Calcite nucleation

Lebron, I. Suarez, D.L. 1996. Calcite nucleation and precipitation kinetics as affected by dissolved organic matter at 25°C and pH > 7.5. Geochimica et Cosmochimica Acta, 60(15), 2765-2776. 

In natural waters CaC03 (calcite) nucleation occurs primarily heterogeneously. Many surfaces such as algae, biomass, aluminum silicates, aluminum oxides serve... 

Carlson W.D. (1983) Aragonite-calcite nucleation kinetics an application and extension of Avrami transformation theory. /. Geol. 91, 57-71. 

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. 

Fig. 27. Optical micrographs of (A) oriented calcite nucleated under stearate mono-layers at [Ca] = 10 mM bar = 100 p,m. (B) Oriented vaterite crystals nucleated under octadecylamine monolayers. Arrows indicate crystals of different crystallographic orientation bar =100 /im.

Geometric correspondence cannot, however, be solely responsible for (llO)-oriented calcite nucleation. For example, the (001) face of calcite comprises a hexagonal lattice of coplanar Ca atoms of 4.96-A periodicity and such an arrangement matches the monolayer-binding sites almost exactly. A significant difference between the (110) and (001) faces is the orientation of the carbonate anions they lie perpendicular to the (110) surface but parallel to (001). Thus the stereochemistry of the carboxylate headgroups mimics that of the anions in the (110) crystal face but not in the (001) face. 

Jenne (1992) suggests that in the presence of a few mg/L of inhibiting DOC, which is common in many groundwaters, SI, values may have to exceed +0.3 or more for calcite nucleation and precipitation. (See also Appelo and Postma 1993.) Jacobson et al. (1971) reported no carbonate precipitation in a stream in which SI values averaged 0.74 diurnally and were as high as 1.5 (see below). The.se experiences, presumably reflect both nucleation-inhibition effects and the relatively slow rate of calcite precipitation compared to the re.sidence times of streams. 

The analysis of surface seawater shows it to be moderately supersaturated with respect to both calcite (SI - 0.70) and disordered dolomite (Sl,i = 0.88) and more supersaturated with respect to ordered dolomite (SI = 1.65). It is also supersaturated with respect to magnesite (5/ , = 0.76 assuming K, = 10 ), and aragonite (5/ = 0.56). Equilibration of seawater with re.spect to ordered dolomite, magnesite, and pure calcite is kinetically inhibited (high Mg " may inhibit calcite nucleation in seawater), although aragonite does readily precipitate from seawater. 

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... 

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. 

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. 

Several crystals, such as vaterite and calcite forms of CaC03, or a-glycine, have been nucleated (induced oriented crystallization) at the water surface covered with a monolayer film of carboxylic acids or aliphatic alcohols (compressed to "suitable" distances of the hydrophilic groups with a Langmuir balance) (Mann et al., 1988). 

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. 

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 ). 

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). 

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]. 

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... 

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... 

Calcite embryos nucleate Crystals nucleate Bubbles nucleate... 

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