A nanocrystalline monoclinic CaCO
3 precursor of metastable aragonite.
SCIENCE ADVANCES 2018;
4:eaau6178. [PMID:
30547088 PMCID:
PMC6291313 DOI:
10.1126/sciadv.aau6178]
[Citation(s) in RCA: 14] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/28/2018] [Accepted: 11/15/2018] [Indexed: 05/29/2023]
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.
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