Examination of the Polymorphs and Particle Size of Calcium

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Ind. Eng. Chem. Res. 2006, 45, 5223-5230

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Examination of the Polymorphs and Particle Size of Calcium Carbonate Precipitated Using Still Effluent (i.e., CaCl2 + NaCl Solution) of Soda Ash Manufacturing Process Rajesh S. Somani, Kartik S. Patel, Alpesh R. Mehta, and Raksh Vir Jasra* Silicates & Catalysis Discipline, Central Salt and Marine Chemicals Research Institute, G. B. Marg, BhaVnagar - 364002, India

The precipitation of calcium carbonate was studied by adding the sodium carbonate solution to a mixed system such as Still effluent (i.e., CaCl2 + NaCl solution) of the soda ash manufacturing (Solvay) process, and the effects of controlling factors on calcium carbonate polymorphs were investigated. The products were characterized with scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and powder X-ray diffraction analysis. The particle size distribution of products was determined as dry powder using laser diffraction technique. It was found that the dilute solutions of the reactants favor the aragonite formation without affecting much the particle size. The formation of calcite is favored with the increase in sodium carbonate concentration. The reaction temperature has no significant effect on polymorphism as well as particle size of calcium carbonate precipitated from solutions having concentration 1.25 M Ca2+ and 1.25 M CO32- and reaction time of 30 min. However, the mode of mixing the reactants has a remarkable effect on the type of polymorph formed and its morphology. When sodium carbonate solution is added to Still effluent maintained at 95 °C, mostly calcite is formed. When the mode of mixing the reactants is reversed, a mixture of calcite and aragonite is formed under identical experimental conditions. In the case of simultaneous addition of both the reactants, aragonite formation is favored. These results indicated that a selective type of polymorph can be produced simply by changing the mode of mixing the reactants. Various morphologies of calcium carbonate crystals such as rhombohedral and needlelike were produced, depending on the preparation conditions. 1. Introduction In recent decades, extensive research efforts have been focused on the preparation of high-performance inorganic materials and their use in paint, textile, plastic, adhesive, rubber, ceramic, and paper industries. Calcium carbonate, both ground and precipitated, finds numerous usages in these applications. However, precipitated calcium carbonate has received more attention because of better control over its size, shape, crystallinity, and architecture during precipitation. Calcium carbonate is also of significance to many research disciplines involving, for instance, medicines, electronics, household, and environment. Since, the morphology and physicochemical properties depend on each polymorphous crystal, the polymorphic control of the crystals is very important from an industrial perspective. Calcium carbonate has three documented polymorphs, namely, calcite, aragonite, and vaterite, in order of increasing solubility and decreasing thermodynamic stability.1 The basic difference among these phases lies in the disposition of the carbonate ions with respect to the central calcium ions in their respective unit cells.2 Aragonite and calcite have very similar crystal structures and thermodynamic stabilities.3 The crystal structures of both aragonite and calcite are composed of alternating layers of calcium and carbonate ions perpendicular to the c axis, i.e., in the ab plane. In both structures, the carbonate ions lie with their molecular planes parallel to the ab layer and the calcium ions occupy almost the same lattice positions in this plane. However, in aragonite, some of the carbonate ions are raised in the c direction to form two layers separated by 0.96 Å, and their orientations in two layers are different, which imparts very different properties to these two phases.4 The rhombohedral * To whom correspondence should be addressed. Tel.: +91-2782471793. Fax: +91-278-2567562. E-mail: [email protected].

