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Title: On the transformation of amorphous calcium carbonate into calcite
Author: Stephens, Christopher James
ISNI:       0000 0004 2702 8484
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2010
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The work presented in this thesis considers the formation mechanisms of the mineral CaCO₃. The role of amorphous calcium carbonate (ACC) as a precursor to calcite, the most thermodynamically stable CaCO₃ polymorph, is investigated in several systems. Precipitation of CaCO₃ in bulk solution proceeds via ACC, identified using Raman spectroscopy, SEM, TEM and spectrophotometry characterisation techniques. The formation rate and stability of ACC was shown to be dependent on the initial solution concentration in the range of [Ca²⁺]= 2-10 mM. The amorphous phase of CaCO₃ is stabilised under moderate degrees of confinement. Precipitation of CaCO₃ within an annular wedge, formed around the region of contact of crossed cylinders, resulted in different crystal morphologies dependent upon the surface separation. Single calcite rhombohedra formed at surface separations greater than 5 μm, identical to those observed on equivalent surfaces in bulk solution. Irregular, multifaceted calcite particles formed at intermediate surface separations, whereas only ACC particles were observed at separations below 1 μm. There was a rapid transition from ACC into calcite upon the removal of the confining surface and subsequent incubation in depleted solution. Furthermore, heating of an amorphous particle after separation resulted in the transformation into calcite. The stabilisation of the amorphous phase was attributed to kinetic factors, since it was shown that ACC would be thermodynamically preferable to calcite at only sub-nanometre surface separations, at least three orders of magnitude smaller than observed here. The precipitation of CaCO₃ is influenced by the substrate properties, which afford control over number density, polymorphism and crystallographic orientation. On homogeneous functionalised self-assembled monolayers (SAMs) of mercaptohexadecanoic acid (MHA) on gold, face selective growth of calcite parallel to the (012) and (015) growth planes was observed, ascribed to favourable interactions between the inorganic CaCO₃ and the organic monolayer. Using patterned SAMs, formed using the deep-UV photo-lithography method, crystallisation at carboxylterminated regions resulted in localised undersaturation at polar-terminated regions, demonstrating that crystallisation is dependent upon concentration gradients. CaCO₃ crystallisation on weathered mica substrates yielded epitaxial growth of calcite in a (001) orientation, parallel to the mica basal cleavage plane. Despite a close lattice match between the Ca-Ca spacing parallel to the (001) plane (0.499 nm) and the adjacent K⁺ site spacing on the basal cleavage plane (0.512 nm), epitaxial overgrowth did not occur on freshly cleaved mica surfaces. It is proposed that the presence of naturally formed surface crystallites of K₂CO₃ are a necessary intermediary for the occurrence of epitaxial calcite. Amorphous-type particles, morphologically similar to those characterised in bulk solution, were observed during the first 60 s after nucleation on both these substrates, before transformation into the crystalline state. Crystallisation of CaC03 within arrays of sub-picolitre droplets supported on patterned SAMs was affected by the limited solution volume. Calcite crystals formed almost exclusively in a tetrahedral configuration, in contrast to the rhombohedral particles observed during precipitation from bulk solution. Multiple occupancy of droplets was rare after a 24 h growth period, attributed to concentration depletion. As in the other studied systems, sub-micron amorphous-type particles were present after short growth times, presenting further evidence for the crystallisation of CaCO₃ through an amorphous precursor phase.
Supervisor: Christenson, Hugo Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available