Amorphous CaCO

Thermodynamically more stable crystalline phases will be favoured as the nuclei grow in size since the bulk lattice energy terms, rather than the surface energy terms, become more important in stabilising the solid phase, particularly (as in the case of amorphous CaCOs) when the amorphous phase is relatively soluble. In cases where the amorphous phase is relatively insoluble, for example, amorphous calcium phosphate, the transformation to crystalline states may be slow at ambient temperature and pressure. In other cases, for example, biogenic silica, the amorphous phase is metastable and not transformed into a crystalline phase under normal conditions due to the high activation energies required to be overcome for this transformation. ... 

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. 

CaC03 (Amorphous) CaCOs (Hydrated) CaCOa (Calcite)... 

Sediment type %CaCO % clastic and clayey material % amorphous silica % pelagic sed. Composition... 

Bound H20 and OH". Spectral data (2 11-13. 18, 21) thermodynamic data (e.g. 86, 87), alkalinity determinations (16), DTA (23). NMR (21), and X-ray diffraction data (21, 23) have all indicated the presence of bound H20 and OH in many carbonate skeletons, particularly high I g calcites. Lowenstam (88)) has reported monohydrocalcite and amorphous hydrated CaCO in some mollusk and arthropod samples. 

The term amorphous silica encompasses an almost infinite variety of structural forms, from ordered opaline aggregates to extended gel-like materials. In all these structures, the mineral exists as a hydrated, covalent inorganic polymer of general formula [SiO /2 (OH)4- ]m 4). This formula, where n = 0 to 4 and m is a large number, indicates the variation in residual functional groups within the condensed structure. There is similar variation in the extent of hydration. This flexibility in composition and reactivity indicates that biogenic silica is not a stoichiometric mineral (in the way that CaCOs is, for example) and that the nature (density, hardness, solubility, viscosity) and composition of siliceous structures in biology may vary consid-... 

Calcium carbonate (precipitated chalk), CaCOs Molecular weight 100.09 colourless, amorphous and heavy powder hardly soluble in water 0.008 grams at 0°C and O.OOSgrams at 20°C per 100... 

Calcium Chloride—Cdcii chhridum (U. S, Hr.)—CaCl,—111—is obtained by dissolving marble in HCl CaCO. 4 2HC1 = CaCI, 4- H,0 4-OO,. It is bitter- deliquescent veiy soluble in H,0 crystallizes with 6 Aq, which it loses when fused, leaving a white, amorphous mas.s used as a drying agent. 

It occurs in white or grayish, amorphous masses odorless . alkahne caustic almost infusible sp. gr. 2.3. With HaO it gives off great heat and is converted into the hydroxid (slacking). In iiir it becomes air-slacked, falling into a white powder, having the composition CaCOs.CaH-aOa. 

The above-listed features of the formation of X-ray amorphous and crystalline products are observed in the decompositions of many other compounds. Haul and Schoning [41] applied an X-ray method (line-width technique) to study the structure of the decomposition products of dolomite as a function of temperature. The experiments were carried out both in vacuum and in a CO2 environment. In vacuum, decomposition proceeded to the CaO and MgO oxides, and in a CO2 environment, to CaCOs and MgO. This difference is reasonable. It originates from the calcium carbonate being more stable than the magnesium carbonate (the enthalpies of formation of CaCOs and MgCOg at 800 K are —1,154 and —1,045 kJ mol respectively). The size of the crystallites formed in vacuum [41] increased monotonically from 6nm for MgO and 13 nm for CaO to 120-140 nm with the temperature increasing within the range 700-1,000° C. (Oversaturation decreases in these conditions from 10 to 10 for MgO, and from 10 to 10 for CaO.)... 

However, the most intensively studied system with Hquid precursors is still CaCOs. If PILP droplets are deposited on a substrate, they coalesce and form a coating, which subsequently transforms into patchwork-Hke calcite films with different single crystalUne domains via an amorphous to crystalline transition [229]. 

Fig. 13 Three-step procedure for the morphosynthesis of nacre-type laminated CaCOs coatings. In the first step, an amorphous highly hydrated CaCOs thin film is deposited on a glass substrate. Upon heating, this precursor film is transformed into a polycrystalline thin film consisting of a mosaic of flat single-crystalline calcite domains. In the last step, highly oriented single and multiple layers of calcite crystals are grown epitaxially on the underlying polycrystalline thin film. Reproduced from [235] with permission of Wiley...

CaCOs shows a broader morphological variety than BaS04 when crystallized in the presence of DHBCs. This is partly a reflection of the fact that CaCOs already has three different anhydrons polymorphs with different morphologies. Most of the reported stndies on DHBC-controUed CaCOs crystallization deal with self-assembled stmctnres whereas mesocrystal formation, oriented attachment, or PILP mechanisms have not yet been reported with DHBC additives. However, the formation of amorphous precinsor particles is omnipresent in the DHBC-controUed crystallization of CaCOs. 

Nevertheless, the rod-to-dumbbell-to-sphere transition seems to be a general crystal growth phenomenon and was observed for several other carbonate systems in the presence of DHBC additives (CaCOs, BaCOs, MnCOs, CdCOs) [61] as well as in rare cases even without additives. Thus it appears that the polymer additive does not only play a structme-directing role as the above discussion may imply, but also a controlling role for the release of building material consisting of amorphous particles. 

Fig. 27 Top SEM images of CaCOs particles grown on a glass slip in the early reaction stage, PEO-6-PMAA, [CaCb] = 10 mM, lgL , 5h. a CaCOs particles with either spherical or hollow structures, b Zoom showing the calcite rhombohedral subunits grown on the surface of the hollow structure and the inner part consisting of tiny primary nanocrystals with a grain size of about 320 nm as indicated by the arrow (sacrificial vaterite template). Bottom Proposed formation mechanism of the calcite hollow spheres a polymer-stabilized amorphous nanoparticles b Formation of spherical vaterite precursors c aggregation of the vaterite nanoparticles d vaterite-calcite transformation starting on the outer sphere of the particles e formation of calcite hollow spheres under consumption of the sacrificial vaterite precursors. Reproduced in part from [61] with permission of the American Chemical Society...

Fig. 21 a Schematic illustration of the new approach for the formation of microperfo-rated single crystals deposition of the ACC mesh from the CaCOs solution, oriented nucleation at the imprinted nucleation site, and the amorphous-to-crystalline transition of the ACC film on the engineered 3D templates. (Reproduced from [247], 2003, American Association for the Advancement of Science)... 

The combination of a surfactant molecule with a colloidal inorganic core results in a micelluar-type structure as shown in the figure. This gives both the ability to solubilize polar materials in a continuous matrix of oil, and provides acid neutralization capacity, which is also intimately contacted with the oil in a dispersed amorphous colloidal phase, shown in the figure as CaCOs for a Ca-detergent. This basic colloidal carbonate neutralizes acids formed during the combustion process, such as nitric acid, sulfuric acid, and... 

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