Vaterite

A significant advantage of the PLM is in the differentiation and recognition of various forms of the same chemical substance polymorphic forms, eg, brookite, mtile, and anatase, three forms of titanium dioxide calcite, aragonite and vaterite, all forms of calcium carbonate Eorms I, II, III, and IV of HMX (a high explosive), etc. This is an important appHcation because most elements and compounds possess different crystal forms with very different physical properties. PLM is the only instmment mandated by the U.S. Environmental Protection Agency (EPA) for the detection and identification of the six forms of asbestos (qv) and other fibers in bulk samples. 

Figure 8.5 SEM micrographs of calcium carbonate agglomerates a, vaterite and calcite h, calcite). After Zauner and Jones, 2000c)...

Figure 8.36 Crystals form the SFTR (a) Vaterite (h) Y-Ba oxalate (courtesy www.huhhletuhe.com)...

Spanos, N. and Koutsoukos, P.G., 1998. The transformation of vaterite to calcite effect of the conditions of the solutions in contact with the mineral phase. Journal of Crystal Growth, 191, 783-790. 

From the more recent reports cited below, further references to the extensive literature concerned with calcite decomposition may be traced. Other modifications of CaC03 (aragonite and vaterite) undergo solid phase transitions to calcite at temperatures of 728 K and 623—673 K respectively [733], below those of onset of decomposition (>900 K). There is strong evidence [742] that the reaction... 

At 1100°C the borate and the hexaboride are formed simultaneously [reaction (b)]. The observed borates are the aragonite type for La, Ce, Nd and the vaterite type... 

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. 

The precipitation of CaCOj in the absence of any additives was carried out under the same nucleation condition (run 1 in Table 1). The crystal phase of the obtained CaCOj was a mixture of calcite and vaterite by IR. The vaterite content... 

Polymorphism and the fraction of vaterite were characterized by FTIR. 

SEM observations showed that the most crystals obtained in the absence and the low concentration of the G4.5 PAMAM dendrimer were rhombohedral. In the high concentration of the G4.5 PAMAM dendrimer, the vaterite products were spherical. Each shape of CaCOj is a typical morphology for each polymorph. The particle sizes of the spherical vaterite particles obtained in the presence of the PAMAM dendrimer were depended on the concentration of the dendrimer. As the concentration of -COONa increased from 5.3 to 10.6 mmol/1, the particle sizes of the spherical vaterite particles were reduced from 8.7 1.0 to 5.2 3.0 pm. 

The precipitations of CaCOj in the presence of the Cl.5, G3.5, and G4.5 PAMAM dendrimers were carried out with constant -COONa unit and calcium ions of 0.1 mol/1. Although vaterite was predominantly formed by the G4.5 dendrimer, relatively high amount of calcite was observed in the case of the G3.5 and G1.5 dendrimers (Table 2). These results suggest that the G4.5 dendrimer effec-... 

The geometry and surface chemistry of the dendrimer assemblies can be varied through the addition of surfactants. These dendrimer/surfactant aggregates can be tuned to template the formation of the different phases of calcium carbonate [40]. In combination with hexadecyltrimethylammonium bromide (CTAB), small spherical aggregates were formed that induce the formation of vaterite. Over a period of five days, the vaterite was transformed into calcite. The use of the negatively charged surfactant, sodium dodecylsulfonate (SDS), result-... 

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. 

Calcium Carbonate CaC03 Calcite Aragonite Vaterite Widespread in animals and plants Exoskeleton Gravity, Ca store Eye lens... 

Morphosynthesis of calcium carbonate (vaterite) microsponges. Advanced Materials, 11, 324—328. 

Fig. 14.15 X -ray diffraction patterns of the CaCC>3/halloysite composite after CaC03 formation. The first peak at 12.3° corresponds to 7.2 A (dehydrated halloysite packing reflection). Arrows indicate peaks corresponding to the vaterite modification of CaC03. Calcite modification of CaC03 was not found. Peak assignment was made using the Powder Diffraction Database.

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