Detection and control of calcium carbonate formation and phase conversion: laboratory and field work

Gao, Boyang (2022) Detection and control of calcium carbonate formation and phase conversion: laboratory and field work. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

This thesis work relates to calcium carbonate (CaCO₃) polymorphism, focusing on crystallization and dissolution in non-equilibrium situations, to show why understanding CaCO₃ chemistry can help control the reactivity of both laboratory-synthesized and naturally occurring carbonate minerals. My laboratory work focused on how two CaCO₃ polymorphs, calcite and aragonite, dissolve and recrystallize in aqueous solution. The experimental results show that aragonite dissolves (and recrystallizes as calcite) faster when there is more calcite present with it. To understand the chemistry behind this observation, the solubility constant and recrystallization rates of each polymorph need to be considered. These results show why putting these two polymorphs together in water causes a non-equilibrium situation that cannot be well-described by their solubility constants alone. In addition, the dissolution-recrystallization behavior of mixed powder is tremendously retarded by the addition of a small amount of polyphosphate in the suspension. Based on bulk and surface characterization data (Infrared Spectroscopy and Solid-state Nuclear Magnetic Resonance), the phosphate species is incorporated only near the CaCO₃ particle surface without forming any separate phosphate-rich phase as a protecting shell. These data show that such dispersed surface-embedded phosphate entities stabilize solids against recrystallization in an aqueous environment. Moreover, polyphosphate treatment can also slightly improve the thermal stability of aragonite in terms of phase conversion. The thesis work also applies similar carbonate chemistry knowledge to characterization method development for field research. Carbonate mineral deposits are a surface manifestation of serpentinization, a water-rock reaction that provides energy and carbon sources that could support microbial life on serpentine-rich areas of Earth, or even on other planets like Mars. Infrared spectra from this study identify calcite and aragonite phases from ground-truthed samples. Based on literature from both laboratory- and field-based studies, the presence of aragonite could be due to high pH water and magnesium ions produced by serpentinization. Similar literature shows that calcite could be formed directly or caused by phase conversion of aragonite over time. However, the period required for the phase transformation is unknown. The thesis work describes how knowledge of carbonate polymorph control in a laboratory setting can help us begin to understand what geochemical factors may be necessary for phase selection during the serpentinization process.

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