in synthetic pyromorphite 72 6 LIST OF TABLES Page Table No. Title 17 Normalized concentrations of REE remaining in solution 74 18 Distribution coefficients for the partition of REE between synthetic pyromorphite and aqueous solution 76 19 Chemical analysis of minerals used in the general survey of REE distribution in the pyromorphite series 99 20 Sodium and chlorine concentrations in synthetic pyromorphite 104 7 LIST OF DIAGRAMS Page Fig. 1 The crystal structure of pyromorphite: projection on to the a-b plane 18 Fig. 2 Sketch map of the geology of the Carrock Fell area 26 Fig. 3 oSketcf thhe maCarrocp showink Felgl minera Complel xvein s at the western end 27 Figs. 4-18 Chondrite-normalized REE concentrations in pyromorphite- group minerals 53-57 Figs.19-23 Chondrite-normalized REE concentrations in pyromorphite and accompanying minerals from the Carrock Fell area 60-64 Fig. 24 Partition of REE between plumbogummite and pyromorphite: distribution coefficients plotted against ionic radius 66 Fig. 25 Partition of REE between psilomelane and pyromorphite: distribution coefficients plotted against ionic radius 67 Figs.26-30 Chondrite normalized REE concentrations in rocks from the Carrock Fell area 70-71 Figs.31-35 REE distribution between synthetic pyromorphite and aqueous solution 77-80 Fig. 31 Variation of partition coefficient with time: REE added after the formation of pyromorphite 77 Fig. 32 Variation of partition coefficient with time: pyromorphite precipitated in the presence of REE 77 Fig. 33 Variation of distribution coefficient with pH 78 Fig. 34 Effect of sulphate ions on the distribution coefficient 1: in neutral saline solution 79 Fig. 35 Effect of sulphate ions on the distribution coefficient 2: in acid solution 80 8- ACKNOWLEDGEMENTS I would like to thank my supervisors, Dr. A. Clark, British Museum (Natural History), and Dr. T.D. Mac Mahon, University of London Reactor Centre, for their advice and constructive criticism. I would also like to thank the staff of the Mineralogy Department, British Museum (Natural History), especially those of the Rock and Mineral Chemistry Section, for making the time I spent at the Museum so enjoyable. I am particularly grateful to Dr. P. Henderson for his interest, encouragement and most helpful discussion, to Dr. C. Stanley for providing rock and mineral specimens, and to Mr. J. Easton and Mr. V. Din for their advice and interest in the chemical analysis and doping experiments. I would also like to thank Mrs. J. Bevan for help with the electron microprobe analysis. For instruction and assistance in the use of the Nuclear Data analyser and data-processing system, I am indebted to Dr. S. Parry (University of London Reactor Centre) and Dr. C.T. Williams (British Museum (Natural History)). Other staff at University of London Reactor Centre to whom I owe special thanks are: Mr. E.A. Caesar and the reactor operators for irradiating samples, Miss M.J. Minski for support and encouragement and, not least, Mrs. J. Vine for typing this thesis. 9- CHAPTER 1 INTRODUCTION 1.1 REASONS FOR THE ANALYSIS OF PYROMORPHITE-GROUP MINERALS BY MEANS OF NEUTRON ACTIVATION /0_ \ rl pyomorpWite Pb^tPO^jU , The pyromorphite series of minerals comprises j^mimetite PbsCAsO^) 3CJI, and vanadinite PbsCVO^)3C2,. The minerals are'found in the oxidized zone of ore deposits containing galena and are thought to be of secondary origin. (The mineralogy and chemical properties of pyromorphite-group are. minerals discussed in Section 2.1.) Extensive substitutional solid solution occurs between pyro- morphite, mimetite and vanadinite and variations in the ratio PO^: AsO^rVO^ are often found within a single mineral grain. Variations in composition in mineral grains are best determined using electron microprobe analysis, but with pyromorphite-group minerals there are two problems associated with microprobe analysis. The first is that the arsenic K X-ray line suffers serious interference from the lead L<* 1 X-ray line and the only other arsenic X-ray which can be used is the K X-ray which is much less PI intense. The second is that, because of the large amounts of lead and arsenic present, large corrections for absorption and atomic number differences have to be applied to the X-ray data. One of the advantages of neutron activation analysis (INAA) is that the detection limits for some elements are very low enabling small sample weights to be used. One of the original purposes of applying instrumental neutron activation analysis to pyromorphite-group minerals was, therefore, to see whether it could be used as an alternative to, or in conjunction with electron microprobe analysis as a means of . t • measuring variations m composition between difference parts of a 10- mineral grain. In addition, it was hoped to discover whether the various colours exhibited by pyromorphite-group minerals could be related to the presence of specific trace elements. 1.2 SURVEY OF RARE-EARTH ELEMENT CONCENTRATIONS IN PYROMORPHITE- GROUP MINERALS The application of INAA to a selection of pyromorphite-group minerals from the British Museum (Natural History) revealed trace amounts of rare-earth elements (REE) in most specimens.