Formation and Transformation of Amorphous Calcium-Magnesium Carbonates in Synthetic Seawater
Date
2013-02
Authors
Journal Title
Journal ISSN
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Publisher
New York State College of Ceramics at Alfred University. Kazuo Inamori School of Engineering.
Abstract
The aqueous chemistry, precipitation, and crystallization of metal-carbonates comprises a
vast field of research that underlies the urgency of CO2 sequestration, ocean-acidification, and
biomineralization. The results of recent experimental and computational studies suggest that
amorphous calcium and magnesium carbonates are precipitated from supersaturated aqueous
conditions by non-classical aggregation of ion pairs, dimers, dynamically-ordered-liquid-likeoxypolymers
(DOLLOPS), and prenucleation clusters (PNCs). We present the first high field
(20 T) 43Ca and 25Mg NMR studies of amorphous calcium-magnesium carbonates (ACC,
ACMC, AMC) materials. Direct integration of computational techniques with experimental
NMR provides a novel step forward toward multi-scale integration of computational and
experimental techniques. Supporting information is derived from X-ray diffraction (XRD),
thermogravimetric/differential thermal analysis (TGA-DTA), and scanning electron microscopy
– energy dispersive spectroscopy (SEM-EDS) and provides important comparison to the bulk
structures and composition.
High field NMR of amorphous carbonates demonstrates that amorphous carbonates
contain various types of local disorder, but does not corroborate the theory of polyamorphism
nor nano scale phase separations postulated by other workers. Carbon (13C) NMR of 13Cenriched
materials indicates a degree of Ca-Mg solid solution in ACMCs, as ACMC 13C
resonances cannot be adequately reconstructed from the pure ACC and AMC 13C resonances.
However, with increasing Mg-content (and therefore H2O content) 13C NMR resonances are
strongly influenced by water-carbonate hydrogen bonding, shifting to lower resonance frequency
and broadening. The 13C-NMR are well-fit with single Gaussian distributions, suggesting that
two-phase models of ACMCs are not required to explain our 13C NMR observations. Protoncarbon
cross polarization indicates that there is a H population proximal to carbonate groups for
all amorphous phases. 43Ca NMR yields line shapes that span the resonance frequency range of
all known crystalline calcium carbonate polymorphs and is well fit with a single Gaussian
distributions. 43Ca NMR does not support a theory of polyamorphisms, but rather suggests an
unstructured, continuous distribution of local environments that is unlike any specific crystalline
phase. The mean 43Ca chemical shifts vary 0.77 ppm from compositions x=0 to 0.5
[x=Mg/(Mg+Ca)], demonstrating that Mg2+ has very little influence on the molecular-scale 43Ca
environment in ACMCs. Through integration of quantum mechanical calculations, classical
MD, and NMR we ascertain a maximum mean Ca-O bond distance in our ACCs/ACMCs of 2.45
± 1 Å that is independent of composition. Unlike the indistinguishable local calcium
environments, 25Mg NMR of amorphous material gives evidence for several distinct overlapping
quadrupolar line shapes. These sites do not generate NMR resonances that are perfect matches
for known crystalline polymorphs of magnesian carbonates and extend toward lower resonance
frequencies far beyond the range of known equilibrium analogs. By comparison to the range of
reference phases, the low frequency singularities of ACMC-AMC resonances are consistent with
some population of Mg-O bond distances greater than 2.10 Å and/or some fraction of sites with
high coordination numbers (up to 8). The local Mg environment of a protodolomite
crystallization [x=Mg/(Mg+Ca)=0.6] exhibits 25Mg NMR parameters most similar to the
asymmetric Mg2+ coordination environment of lansfordite [Mg(CO3)2(H2O)4]2-or huntite.
Although H-C cross polarization indicates no H-bonding with carbonate the XRD gives not longrange
indications of huntite. The large effective radius of strongly hydrated Mg in the
protodolomite likely provides a driving force for cation ordering in dolomite.
Description
Advisory committee members: Scott Misture, S.K. Sundaram, Nathan Mellott. Dissertation completed in partial fulfillment of the requirements for the degree of PHD in Material Science and Engineering at the Kazuo Inamori School of Engineering, New York State College of Ceramics at Alfred University
Type
Thesis