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| Titel |
Coupled dissolution-precipitation as a mechanism for amorphous-to-crystalline calcium carbonate phase transition |
| VerfasserIn |
Carlos Manuel Rodriguez-Navarro, Krzysztof Kudlacz, Encarnacion Ruiz-Agudo |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250100897
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| Publikation (Nr.) |
 EGU/EGU2014-16915.pdf |
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| Zusammenfassung |
| Growing evidence shows that several calcium carbonate biominerals form via an amorphous
precursor phase. Such a biomineralization strategy could also be applicable for the
biomimetic synthesis of novel functional materials. A crucial step in this process is the
transformation of amorphous calcium carbonate (ACC) into calcite. However, controversy
exists as to what is the actual mechanism of this transformation: Is it a solid-solid (solid state)
or a dissolution/precipitation mechanism? Determining the transition mechanism is critical
for example in interpreting the formation of oriented crystalline structures in biominerals
(e.g., echinoderm spicles). We studied calcium carbonate precipitation and phase
transitions according to the overall reaction Ca(OH)2 + CO2 = CaCO3+ H2O. Mineral
phase transformations during this reaction were studied using transmission electron
microscopy (TEM). Our TEM analysis showed that two different types of ACC
are sequentially formed during this reaction. Type I ACC shows no well-defined
short-range order, while Type II ACC shows a short-range order corresponding to
calcite. Following e-beam irradiation, Type I ACC particles transform into randomly
oriented CaO nanocrystals, while irradiation of Type II ACC leads to the formation of
pseudomorphs made up of perfectly oriented aggregates of calcite nanocrystals. Moreover,
calcite crystals formed in solution or in air (85 % relative humidity) after Type
II ACC are also pseudomorphs made up of porous aggregates of preferentially
oriented calcite nanocrystals. Our results give experimental evidence showing that the
ACC to calcite transformation under relevant biomineralization conditions (low T
and P), also applicable in the biomimetic synthesis of calcite, is a pseudomorphic
dissolution-precipitation process. This mechanism involves the tightly interface-coupled
dissolution of the precursor amorphous phase (with the crystalline phase protostructure) and
concomitant deposition of the crystalline product (calcite) on the remaining (not fully
dissolved) precursor phase via epitaxial crystallization. The solubility and molar
volume differences between parent and product phase explain the generation of
porosity, which enables the progress of the reaction leading to a calcite pseudomorph. |
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