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 Mg2+/Ca2+ ratio > 1.5. Here, we provide crystallographic evidence of a nanocrystalline CaCO3 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 CaCO3 that plays a hitherto unrecognized key role in metastable aragonite formation.

Németh, P., Mugnaioli, E., Gemmi, M., Czuppon, G., Demény, A., Spötl, C. (2018). A nanocrystalline monoclinic CaCO3 precursor of metastable aragonite. SCIENCE ADVANCES, 4(12) [10.1126/sciadv.aau6178].

A nanocrystalline monoclinic CaCO3 precursor of metastable aragonite

Mugnaioli, Enrico;
2018-01-01

Abstract

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 Mg2+/Ca2+ ratio > 1.5. Here, we provide crystallographic evidence of a nanocrystalline CaCO3 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 CaCO3 that plays a hitherto unrecognized key role in metastable aragonite formation.
2018
Németh, P., Mugnaioli, E., Gemmi, M., Czuppon, G., Demény, A., Spötl, C. (2018). A nanocrystalline monoclinic CaCO3 precursor of metastable aragonite. SCIENCE ADVANCES, 4(12) [10.1126/sciadv.aau6178].
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11365/1066530