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Nucleation of calcites

In precipitation studies (4 7, 4 ) it has been shown that, below a certain Mg/Ca concentration ratio in the aqueous solution, the rate of nucleation of calcite was faster than that of aragonite. Above that Mg/Ca ratio the order was reversed. This was explained by the effect of Mg2+ ions on the interfacial tension between the solution and precipitate, which apparently is larger for calcite than for aragonite (49). At still higher Mg/Ca ratios dolomite can be formed (50). Such low temperature precipitates of dolomite contain ordering defects. The number of defects increases when precipitation proceeds in a shorter time interval or at lower temperatures C51 ). [Pg.540]

The improvement of enzyme like MIP is currently another area of intense research. Beside the use of the MIP themselves as catalysts, they may also be applied as enhancer of product yield in bio-transformation processes. In an exemplary condensation of Z-L-aspartic acid with L-phenylalanine methyl ester to Z-aspartame, the enzyme thermolysin was used as catalyst. In order to shift the equilibrium towards product formation, a product imprinted MIP was added. By adsorbing specifically the freshly generated product from the reaction mixture, the MIP helped to increase product formation by 40% [130]. MIP can also be used to support a physical process. Copolymers of 6-methacrylamidohexanoic acid and DVB generated in the presence of calcite were investigated with respect to promotion of the nucleation of calcite. Figure 19 (left) shows the polymer surface with imprints from the calcite crystals. When employing these polymers in an aqueous solution of Ca2+ and CO2 the enhanced formation of rhombohedral calcite crystals was observed see Fig. 19 (right) [131]. [Pg.158]

Fig. 19. SEM left a calcite-imprinted polymer surface after HCl/MeOH wash right nucleation of calcite at the imprinted polymer surface in the presence of CaCl2 (1.0 mmol/1), Na2C03 (0.8 mmol/1). Reprinted with permission from D Souza SM, Alexander C, Carr SW, Waller AM, Whitcombe MJ,Vulfson EN (1999) Nature 398 312. Copyright 1999 Macmillan Magazines Ltd... [Pg.160]

Figure 7.17. Schematic representation of nucleation of calcite in a cavity. (After Wollast, 1971.)... Figure 7.17. Schematic representation of nucleation of calcite in a cavity. (After Wollast, 1971.)...
Nucleation of calcite on the (110) face under stearate monolayers can be rationalized in terms of charge, stereochemical and geometric com-... [Pg.192]

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... [Pg.194]

If nucleation of calcite cement is controlled by the presence of certain types of biogenic carbonate which act as favourable nucleation substrates, then calcite cement nuclei will tend to be concentrated where most biogenic carbonate is present, i.e. within biogenic carbonate-rich layers. If nucleation is determined by calcite supersaturations, nuclei will still tend to be concentrated in biogenic carbonate-rich layers, as these contain the dominant source of dissolved calcite, and the supersaturations necessary for calcite cement nucleation will therefore normally be first achieved within these layers. The nucleation of calcite cement may be a result of the difference in solubility between calcite cement and carbonate fossils, i.e. the concentration of dissolved calcite in equilibrium with biogenic carbonate may exceed the supersaturation necessary for nucleating the less soluble calcite cement. [Pg.185]

Fig. 2. The interaction of the ranges of influence for several calcite cement nuclei. Note that nucleation of calcite cement may possibly take place at a dissolved calcite concentration significantly lower than the solubility of biogenic carbonate. Fig. 2. The interaction of the ranges of influence for several calcite cement nuclei. Note that nucleation of calcite cement may possibly take place at a dissolved calcite concentration significantly lower than the solubility of biogenic carbonate.
D Souza, S.M. Alexander, C. Carr, S.W. Waller, A.M. Whitcombe, M.J. Vulfson, E.N. Directed nucleation of calcite at a crystal-imprinted polymer surface. Nature... [Pg.282]

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... [Pg.150]

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. [Pg.156]

