High-Magnesium Calcite Mesocrystals: Formation in Aqueous Solution under Ambient Conditions.

Mesocrystals of high-magnesian calcites are commonly found in biogenic calcites. Under ambient conditions, it remains challenging to prepare mesocrystals of high-magnesian calcite in aqueous solution. We report that mesocrystals of calcite with magnesium content of about 20 mol % can be obtained from the phase transformation of magnesian amorphous calcium carbonate (Mg-ACC) in lipid solution. The limited water content on the Mg-ACC surface would reduce the extent of the dissolution-reprecipitation process and bias the phase transformation pathway toward solid-state reaction. We infer from the selected area electron diffraction patterns and the dark-field transmission electron microscopic images that the formation of Mg-calcite mesocrystals occurs through solid-state secondary nucleation, for which the phase transformation is initiated near the mineral surface and the crystalline phase propagates gradually toward the interior part of the microspheres of Mg-ACC.

Formation of nano-magnesite in the calcareous spicules prepared under ambient conditions.

We report that high-Mg calcite spherulites can undergo a coarsening process to form calcareous spicules of ∼30 microns in width and several hundred microns in length after an aging process in air for a prolonged period. During the aging process, the crystallinity of the calcitic structure has been improved substantially with a significant migration of Mg ions toward the mineral surface. In a thin-foil sample of the spicule aged for 20 months, nanocrystallites of magnesite with minor substitution level of Ca ions have been found near the surface of the spicule.

Anhydrous amorphous calcium carbonate (ACC) is structurally different from the transient phase of biogenic ACC.

Amorphous calcium carbonate (ACC) is an important precursor phase of biogenic calcite. In this work, an in situ Ca L-edge X-ray absorption spectroscopic study has been carried out to monitor the phase transformation process of hydrated ACC from room temperature to 773 K in the presence of water vapor pressure at 0.4 mbar. The L2,3 crystal field splittings of the near edge X-ray absorption fine structure (NEXAFS) spectra acquired for hydrated and anhydrous ACC are indistinguishable. The transformation process from anhydrous ACC to calcite is greatly facilitated by the presence of water moisture. Our data acquired for nano-calcite are in close resemblance to those reported for "type 2" ACC in sea urchin larval spicules. We suggest that "type 2 ACC" or the "transient phase of ACC" is a disordered calcium carbonate phase with a nascent calcitic structure at the nanometer length scale.

Structural characterization of fluoride species in shark teeth.

In shark teeth we have identified the species fluorapatite, hydroxyfluorapatite and its defect site, calcium fluoride, and potassium fluoride. Their relative amounts in teeth at different development stages have been quantified. Calcium fluoride and potassium fluoride may be associated with the fluoridation mechanism in shark teeth.