Research ArticleGEOCHEMISTRY

A nanocrystalline monoclinic CaCO3 precursor of metastable aragonite

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Science Advances  12 Dec 2018:
Vol. 4, no. 12, eaau6178
DOI: 10.1126/sciadv.aau6178
  • Fig. 1 mAra from the drip water sample and from the top layer of the aragonite flowstone.

    (A) Flaky hydromagnesite particles occur together with an elongated mAra crystal (white arrow) in the drip water sample. (B) Tabular mAra grain from the same sample. (C) Aragonite (1) is associated with needle-shaped (2) and flaky (3) mAra crystals in the flowstone sample. The SAED patterns of these crystals are shown in Fig. 2. (D) Mg-bearing elongated mAra crystal from the flowstone sample. Chemical analyses obtained from the white circle area indicate ~10 atomic % Mg (black arrow).

  • Fig. 2 Compared to aragonite (1), the characteristic diffraction features of mAra (2 and 3) are the occurrence of satellite reflections.

    Satellites are marked by white circles. White arrows of (2) show streaking of reflections along {110} indicating disorder. Sample thickness gives rise to {001} reflections. The corresponding crystals of these SAED patterns are shown in Fig. 1C.

  • Fig. 3 Principal two-dimensional slices of the reciprocal lattice highlight diffuse scattering and satellite reflections of aragonite and reveal the crystallographic relationship between aragonite and mAra.

    The slices are reconstructed from the three-dimensional EDT volume. The measured grain is shown in the lower left corner of the first panel. Black and blue lines mark mAra and aragonite unit cells, respectively. mAra and aragonite crystallographic axes are labeled by black and blue indices, respectively. Diffuse scattering runs along the {110} diagonal of aragonite cell, breaks its orthorhombic symmetry, and indicates modulation. Discrete satellite reflections suggest that the unit cell of mAra is monoclinic, and the unit-cell parameter a is 28.6(7) Å.

  • Fig. 4 Structure models of mAra in comparison to aragonite.

    (A) Missing-atoms model of mAra showing incomplete Ca polyhedra. The coordination numbers are nine for Ca1 and Ca4 (blue polyhedra), eight for Ca3 and Ca6 (red polyhedra), seven for Ca2 (green polyhedra), and six for Ca5 (green polyhedra). (B) All-atoms mAra has ninefold coordinated Ca (blue polyhedra). (C) Standard aragonite. Black spheres mark C atoms.

  • Fig. 5 Aragonite and mAra are polytypes, and their structures can be built by OD layers.

    {110} slab is the basic structure unit (OD layer) of aragonite. Red spheres mark Ca atoms, and blue and green triangles correspond to CO3 groups that differ in relative height along the aragonite ⟨001⟩ projection. Direction of the layer shift is marked by + and − with respect to the basic OD unit. (A) Ordinary aragonite (marked by black lines) is a one-layer polytype, in which the OD layer shift is one way (+++). Dotted lines and italicized indices mark the unit cell size and the parameters of the monoclinic modulation-1 structure, respectively. (B) The ordered modulation-6 mAra structure (marked by dotted lines) can be built by a six-layer sequence (+++++−), in which there is an orientation change (marked by a black arrow) in the building OD layers.

  • Fig. 6 Proposed mechanism for metastable aragonite formation from drip water containing a Mg2+/Ca2+ ratio > 1.5.

    Initially precipitated mAra forms by incorporating Mg atoms and hydroxyl groups, which result in a disordered monoclinic structure. mAra is associated with abundant hydromagnesite in the 20-hour collected drip water and some magnesite in the flowstone samples. Abundant hydromagnesite formation is presumably related to the chemical variability of the drip water. Aragonite forms as a result of crystallographic ordering and Mg leaching from mAra. During its formation, depending on the composition of the primary phases, the Ca concentration increases, whereas Mg and water contents decrease.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/12/eaau6178/DC1

    Section S1. Crystallographic relationship between mAra and aragonite

    Fig. S1. Sample collection sites.

    Fig. S2. The EDT measured hydromagnesite grain from the drip water sample.

    Fig. S3. Magnesite coexisting with mAra in the sample collected from the surface of the aragonite flowstone.

    Fig. S4. Chemical analysis of the mAra crystal measured by EDT (Fig. 3) indicates ~2 at % Mg.

    Fig. S5. Satellite reflections of the SAED patterns (2 and 3 of Fig. 2) can be indexed with the mAra (monoclinic-6) unit cell.

    Table S1. Summary of EDT data for hydromagnesite and mAra.

    Table S2. Summary of structure refinements of hydromagnesite and mAra.

    Table S3. EDT-measured and SHELXL-refined hydromagnesite atomic coordinates (x1, y1, z1) versus literature data (x2, y2, z3) of (30).

    Table S4. Atomic coordinates and equivalent isotropic displacement parameters (Uiso in Å2) for the ab initio determined, missing-atoms (x1, y1, z1, Uiso1) and all-atoms refined (x1, y1, z1, Uiso2) mAra structures.

    Table S5. Ca–O bond distances (in angstrom units) for the all-atoms refined mAra structure.

    Data file S1. (Hydromagnesite.cif) cif file of the measured hydromagnesite.

    Data file S2. (Hydromagnesite.hkl) hkl file of the measured hydromagnesite.

    Data file S3. (Missing-atoms mAra.cif) cif file of the missing-atoms mAra.

    Data file S4. (All-atoms mAra.cif) cif file of the all-atoms mAra.

    Data file S5. (mAra.hkl) hkl file of the measured mAra.

  • Supplementary Materials

    The PDF file includes:

    • Section S1. Crystallographic relationship between mAra and aragonite
    • Fig. S1. Sample collection sites.
    • Fig. S2. The EDT measured hydromagnesite grain from the drip water sample.
    • Fig. S3. Magnesite coexisting with mAra in the sample collected from the surface of the aragonite flowstone.
    • Fig. S4. Chemical analysis of the mAra crystal measured by EDT (Fig. 3) indicates ~2 at % Mg.
    • Fig. S5. Satellite reflections of the SAED patterns (2 and 3 of Fig. 2) can be indexed with the mAra (monoclinic-6) unit cell.
    • Table S1. Summary of EDT data for hydromagnesite and mAra.
    • Table S2. Summary of structure refinements of hydromagnesite and mAra.
    • Table S3. EDT-measured and SHELXL-refined hydromagnesite atomic coordinates (x1, y1, z1) versus literature data (x2, y2, z3) of (30).
    • Table S4. Atomic coordinates and equivalent isotropic displacement parameters (Uiso in Å2) for the ab initio determined, missing-atoms (x1, y1, z1, Uiso1) and all-atoms refined (x1, y1, z1, Uiso2) mAra structures.
    • Table S5. Ca–O bond distances (in angstrom units) for the all-atoms refined mAra structure.
    • Legends for data files S1 to S5

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    Other Supplementary Material for this manuscript includes the following:

    • Data file S1. (Hydromagnesite.cif) cif file of the measured hydromagnesite.
    • Data file S2. (Hydromagnesite.hkl) hkl file of the measured hydromagnesite.
    • Data file S3. (Missing-atoms mAra.cif) cif file of the missing-atoms mAra.
    • Data file S4. (All-atoms mAra.cif) cif file of the all-atoms mAra.
    • Data file S5. (mAra.hkl) hkl file of the measured mAra.

    Files in this Data Supplement:

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