The Development of Morphology and Structure in Hexagonal Vaterite

Abstract

Inspired by the remarkable shapes and properties of CaCO(3) biominerals, many studies have investigated biomimetic routes aiming at synthetic equivalents with similar morphological and structural complexity. Control over the morphology of CaCO(3) crystals has been demonstrated, among other methods, by the use of additives that selectively allow the development of specific crystal faces, while inhibiting others. Both for biogenic and biomimetic CaCO(3), the crystalline state is often preceded by an amorphous precursor phase, but still limited information is available on the details of the amorphous-to-crystalline transition. By using a combination of cryoTEM techniques (bright field imaging, cryo-tomography, low dose electron diffraction and cryo-darkfield imaging), we show for the first time the details of this transition during the formation of hexagonal vaterite crystals grown in the presence of NH(4)(+) ions. The formation of hexagonal plate-like vaterite occurs via an amorphous precursor phase. This amorphous phase converts into the crystalline state through a solid state transformation in which order and morphology develop simultaneously. The mineral initially develops as polycrystalline vaterite which transforms into a single crystal directed by an NH(4)(+)-induced crystal plane that acts as a templating surface.

A classical view on nonclassical nucleation

“…image 30). For this system, the observed low contrast implies a phase of low density, and therefore it is sensible to assume that the objects contain a high degree of hydration.…”

Calcium carbonate polymorph control using droplet-based microfluidics

“…Additives including certain polymers, [28][29][30] small molecules, 31 and the ammonium ion [32][33][34] can uniquely promote the formation of vaterite, and vaterite has been precipitated in microemulsions of CTAB (cetyl trimethylammonium bromide) and water-in-oil microemulsions of sodium dodecyl sulphate (SDS). 35 A microwave-assisted method was applied to produce vaterite with a range of morphologies from water/ethylene glycol mixtures in the presence of the additives CTAB and SDS, 36 while stacks of vaterite platelets were produced on precipitation from a viscous solution of hydroxyethyl cellulose (HEC).…”

Crystallization by particle attachment in synthetic, biogenic, and geologic environments

“…In the bulk, hydrated ACC is stable in dry conditions but crystallizes in humid conditions or upon heating with the release of water (46,70). Although the observed coexistence of crystalline and amorphous material within early stage nanoparticles both in solution (69) and under Langmuir monolayers (47) suggests that solid-state crystallization may occur at the onset of the transition, ACC confined in small volumes remains stable for very long times even in the presence of bulk water, indicating that a heterogeneous nucleator for one of the crystalline polymorphs may be required (71,72). Transformation then typically occurs through local dissolution and reprecipitation (73).…”

“…Mineral systems may crystallize through an amorphous precursor at sufficiently high supersaturation (5,20,46,(66)(67)(68)(69), but the mechanism of the transition is unclear for most systems. For calcium carbonate formed abiotically from aqueous solution, the ACC precursor phase is initially hydrated (64,66).…”