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A nanocrystalline monoclinic CaCO(3) precursor of metastable aragonite

Despite its thermodynamical metastability at near-surface conditions, aragonite is widespread in marine and terrestrial sediments. It abundantly forms in living organisms, and its abiotic formation is favored in waters of a Mg(2+)/Ca(2+) ratio > 1.5. Here, we provide crystallographic evidence of...

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Detalles Bibliográficos
Autores principales: Németh, Péter, Mugnaioli, Enrico, Gemmi, Mauro, Czuppon, György, Demény, Attila, Spötl, Christoph
Formato: Online Artículo Texto
Lenguaje:English
Publicado: American Association for the Advancement of Science 2018
Materias:
Acceso en línea:https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6291313/
https://www.ncbi.nlm.nih.gov/pubmed/30547088
http://dx.doi.org/10.1126/sciadv.aau6178
Descripción
Sumario:Despite its thermodynamical metastability at near-surface conditions, aragonite is widespread in marine and terrestrial sediments. It abundantly forms in living organisms, and its abiotic formation is favored in waters of a Mg(2+)/Ca(2+) ratio > 1.5. Here, we provide crystallographic evidence of a nanocrystalline CaCO(3) polymorph, which precipitates before aragonite in a cave. The new phase, which we term monoclinic aragonite (mAra), is crystallographically related to ordinary, orthorhombic aragonite. Electron diffraction tomography combined with structure determination demonstrates that mAra has a layered aragonite structure, in which some carbonates can be replaced by hydroxyls and up to 10 atomic % of Mg can be incorporated. The diagnostic electron diffraction features of mAra are diffuse scattering and satellite reflections along aragonite {110}. Similar features have previously been reported—although unrecognized—from biogenic aragonite formed in stromatolites, mollusks, and cyanobacteria as well as from synthetic material. We propose that mAra is a widespread crystalline CaCO(3) that plays a hitherto unrecognized key role in metastable aragonite formation.