Amorphous Calcium Carbonate: Synthesis and Potential Intermediate in Biomineralization

Abstract

gree of amorphous character. This would be expected as the deposition occurs at room temperature and the presence of any crystalline material is in itself remarkable. Different polymorphs have differing optical properties; hence this could provide a simple route to a generally inaccessible material with potentially interesting applications. Scanning electron microscopy (SEM) revealed the films (14 days deposition) were composed of nanosized HgS spheres, from 20 to 250 nm in diameter (Fig. 3). The nanodimensions of the particles could explain the stability of the b-polymorph film, as small particles can often exist in energetically unfavorable crystal phases.[20] Absorption spectroscopy on the same film showed an onset of absorption at ca. 1 eV with an excitonic feature at ca. 1.3 eV (Fig. 2b). Although this does not fit with any theoretical data, other studies have shown that band-edge measurement of HgS materials do not correlate with predicted bandgaps.[21]In conclusion, we have prepared a novel mercury(II) dithiocarbamate complex with an eight-membered ring structure. The molecule has been used as a room-temperature precursor to a metastable phase of mercury sulphide not normally accessible via low-temperature reaction pathways. ExperimentalIn a typical synthesis, [Hg(S 2 CNMe(EtPh)) 2 ] 2 was prepared by dissolving 1.95 g (6.1 10 ±3 M) mercury acetate in 150 mL ethanol, followed by filtration. To this was added dropwise a mixture of carbon disulfide (0.8 mL, 0.012 M) and N-methylphenethylamine (2 ml, 0.012 M) in 50 mL ethanol, producing an immediate white precipitate. After 4 h stirring, the precipitate was filtered, air dried, and redissolved in 50 mL dichloromethane. Storage at 4 C resulted in yellow needle-like crystals, the single-crystal X-ray structure (120 K) of which is shown in Figure 1. It is noteworthy that the precursor started to decompose in solution and a black material was observed (HgS). Once isolated, the yellow crystals were indefinitely stable at room temperature. To produce a b-HgS thin film, 0.05 g of [Hg(S 2 CNMe(EtPh)) 2 ] 2 was dissolved in 10 mL acetone producing a optically clear colorless solution, into which a glass substrate was placed vertically.

A classical view on nonclassical nucleation

“…In contrast to the scenarios proposed previously for other systems (11,14), we conclude that liquid-liquid separation in the studied system happens via an activated process. Our total calcium and carbonate concentrations of ∼1 mM at t σ max are well below the spinodal limit reported by Zou et al (15), although we note that their solutions were not prepared identically.…”

“…Liquid-liquid phase separation has been proposed to occur in CaCO 3 solutions. Faatz et al (11) presented the basis for a phase stability diagram including liquid-liquid phase separation. Wolf et al (12) performed experiments in acoustically levitated droplets and observed the formation of emulsion-like structures in transmission electron microscopy (TEM), which were proposed to be a dense liquid phase (DLP).…”

Reconciling disparate views of template-directed nucleation through measurement of calcite nucleation kinetics and binding energies

“…4). Consequently, despite the recent reports of prenucleation clusters (41)(42)(43), liquid-liquid separation (44)(45)(46), and multiphase aggregationbased pathways of calcite formation (41,42,47) in bulk solutions, the results presented here provide strong evidence that the classical theory of nucleation provides a good description of calcite formation on ionized surfaces and that interfacial energy is a useful concept even when critical nuclei are in the nanometer range. Calculations of γ assume rhombohedral nuclei.…”

Testing the cation-hydration effect on the crystallization of Ca–Mg–CO 3 systems

“…This observation is consistent with the hypothesized role of water in limiting Mg-rich carbonate formation and likely constitutes direct evidence that quantifies the hindrance of cation hydration on Mg 2+ incorporation into calcite lattice. Lastly, our experimental approach provided a synthetic route to produce anhydrous amorphous magnesium carbonate at room temperature and atmospheric pCO 2 , which may help to extend the current effort in understanding amorphous calcium carbonate (20,(53)(54)(55) to amorphous magnesium carbonate (AMC), both of which provide a low-energy pathway for carbonate mineralization in biological (organic-rich) environments. To the best of our knowledge, no literature report is available to date documenting the formation of anhydrous AMC, although the synthesis of hydrous forms, Ca (1−x) Mg x CO 3 ·nH 2 O (0 ≤ x ≤ 1), was achieved at elevated temperature (19).…”