Various new strategies have been recently developed to produce CaCO ₃ micro-/nanoparticles with controlled size, morphology, polymorphism and crystallinity, which are then surface modified, functionalized and hierarchically assembled to yield medical, environmental, and energy materials.

Calcium carbonate polymorphs were crystallized in alginate and xanthan hydrogels in which a degree of entanglement was altered by the polysaccharide concentration. Both hydrogels contain functional groups (COOH and OH) attached at diverse proportions on saccharide units. In all systems, the precipitation process was initiated simultaneously with gelation, by the fast mixing of the calcium and carbonate solutions, which contain the polysaccharide molecules at respective concentrations. The initial supersaturation was adjusted to be relatively high in order to ensure the conditions suitable for nucleation of all CaCO₃ polymorphs and amorphous phase(s). In the model systems (no polysaccharide), a mixture of calcite, vaterite and amorphous calcium carbonate initially precipitated, but after short time only calcite remained. In the presence of xanthan hydrogels, precipitation of either, calcite single crystals, porous polyhedral aggregates, or calcite/vaterite mixtures were observed after five days of ageing, because of different degrees of gel entanglement. At the highest xanthan concentrations applied, the vaterite content was significantly higher. In the alginate hydrogels, calcite microcrystalline aggregates, rosette-like and/or stuck-like monocrystals and vaterite/calcite mixtures precipitated as well. Time resolved crystallization experiments performed in alginate hydrogels indicated the initial formation of a mixture of calcite, vaterite and amorphous calcium carbonate, which transformed to calcite after 24 h of ageing.

Amorphous calcium phosphate (ACP) attracts attention as a precursor of crystalline calcium phosphates (CaPs) formation in vitro and in vivo as well as due to its excellent biological properties. Its formation can be considered to be an aggregation process. Although aggregation of ACP is of interest for both gaining a fundamental understanding of biominerals formation and in the synthesis of novel materials, it has still not been investigated in detail. In this work, the ACP aggregation was followed by two widely applied techniques suitable for following nanoparticles aggregation in general: dynamic light scattering (DLS) and laser diffraction (LD). In addition, the ACP formation was followed by potentiometric measurements and formed precipitates were characterized by Fourier transform infrared spectroscopy (FTIR), powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The results showed that aggregation of ACP particles is a process which from the earliest stages simultaneously takes place at wide length scales, from nanometers to micrometers, leading to a highly polydisperse precipitation system, with polydispersity and vol. % of larger aggregates increasing with concentration. Obtained results provide insight into developing a way of regulating ACP and consequently CaP formation by controlling aggregation on the scale of interest.

In this study, the influence of Mg²⁺ on the aggregation and morphology of precipitated aragonite crystals is investigated at different temperatures. Different from some investigations described in the literature, aragonite precipitates in chemical systems in which Mg²⁺ acts specifically as crystal habit modifiers and not as polymorphic selectors. The results show that at increased Mg²⁺ concentration and temperature the aragonite crystals are less aggregated and that the amount of crystals sharing {110} faces and having larger extension of {001} faces increases. These outcomes may be relevant in biological, geochemical and technological contexts.

CaCO₃ precipitation is physical-chemical basis of biomineral formation of hard tissue (shells, skeletons) in marine calcifying organisms (=biomineralization). Processes controlling biomineralization are still not fully clarified, so the study of influence of pH on basic processes of CaCO₃ precipitation should contribute to better understanding of biomineralization under climate change. This paper reports on the effect of initial pH (pH₀) and type of stirring (mechanical and magnetical) on spontaneous precipitation and phase composition, size and morphology of spontaneously precipitated CaCO₃ formed at the identical initial supersaturation, ionic strength and a(Ca²⁺)/a(CO₃²⁻) ratio. The initial pH varied in a range 8.50 ≤ pH₀ ≤ 10.50 and included values relevant for mimicking the conditions related to biomineralization in marine organisms. In all systems two CaCO₃ polymorphs were found: calcite and/or vaterite. The increase of pH₀ favoured the formation of rhombohedral calcite no matter the type of stirring. This was exclusively influenced by the systems’ pH₀ (other relevant initial parameters were identical). Furthermore, increase of pH₀ caused change of vaterite morphology from cauliflower-like spheroids to regular spherulites. The mechanically stirred systems produced larger calcite and vaterite particles and higher content of calcite.