The Multiple Structures of Vaterite

Abstract

For many years the nature of the disordered structure of vaterite, a calcium carbonate polymorph that plays a significant role in biomineralisation, has been debated. In the past year, two independent studies (Angew. Chem., Int. Ed., 51, 7041; CrystEngComm, 14, 44) have proposed new, yet different models on the basis of electron diffraction experiments and ab initio calculations, respectively. Here, it is shown that there are at least three effectively isoenergetic models that are equally likely to describe the vaterite structure at room temperature. These distinct models, each consisting of multiple structures, can explain the disorder of vaterite in terms of different orientations of the carbonate anions, multiple stacking sequences of the carbonate layers, and possible chiral forms. Hence, vaterite is not a single “disordered” structure but should instead be considered as a combination of different forms, each of which can exhibit rapid interchange between multiple structures that exhibit similar average properties.

Nucleation of metastable aragonite CaCO 3 in seawater

“…For the more soluble phases to precipitate first, their nucleation barriers, and thus surface energies, must be inversely ordered ACC < vaterite < aragonite-calcite. Indeed, molecular dynamics simulations have shown that ACC formation might be barrierless (39), although the nucleation barrier of vaterite is challenging to quantify computationally, due to the complexity of the vaterite structure (44,45). Nevertheless, such inverse relationships between surface energy and bulk metastability would be consistent with other polymorphic oxides (8).…”

Calcium Carbonate Precipitation for CO2 Storage and Utilization: A Review of the Carbonate Crystallization and Polymorphism

“…Ševčík et al (2015) attempted to reveal the optimal synthesis conditions for preparing pure vaterite using CaCl2⋅2H2O and K2CO3 solutions. They performed a quantitative analysis by the Rietveld method employing powder X-ray diffraction (PXRD) with the assumption that vaterite exhibits two crystal structures, the well-established hexagonal structure (space group: P63/mmc) (Meyer, 1959) and triclinic structure (space group C1) proposed by Demichelis et al (2013). However, the exact crystal structure of vaterite is still under debate due to the difficulties in obtaining large, pure, single crystals of vaterite (Kamhi, 1963;Mugnaioli Ca ratios using CaCl2 and K2CO3 solutions via ATR FT-IR and PXRD spectroscopies at 25, 50, and 80°C, stirring at 350 rpm (Chang et al, 2017).…”

Enhancement of HPO₄&ndash;OH layered structure in octacalcium phosphate and its morphological evolution by acetic acid

“…38),39) For example, vaterite, a metastable phase of calcium carbonate, formed several different structures through cluster-based mineralization. Using ab initio calculation, Demichelis et al 38) showed that vaterite exhibits multiple types of crystal structures with essentially the same Ca 2+ ions locations but different CO 3 2¹ ions locations. Analysis of XRD patterns for the crystals obtained by cluster-based mineralization showed that while the main peaks of these crystals coincided well with the standard vaterite peaks and were identified as vaterite, worse agreement was observed for the minor peaks.…”