INSIGHTS INTO THE KINETICS OF SOLID GYPSUM DEHYDRATION FROM WIDE- AND SMALL-ANGLE SYNCHROTRON X-RAY SCATTERING Katherine C. M. Gioseffi B. Sc. (Earth Science), Queensland University of Technology, Brisbane, 2015 Grad. Dip. (Psychology), Queensland University of Technology, Brisbane, 2009 B. A. Sc. (Biotechnology), Queensland University of Technology, Brisbane, 2009 Thesis submitted in fulfilment of the requirements for the degree of Masters of Applied Science School of Earth, Environmental and Biological Sciences Science and Engineering Faculty Queensland University of Technology 2019 Keywords Dehydration kinetics, dissolution-precipitation, gypsum, microstructure, polycrystalline, pre-stress, pseudomorphs, SAXS/WAXS, solid-state kinetics, synchrotron. i Abstract Fluids liberated through the dehydration of hydrous minerals play a major role in the occurrence of earthquakes, metasomatism, and metamorphism in the lithosphere. Dehydration reactions often create porosity because the anhydrous solid product is generally denser than its hydrous counterpart. The liberated fluids then drain through the newly established pore network. Pore pressure can build up when fluid release rate outpaces drainage. Subsequent over-pressure can result in hydraulic fracturing, weakening the solid phase. Gypsum is an ideal experimental analogue for hydrous minerals found in fault zones and subducting slabs due to its relatively low dehydration temperatures (~100°C). Previous studies on gypsum dehydration have predominantly used ‘black-box’ experimental set-ups where reaction progression is assessed using proxy measurements. Furthermore, in-situ gypsum dehydration kinetics have only been obtained using powdered starting materials. This thesis presents a unique set of novel in-situ dehydration experiments performed on polycrystalline gypsum discs (Volterra alabaster) using X-ray scattering techniques at the Australian Synchrotron. The dehydration reaction is tracked in-situ in real-time using: [1] Wide-angle X-ray Scattering (WAXS) and [2] Small-angle X-ray Scattering (SAXS), which monitor in-situ changes in mineral phase and nano-porosity, respectively. The primary focus is on the kinetics of polycrystalline gypsum dehydration. Results are reported for five different dehydration temperatures (120/128/141/151/170˚C) recorded at two different constant axial pre-stress states under radially drained conditions. These data show that [1] solid polycrystalline gypsum dehydrates slower than finer-grained powder, and [2] an increase in axial pre-stress enhances reaction rate. Activation energies of 73.8 kJ/mol (R2: 0.97) and 50.4 kJ/mol (R2: 0.98) were calculated for low- and high-pre-stressed states of gypsum, respectively (T: 128°C - 173°C). Three distinct bassanite morphologies were observed from post-mortem microstructural analysis: single-crystal pseudomorphs, multi-crystal pseudomorphs, and idiomorphic prismatic bassanite. Dehydration microstructures were highly sensitive to the sample chamber’s water vapour pressure. Morphologies are hypothesised to form along a spectrum of rate-coupled dissolution-precipitation mechanisms, where the dominant microstructure is related to the phase of water in the system. These results provide key insights into the complexity of polycrystalline gypsum dehydration. This has important implications when upscaling dehydration kinetics to understand the reaction-induced embrittlement and weakening of hydrous mineral host rock, and the migration pathways of fluids in the Earth’s crust. ii Table of Contents Keywords .................................................................................................................................. i Abstract .................................................................................................................................... ii Table of Contents .................................................................................................................... iii List of Figures ...........................................................................................................................v List of Tables .......................................................................................................................... xi List of Abbreviations ............................................................................................................ xiii List of Symbols ..................................................................................................................... xiv Statement of Original Authorship ...........................................................................................xv Acknowledgements ............................................................................................................... xvi Chapter 1: Introduction .......................................................................................1 1.1 Context, purpose and significance ..................................................................................2 Chapter 2: Literature Review ..............................................................................4 2.1 Gypsum Dehydration ......................................................................................................4 2.2 Kinetics .........................................................................................................................32 2.3 X-ray Scattering ............................................................................................................42 Chapter 3: Methodology ....................................................................................47 3.1 Starting materials ..........................................................................................................49 3.2 The Blach cell ...............................................................................................................49 3.3 Stage 1: Preliminary microstructural and chemical characterisation. ...........................51 3.4 Stage 2: In-situ time-resolved dehydration experiments using X-ray scattering ..........52 3.5 Stage 3: Post-Synchrotron microstructural and chemical characterisation. ..................62 3.6 Stage 4: Ex-situ dehydration experiments ....................................................................65 3.7 Stage 5: Temperature calibrations of the Blach cell .....................................................66 3.8 Ethics and Limitations ..................................................................................................67 Chapter 4: Results ..............................................................................................68 4.1 Starting material volterra alabaster: Microstructure and composition ..........................68 4.2 Cell loading conditions: calibration of temperature and axial pre-stress ......................70 4.3 In-situ time-resolved dehydration experiments using X-ray scattering ........................73 4.4 Post-experimental characterisation ...............................................................................79 Chapter 5: Discussion .......................................................................................101 5.1 Bassanite microstructures ...........................................................................................101 5.2 The effect of pre-stress on polycrystalline gypsum dehydration kinetics ...................114 5.3 The effect of microstructure on mineral dehydration .................................................120 5.4 Limitations ..................................................................................................................125 iii Chapter 6: Conclusions ....................................................................................128 Bibliography ............................................................................................................131 Appendices ...............................................................................................................146 Appendix A: Dehydration contour plots ...............................................................................146 Appendix B: WAXS 1D Scattering profiles: Final vs. Initial ...............................................152 Appendix C: Calculated Q Values For Bassanite .................................................................158 Appendix D: Calculated Q Values For Gamma-Anhydrite ..................................................162 iv List of Figures Figure 2-1: The crystallographic structures of the monoclinic gypsum (a) and bassanite (b), and the orthorhombic insoluble anhydrite (c), viewed along their c-axes. Image made using Vesta (Momma and Izumi, 2011). ....................................8 Figure 2-2: Post-dehydration SEM micrographs (a, b) and reaction curves for a drained gypsum dehydration experiment under constant pore pressure (128°C, Pc = 150MPa, Pp = 10 MPa); where (c): Volume of fluid expelled (ΔV/V0 versus time), and (d) the first derivative of (ΔV/V0). The 3 stages show the initial slow increase of fluid expelled (linked to isolated porosity pockets, (a)), followed by a period of rapid increase where the majority of fluid is expelled (linked to coalescence of pore network (b); followed by a final stage of fluid expulsion decrease indicating reaction cessation. Figures from Ko et al. (1997). ...........................................................................................................15 Figure 2-3: Photomicrographs from partially dehydrated polycrystalline gypsum