Establishing the Physical Basis for Calcification by Amorphous Pathways
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Establishing the Physical Basis for Calcification by Amorphous Pathways Christina R. Blue Dissertation submitted to the faculty of the Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy In Geosciences Patricia M. Dove, chair J. Donald Rimstidt Michael F. Hochella, Jr. Shuhai Xiao May 2, 2014 Blacksburg, Va Keywords: ACC, magnesium, pH, partitioning, mixed flow reactor, calcite, polymorphs © Christina R. Blue, 2014 ESTABLISHING THE PHYSICAL BASIS FOR CALCIFICATION BY AMORPHOUS PATHWAYS Christina R. Blue ABSTRACT The scientific community is undergoing a paradigm shift with the realization that the formation of carbonate minerals with diverse compositions and textures can be understood within the framework of multiple pathways to mineralization. A variety of common minerals can form via an amorphous pathway, where molecules or clusters aggregate to form a metastable amorphous phase that later transforms to one or more crystalline polymorphs. Amorphous calcium carbonate (ACC) is now recognized in a wide variety of natural environments. Recent studies indicate the chemical signatures and properties of the carbonate polymorphs that transform from an ACC pathway may obey a different set of dependencies than those established for the “classical” step-growth process. The Mg content of ACC and calcite is of particular interest as a minor element that is frequently found in ACC and the final crystalline products of calcified skeletons or sediments at significant concentrations. Previous studies of ACC have provided important insights into ACC properties, but a quantitative understanding of the controls on ACC composition and the effect of mineralization pathway on Mg signatures in calcite has not been established. This study utilized a new mixed-flow reactor (MFR) procedure to synthesize ACC from well-characterized solutions that maintain a constant supersaturation. The experimental design controlled the input solution Mg/Ca ratio, total carbonate concentration, and pH to produce ACC with systematic chemical compositions. Results show that ACC composition is regulated by the interplay of three factors at steady state conditions: 1) Mg/Ca ratio, 2) total carbonate concentration, and 3) solution pH. Findings from transformation experiments show a systematic and predictable chemical framework for understanding polymorph selection during ACC transformation. Furthermore, results suggest a chemical basis for a broad range of Mg contents in calcite, including high Mg calcite. We find that the final calcite produced from ACC is similar to the composition of the initial ACC phase, suggesting that calcite composition reflects local conditions of formation, regardless of the pathway to mineralization. The findings from this study provide a chemical road map to future studies on ACC composition, ACC transformation, polymorph selection, and impurities in calcite. ACKNOWLEDGEMENTS The work presented in this dissertation is truly a group effort, and I would be remiss if I did not mention the individuals who assisted in this project. I would like to thank undergraduates Lisa Scott and Morgan Distad for spending countless hours in the lab assisting in data collection. I would also like to thank Athena Tilley for analyzing all the ICP-AES samples collected for this study. A huge amount of thanks go to Nizhou Han for making my life a lot easier by keeping the lab in working order and making sure that I always had everything I needed for my research. Also, I would like to thank my committee members: Patricia Dove, Don Rimstidt, Mike Hochella, and Shuhai Xiao for their guidance throughout my project. I truly stood on the shoulders of giants. Last, and certainly not least, I would like to thank my colleagues in the Geosciences Department and in the Biogeochemistry of Earth Processes research group, both past and present, for their helpful discussions, support, and general good company. iii TABLE OF CONTENTS ABSTRACT………………………………………………………………………………………ii ACKNOWLEDGMENTS………………………………………………………………………..iii LIST OF FIGURES………………………………………………………………………………vi LIST OF TABLES……………………………………………………………………………...viii PREFACE………………………………………………………………………………………..ix CHAPTER 1. INTRODUCTION 1.1 MOTIVATION FOR STUDY………………………………………………………..1 1.2 STUDY OBJECTIVES……………………………………………………………….4 1.3 REFERENCES………………………………………………………………………..6 CHAPTER 2. A MIXED FLOW REACTOR METHOD TO SYNTHESIZE AMORPHOUS CALCIUM CARBONATE UNDER CONTROLLED CHEMICAL CONDITIONS ABSTRACT………………………………………………………………………………8 2.1 INTRODUCTION…………………………………………………………………….9 2.2 METHOD……………………………………………………………………………10 2.3 CHARACTERIZATION…………………………………………………………….13 2.3.1 Scanning Electron Microscopy (SEM)…………………………………….13 