Vaterite structure

Studies seeking evidence for the multi-step in vitro crystalhzation process have been carried out. Li and Mann were able to demonstrate that in inverse microemulsions, surfactant-vaterite structures with different shapes are formed by a phase transition from stabihzed ACC nanoparticles [76]. The... 

There is also a high temperature form of the vaterite structure where BO assumes a planar structure (Bradley et al., 1966) and which is thus more similar to CaC03 vaterite than is the low-temperature form. In YbB03 the transition from the... 

Besides research on thin films obtained by sol-gel, powders can also be obtained by this method and they are of particular interest in the production of phosphors. In a study of LuBOs phosphors (Boyer, 2002), the evolution of the Lu site as a function of thermal treatment was carried out by the EXAFS technique, from the xerogel to the conpletely crystallized samples. In this study, EXAFS revealed, in agreement with crystallographic results, that a vaterite structure is formed, instead of the calcite structure observed in the conventional solid-state reaction method. Also, Eu-doped Y2SiOs phosphors have been studied (Tao, 1996), relating the Eu-O bond length variation upon thermal treatment with the luminescence properties. 

Vaterite is thermodynamically most unstable in the three crystal structures. Vaterite, however, is expected to be used in various purposes, because it has some features such as high specific surface area, high solubility, high dispersion, and small specific gravity compared with the other two crystal systems. Spherical vaterite crystals have already been reported in the presence of divalent cations [33], a surfactant [bis(2-ethylhexyl)sodium sulfate (AOT)] [32], poly(styrene-sulfonate) [34], poly(vinylalcohol) [13], and double-hydrophilic block copolymers [31]. The control of the particle size of spherical vaterite should be important for application as pigments, fillers and dentifrice. 

Spherical vaterite crystals were obtained with 4-mercaptobenzoic acid protected gold nanoparticles as the nucleation template by the carbonate diffusion method [51]. The crystallization of calcium carbonate in the absence of the 4-MBA capped gold nanoparticles resulted in calcite crystals. This indicates that the polymorphs of CaCOj were controlled by the acid-terminated gold nanoparticles. This result indicates that the rigid carboxylic acid structures can play a role in initiating the nucleation of vaterite as in the case of the G4.5 PAMAM dendrimer described above. 

The packing arrangement of atoms or molecules in a crystalline solid phase is generally not unique, and for organic molecules in particular, it is common for two or more crystalline forms of the same substance to exist. The most familiar example in elemental terms is Graphite and Diamond. Both are composed entirely of the element Carbon, however their ciystal structures are very different, and so too are their physical properties. Calcium Carbonate is another common example with three polymorphic forms Calcite, Aragonite and Vaterite. 

EuBOz. — Rare earth borates of the type MBOs, where M = Sm—Lu and Y, are isostructural with vaterite ( x—CaCOs). The lattice constants [325] of EuBOs are a = 3.842 and c = 8.937 A. Lanthanum borate, however, crystallizes in the aragonite structure [306]. Europium borate and the above mentioned borates can be obtained by the reaction of the metal oxide and B2O3 in an equimolar ratio at 1200—1400° C in air. 

Calcium carbonate occurs naturally in three crystal structures calcile. aragonite, and. although rarely, vaterite. Calcite is ihertnndynamically stable aragonite is melastable and irreversibly changes to calcite when heated in dry air to about 400 C. 

On a relative basis, i.e. residues per 1000, there is virtually no one species like the other. In contrast, different shell samples from the same species and obtained from the same natural habitat yield identical amino acid patterns. It is of interest that (1) the structure of carbonates (aragonite-calcite-vaterite), (2) the content in trace elements, and (3) the stable isotope distribution are markedly effected by fluctuations in salinity, water temperature, Eh/pH conditions, and some anthropogenic factors. The same environmental parameters determine to a certain degree the chemical composition of the shell organic matrix. This feature suggests a cause-effect relationship between mineralogy and organic chemistry of a shell. In the final analysis, however, it is simply a reflection of the environmentally-controlled dynamics of the cell. 

A phase is that portion of a system which is homogeneous. A complicated system can have more than one solid phase corresponding to different crystal structure variations (e.g. pleomorphic forms of CaCXf calcite, aragonite and vaterite). Sometimes it can also have more than one liquid phase but usually contains only one gaseous phase. We can have a system having a total of p phases with c components (i.e. substances) present. Thus many of the important phase systems are more complicated than the simple one-component systems so far discussed. 

Calcium carbonate minerals are deposited in a wide variety of bacteria, protozoa, algae, higher plants, and invertebrates (Table I). They are also formed, although not as frequently, in vertebrates. The major structural polymorphs identified in biological systems are calcite, aragonite, and vaterite, although there is also evidence for monohydrate, amorphous, and a range of Ca/Mg carbonate phases. 

The presence of both oriented vaterite and calcite crystals on charged monolayers indicates that the electrostatic interactions between nuclei and the organic surface are influenced by structural relationships at the interface. The nucleation of vaterite on films of positive and negative charge indicates that Ca binding is not a prerequisite for stabilization of this metastable phase. Oriented calcite, on the other hand, requires Ca binding at the carboxylate head-groups. ... 

The change from calcite to vaterite nucleation on stearate films at low [Ca] suggests that the extent of Ca binding is important for polymorph selection. The nucleation of calcite is favored by the formation of a well-defined Ca-carboxylate layer that mimics the first layer of the (110) face of the unit cell. By contrast, the structural requirements for vaterite formation are less precise. This is consistent with vaterite being the dominant phase on amine monolayers where no Ca binding is present, and suggests that kinetic factors of charge accumulation... 

An additional possible reason for differences in behaviour is that CaCOj exists in three crystalline forms. Calcite is the most stable and aragonite and vaterite undergo transitions to calcite at 728 K and between 623 and 673 K, respectively. Consequently, at decomposition temperatures (above 900 K) the reactant CaCOj would normally be in the calcite structure [2]. Salvador et al. [3] concluded that the... 

Polymorphism occurs when different structures can occur for the same chemical formula. The atoms are the same but their atomic arrangement differs between the structures. Although ultimately thermodynamics (the minimum Gibbs free energy) dictates which of the probable structures is formed at a particular temperature and pressure, other factors such as electrostatic interaction mean that a variety of different structures is possible. Polymorphism applies not only to elements, e.g. black and red phosphorus, but also to compounds such as calcium carbonate, which can exist in a number of forms including calcite, aragonite and vaterite. 

Materials are traditionally classified in three states of matter gases, liquids and solids distinguished by their properties. However, the solid phase of a material can exhibit different structures, which in turn can show different properties. In addition to different crystal structures, called polymorphs, which are characterized by long-range order, a material may appear as an amorphous solid, characterized by the lack of long-range order. The polymorphism of calcium carbonate (calcite, vaterite and aragonite) was identified more than 200 years ago by Klaproth in 1788... 

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