Pronounced discrepancies between observed composition and the calculated equilibrium composition illustrate that the formation of the solid phase, for example, the nucleation of dolomite and calcite in seawater, is often kinetically inhibited, and the formation of phosphates, hydrated clay and pyrite is kinetically controlled. [Pg.211]

Dolomite is one of the most abundant sedimentary carbonate minerals but its mode of formation and its surface properties are less well known than for most other carbonate minerals. As we have mentioned, the nucleation of dolomites and its structural ordering is extremely hindered. There is a general trend for the "ideality" of dolomite to increase with the age of dolomite over geological time (Morse and Mackenzie, 1990). Most dolomites that are currently forming in surfacial sediments and that have been synthesized in the laboratory are calcium-rich and far from perfectly ordered. Such dolomites are commonly referred to as "protodolomites . Morse and Mackenzie (1990) have reviewed extensively the geochemistry (including the surface chemistry of dolomites and Mg-calcites. [Pg.303]

Calcium phosphate precipitation may also be involved in the fixation of phosphate fertilizer in soils. Studies of the uptake of phosphate on calcium carbonate surfaces at low phosphate concentrations typical of those in soils, reveal that the threshold concentration for the precipitation of the calcium phosphate phases from solution is considerably increased in the pH range 8.5 -9.0 (3). It was concluded that the presence of carbonate ion from the calcite inhibits the nucleation of calcium phosphate phases under these conditions. A recent study of the seeded crystal growth of calcite from metastable supersaturated solutions of calcium carbonate, has shown that the presence of orthophosphate ion at a concentration as low as 10-6 mol L" and a pH of 8.5 has a remarkable inhibiting influence on the rate of crystallization (4). A seeded growth study of the influence of carbonate on hydroxyapatite crystallization has also shown an appreciable inhibiting influence of carbonate ion.(5). [Pg.650]

The nucleation of these decomposition processes was studied by means of thermomicroscopy on single crystal cleavage plates of calcite, magnesite, dolomite and smithsonite (Fig. 59). The shape of the nuclei was found to be different for these carbonates, which might be also of importance for the decomposition mechanism. The partial pressure of water vapor has a pronounced effect on the decomposition of transition metal carbonates such as ZnC03 and CdC03. The evolution of C02 is probably catalyzed in the presence of water vapor and shifted to considerably... [Pg.130]

Acidic amino acids seem to play a key role in (1) the fixation of calcium, (2) the nucleation of CaC03 crystals, and (3) the oriented growth of the mineral phase. There appears to be no essential difference between biochemical or geochemical template-induced mineral formation. The only requirement is the presence of a calcium-specific template and an environment suitable for the deposition of calcite or aragonite. [Pg.17]

It is interesting to note that the uptake patterns found for Mn2+ on the surface of calcite are similar to those observed for orthophosphate uptake on calcite. Stumm and Leckie (1970) observed a three-stage process for phosphate uptake on calcite surfaces from dilute solutions, representing adsorption, nucleation, and apatite growth. However, deKanel and Morse (1978) found a two-stage pattern of phosphate uptake on calcite from seawater, with an initial rapid adsorption phase followed by slow but steady uptake. [Pg.71]

The rate of reaction is dependent on the nucleation and growth rates of calcite, not the dissolution rate of aragonite. Curiously, it has also been observed that absolute rates are strongly dependent on the aragonitic material used. This observation appears to contradict the generally held conclusion that rates are strictly dependent on calcite nucleation and precipitation rates, not the dissolution rate of aragonite. [Pg.294]

It can be demonstrated that grain boundaries or microcracks in sediments are preferred environments of cement nucleation. Consider the environment between two grains of carbonate separated by a void area of radius r (Figure 7.17 ) the energy (AFcav) required for the formation of calcite cement domains in such a cavity of height h and radius r is ... [Pg.323]


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See also in sourсe #XX -- [ Pg.323 ]




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