A critical analysis of calcium carbonate mesocrystals

Abstract

The term mesocrystal has been widely used to describe crystals that form by oriented assembly, and that exhibit nanoparticle substructures. Using calcite crystals co-precipitated with polymers as a suitable test case, this article looks critically at the concept of mesocrystals. Here we demonstrate that the data commonly used to assign mesocrystal structure may be frequently misinterpreted, and that these calcite/polymer crystals do not have nanoparticle substructures. Although morphologies suggest the presence of nanoparticles, these are only present on the crystal surface. High surface areas are only recorded for crystals freshly removed from solution and are again attributed to a thin shell of nanoparticles on a solid calcite core. Line broadening in powder X-ray diffraction spectra is due to lattice strain only, precluding the existence of a nanoparticle sub-structure. Finally, study of the formation mechanism provides no evidence for crystalline precursor particles. A re-evaluation of existing literature on some mesocrystals may therefore be required.

The Effect of Additives on the Early Stages of Growth of Calcite Single Crystals

“…The size at which an additive begins to affect the morphology of calcite depends on the additive binding strength, the concentration, and the supersaturation, which was rationalized by considering additive binding to available kink sites. Our data also confirm that calcite grows by a classical ion‐by‐ion mechanism in the presence of PSS18 rather than a non‐classical assembly of nanoparticles 27. These results provide insight into the growth mechanisms of sparingly soluble crystals such as calcite, and show that it is important to consider the action of additives on nucleation and growth to obtain product crystals with the desired properties.…”

“…During crystal growth, the concentration of Ca 2+ decreases significantly, while there is little change in the additive concentrations (as they are only sparingly incorporated into the calcite crystals). The additive/Ca 2+ ratio therefore increases during crystal growth and reaches very high levels shortly before growth terminates 18. Rough calculations showed how the solution composition changes during crystallization (Figure 3 a).…”

What Can Electron Microscopy Tell Us Beyond Crystal Structures?

“…On an opposite note, data used to assign CaCO 3 mesocrystals, which are a 3D array of iso-oriented single crystal particles are commonly misinterpreted. [18] Roughness on the calcite/polymer crystal surface can be falsely interpreted as homogeneous nanoparticle substructure when in fact, these nanoparticles are only present as a thin surface layer.…”

“…[16] They may also be hollow crystals, or crystals containing many defects or have incorporated organic matrix. [17,18] All types of crystal imperfections would lead to extra XRD peak broadening due to microstrain, resulting in a smaller value of crystal size. Figure 1.…”

Removal of Hardness from Water Samples by a Carbonation Process with a Closed Pressure Reactor

“…Under super-saturated conditions the Ca 2+ and Mg 2+ nucleated and formed calcium and magnesium carbonates [12,13]. Consequently, small calcite crystallites appeared on the surface, as shown in Figure 10, which presents the crystal growth mechanism of the carbonation process.…”