Amorphous calcium carbonate

Chalk, A native amorphous calcium carbonate composed of calcareous remains of minute marine organisms. It can be used as antacid or stabilizer in expl and proplnt compns(See Calcium Carbonate, under Carbonates)... 

Sea urchin larvae are about a hundred micrometers in diameter. They have an internal skeleton that supports the soft tissues. The skeleton is composed of one or several pairs of intricately shaped spicules, the morphologies of which vary among different species. The spicule is composed of two different minerals, amorphous calcium carbonate and calcite [66]. The mineral phases in the adult skeleton are thought to be similar to those in the larval skeleton [67]. In fact almost the whole echinoderm phylum appears to use this type of material for constructing a large variety of skeletal elements. 

The cells surrounding the spiculogenic cavity have granules that contain amorphous calcium carbonate [74, 79]. It is suspected that this mineral phase forms initially within the cells and is then transported as such to the spiculogenic cavity. 

An interesting analogous in vitro experiment was performed by Xu el al. [92] in which the presence of a polyelectrolyte in solution resulted in a layer of amorphous calcium carbonate forming under a structured monolayer. This subsequently transformed into a thin layer of polycrystalline calcite. The crystal growth in this in vitro system occurs by phase transition of amorphous calcium carbonate into calcite and not by dissolution of amorphous phase and reprecipitation of calcite crystals. 

Aizenberg, J. 1996. Stabilization of amorphous calcium carbonate by specialized macromolecules in biological and synthetic precipitates. Advanced Materials, 8 222-5. 

The non-pattemed CaCOj films could be observed to crystallize within 1 h by optical microscopy. However the patterned films stayed amorphous for 2-3 h under ambient conditions and were only completely crystalline after 24 h, which is probably due to the use of ethanol in the patterning procedure, as this is known to stabilize ACC (amorphous calcium carbonate). Subsequently cell culture experiments were performed and the results indicated that the CaCOj substrates support rat bone marrow stromal cell attachment, proliferation and differentiation into osteoblast and osteoclast-like cells. Moreover, mineral formation by the osteoblast-like cells was favored on the CaCOj films compared to the developed polymer films. Also, the osteoclast-like cells can degrade the CaC03 films. Therefore, these patterns of CaCOj films can be regarded as suitable 2D model substrates for bone cells. 

ACC = Amorphous calcium carbonate RNA = Ribonucleic Acid TEM = Transmission Electron Microscope XANES = X-ray Absorption Near Edge Stmcture. 

Most shells or exoskeletons are made of crystalline minerals, but amorphous calcium carbonate (ACC) is also present. These biological ACC contain significant amounts of Mg, P and rich glutamic acid proteins. ... 

PoLiTi, Y., Arad, T., Klein, E., Weiner, S. Addadi, L. 2004. Sea urchin spine calcite forms via a transient amorphous calcium carbonate phase. Science, 306, 1161-1164. 

Brecevic, Lj. and Nielsen, A.E. (1990) Precipitation and properties of an amorphous calcium carbonate. In Industrial Crystallization 90 (11th Symposium, Garmisch-Partenkirchen), A. Mersmann (ed.), GVC-VDI, Dusseldorf, 241-246. 

Li, C., Hong, G.S., Yu, H., Qi, L.M. Facile fabrication of honeycomb-patterned thin films of amorphous calcium carbonate and mosaic calcite. Chem. Mater. 22, 3206-3211 (2010)... 

Hild, S., Marti, O., and A. Ziegler. 2008. Spatial distribution of calcite and amorphous calcium carbonate in the cuticle of the terrestrial crustaceans Porcellio scaber and Armadillidium vulgare. Journal of Structural Biology 163 100-108. 

Amorphous calcium carbonate (ACC) is a metastable precursor to crystalline CaCOa phases that precipitates by aggregation of ion pairs and prenucleation clusters. Ca solid-state NMR spectroscopy was used to probe the local structure and transformation of ACC synthesized from seawater-like solutions with and without... 

Fig. 5.4 Parameters observed during the crystallisation of composite calcium carbonate fillta- with colloidal morphology. AmCC amorphous calcium carbonate...

Alternatively, due to differences in the nucleation mechanism, rhombohedral PCC (r-PCC) could be obtained through amorphous calcium carbonate (AmCC) and partially carbonated basic calcium carbonate (BCC), as shown below (CamiOTia et al. 2004). 

In a further study, DiMasi and colleagues investigated the kinetics of amorphous CaCOs formation at a fatty acid monolayer interface using synchrotron X-ray reflectivity measurements [173]. In-situ experiments found three different parameters that control CaCOs mineralization in the presence of arachidic acid monolayers, PAA, and Mg + ions. Firstly, the crystal growth rate depends on the concentration of counterions and not on the polymer concentration in solution. Secondly, the soluble polymer only affects the lifetime of the amorphous calcium carbonate. And finally, the sole effect of Mg + is to delay the mineral film formation. These data thus suggest that competitive adsorption (e.g. Mg + vs. Ca +) is another parameter to consider in controlled mineralization processes. 

X-ray diffraction analysis of the films showed calcite, vaterite, and a broad amorphous region due to the organic material and/or amorphous calcium carbonate. 

From the experimental data it can be concluded that the films have a diameter of about 5-10 xm and consisted of stearic acid, calcium stearate, and calcium carbonate (mainly calcite, no experimental information could be given concerning amorphous calcium carbonate at this point). The viscoelastic data pointed to a glassy state with a crystalline hardness. It is important to note that the films did not consist of a stearate monolayer, cross-linked by calcium ions (chalk soaps) as control experiments with CaCl2 observed. Derived from other control experiments it became clear that the growth of the films was due to a specific interaction of crystalline (or precrystalline) calcium carbonate and stearic acid. 

In analogy to the studies at the air/water interface we suggest a growth mechanism involving calcium carbonate nanoparticles, which served as precursor particles. Osmotic draining of locally constrained emulsion droplets could also lead to crystallization processes. Further experiments shall involve high resolution X-ray diffraction and FTIR studies in order to evaluate the role of amorphous calcium carbonate in the crystallization processes. 

Kinetically driven crystallization often involves an initial amorphous phase that may be non-stoichiometric, hydrated, and susceptible to rapid phase transformation. Amorphous calcium carbonate (ACC) for instance is highly soluble, has a low density of almost half of the crystalline mineral indicating a high hydration [62], and rapidly transforms to calcite, vaterite, or aragonite unless kinetically stabilized. In aqueous solution, this transformation into vaterite or calcite takes place within seconds or less even if additives are present, as shown by recent SAXS/WAXS measurements of ACC transformation in the presence of a DHBC [63]. 

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