2.3.2 Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES)...14 2.3.3 Raman Spectroscopy……………………………………………………….15 2.3.4 Fourier-Transform Infrared Spectroscopy…………………………………15 2.3.5 Thermogravimetric Analysis (TGA)……………………………………….16 2.4 RESULTS……………………………………………………………………………16 2.5 REFERENCES………………………………………………………………………20 CHAPTER 3. CHEMICAL CONTROLS ON THE MAGNESIUM CONTENT OF AMORPHOUS CALCIUM CARBONATE ABSTRACT……………………………………………………………………………..22 3.1INTRODUCTION……………………………………………………………………23 3.2 EXPERIMENTAL METHODS……………………………………………………..26 3.2.1 Sample Synthesis…………………………………………………………..26 3.2.2 Sample Characterization…………………………………………………...29 3.3 RESULTS…………………………………………………………………………....30 3.3.1 Sample Characterization…………………………………………………...30 iv 3.3.2 Controls on Mg Content of ACMC………………………………………..32 3.4 DISCUSSION………………………………………………………………………..38 3.5 REFERENCES………………………………………………………………………42 CHAPTER 4. THE EFFECT OF SOLUTION CHEMISTRY ON THE TRANSFORMATION OF AMORPHOUS CALCIUM CARBONATE ABSTRACT……………………………………………………………………………..46 4.1 INTRODUCTION.…………………………………………………………………..47 4.2 METHODOLOGY…………………………………………………………………..51 4.2.1 ACC Synthesis…………………………………………………………….51 4.2.2 ACC Transformation………………………………………………………52 4.2.3 Sample Characterization.…………………………………………………..53 4.3 RESULTS……………………………………………………………………………57 4.3.1 Polymorph Characterization……………………………………………….57 4.3.2 Control of Solution Chemistry on Polymorph Selection.………………….58 4.3.3 Solution Chemistry and an Indicator of Transformation Process………….59 4.3.4 ACC Transformation Time and Polymorph Formation……………………64 4.3.5 Influence of Mixing vs. Non-mixing………………………………………66 4.3.6 Consequence of ACC Transformation for Calcite Composition.………….68 4.4 DISCUSSION………………………………………………………………………..71 4.5 CONCLUSIONS.……………………………………………………………………77 4.6 REFERENCES………………………………………………………………………78 APPENDIX A. SUPPLEMENATRY INFORMATION FOR CHAPTER 2 A1. ADDITIONAL CHARACTERIZATION FIGURES……………………………….83 A2. DISTRIBUTION COEFFICIENTS…………………………………………………86 APPENDIX B. SUPPLEMENTARY INFORMATION FOR CHAPTER 3 B1. ACC SYNTHESIS DATA…………………………………………………………..87 B2. SUPPLEMENTARY FIGURES…………………………………………………….88 APPENDIX C. SUPPLEMENTARY INFORMATION FOR CHAPTER 4 C1. ACC TRANSFORMATION DATA.………………………………………………..91 C2. SUPPLEMENTARY FIGURES.…………………………………………………..101 v LIST OF FIGURES Figure 2.1 Schematic of the mixed flow reactor system…………………………………….10 Figure 2.2 SEM images and Raman spectra of ACC synthesized from 2:1, 5:1, and 8:1 Mg/Ca solutions using the new MFR method…………………………………...14 Figure 2.3 ACC compositions are dependent upon the Mg/Ca ratio of the reactant solution…………………………………………………………………………..17 Figure 2.4 ACC synthesized at a constant Mg/Ca ratio (5:1) but with different carbonate concentrations and pH contain variable amounts of Mg………………………...19 Figure 3.1 SEM images show similar morphology and particle sizes of ACMC for all Mg contents (25–65 mole %) and reactant solution compositions…………………..31 Figure 3.2 Early experiments suggested the Mg content of ACMC correlates with solution alkalinity…………………………………………………………………………33 Figure 3.3 The interplay of carbonate concentration, pH, and solution Mg/Ca ratio on Mg content in ACMC………………………………………………………………..34 Figure 3.4 Mg partitioning into ACMC correlates with the steady state pH of the local precipitation environment………………………………………………………..37 Figure 4.1 SEM images show the morphologies and sizes of synthesized products………..55 Figure 4.2 XRD confirms the mineralogy of synthesized products…………………………56 2+ 2+ 2-/ 2+ Figure 4.3 Plotting the solution activity ratios of Mg /Ca and CO3 Ca during steady state precipitation of ACC gives a stability diagram for initial polymorph formation after ACC transformation……………………………………………..59 Figure 4.4 The evolution of solution chemistry differs between calcite and monohydrocalcite transformation……………………………………………………………………61 Figure 4.5 Higher solution Mg/Ca ratios result in longer transformation times and a transition to different polymorphs for mixed conditions.………………………..65 Figure 4.6 The stability diagram for initial polymorph formation after ACC transformation is different for unmixed conditions.………………………………………………..66 Figure 4.7 The transformation time for calcite increases with the output solution Mg/Ca ratio………………………………………………………………………………68 Figure 4.8 The output solution Mg/Ca ratio during ACC precipitation controls the Mg content of transformed calcite..………………………………………………….69 Figure 4.9 The Mg content of ACC and subsequent calcite are similar.……………………70 Figure A1 FTIR characterization of pure ACC and ACC with 32 mole % Mg……………..83 Figure A2 FTIR spectra of ACC synthesized from solutions with an initial Mg/Ca ratio of 2- 5:1 shows a systematic shift to higher wavenumbers in the ν1 (CO3 symmetric stretch) peak with increasing