calcite structure can be visualized as alternating planes of carbonate and calcium ions, perpendicular to the c axis. The calcium ion in calcite is 6-coordinated, forming a slightly distorted octahedron with a Ca2+-O distance of 2.38 Å.5 The hexagonal vaterite structure is more complicated and less wellestablished. Unlike in calcite, the carbonate groups in vaterite are oriented perpendicular to the basal plane of the structure, i.e., parallel to the c axis. The calcium ions are surrounded in a first coordination sphere by six oxygen atoms in an octahedral fashion with a Ca2+-O distance of 2.59 Å. Different configurations in the second coordination sphere leave two sets of Ca2+ ions with a molar ratio of 2:1, respectively. The vaterite structure is loosely packed, which explains the higher solubility (pK′o ) 7.9) as compared to that for calcite (pK′′o ) 8.5) as well as the lower density of 2.54 g/cm3 vs 2.71 g/cm3 for calcite.6 Precipitation processes of calcium carbonate polymorphs of different sizes and shapes have been intensively studied because of their commercial importance.7-9 The precipitation behavior of the polymorphs has been investigated in a sodium carbonate and calcium chloride reaction system in the absence or presence of impurity in various systems. For example, Parsiegla and Katz10 studied calcite inhibition by Cu(II) and the effect of solution composition. Tracy et al.11 studied the growth of calcite spherullites from solution and kinetics of formation in the presence of Mg2+ and SO42-. Yu et al.12 observed that the acidity of organic macromolecules such as poly(maleic anhydride) or poly(sodium 4-styrenesulfonate) is an important parameter for the morphological control and the preparation of monodispersed CaCO3 micro spheres with a size of ∼1-2 µm by precipitation reaction of sodium carbonate with calcium chloride in the presence of such additives at room temperature. Gehrke et al.13 concluded that the ammonium ions act as effective additives for the control of CaCO3 morphology, i.e.,

10.1021/ie0513447 CCC: $33.50 © 2006 American Chemical Society Published on Web 06/21/2006

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Figure 1. Effect of variation in both reactant concentrations on polymorphs content of precipitated calcium carbonate.

Figure 2. Effect of variation in both reactant concentrations on average particle size of calcium carbonate.

formation of hexagonal, lens- and rosetta-shaped superstructures, by a mesoscale assembly process in which, in a first step, {001} oriented vaterite nanosheets presumably stabilized by ammonium are stacked to hexagonal superplates with subsequent fusion to a single crystal by oriented attachment. In many chemical industries, acidic effluents are generated that are neutralized by adding calcium hydroxide, calcium carbonate, or limestone, which produce mainly calcium chlorides according to eqs 1 and 2. Other soluble halides may also be present in such effluents along with calcium chloride.

Ca(OH)2 + 2HCl f CaCl2 + 2H2O

(1)

CaCO3 + 2HCl f CaCl2 + H2O + CO2

(2)

The well-known example of such an effluent is Still effluent generated during the soda ash manufacturing (Solvay) process, which involves the following chemical reactions.

NaCl + NH3 + CO2 + H2O f NaHCO3 + NH4Cl (3) heat

2NaHCO3 98 Na2CO3 + CO2 + H2O

(4)

2NH4Cl + Ca(OH) 2 f CaCl2 + 2NH3 + 2H2O

(5)

Because the Still effluent is a mixture of calcium chloride and sodium chloride, it can be used to prepare precipitated calcium carbonate as per the following equation.

Figure 3. Effect of mode of mixing the reactants on the polymorphs of calcium carbonate: usual ) sodium carbonate solution is added to Still effluent (i.e., CaCl2 + NaCl solution); reverse ) Still effluent (i.e., CaCl2 + NaCl solution) added to sodium carbonate solution; and simultaneous ) both reactants added simultaneously to 100 mL of water.

As of now, Still effluent of the soda ash manufacturing process is discharged into sea or river waters and does not find any use. Owing to the commercial value of precipitated calcium carbonate as a filler and pigment in a variety of applications, it will help to have a favorable impact on soda ash manufacturing economics if precipitated calcium carbonate is produced using Still effluent (i.e., CaCl2 + NaCl solution). However, only a few reports have been published for the crystallization behavior of calcium carbonate polymorphs from such mixed systems. In this work, the effect of synthesis parameters for the precipitation of calcium carbonate from Still effluent (i.e., CaCl2 + NaCl solution) of the soda ash manufacturing (Solvay) process on the type of polymorphs formed and their particle size distribution was investigated. The experiments were performed in order to study the influence of varied concentrations of both reactants, i.e., Still effluent and sodium carbonate solution (1.25-0.15 M), concentration of sodium carbonate solution only (3.0-0.4 M), and reaction temperature (30-95 °C). Some additional experiments were performed by changing the mode of mixing the reactants at 95 °C. 2. Experimental Section The Still effluent of the soda ash manufacturing process (Nirma Chemical Works, Kalatalav, Bhavnagar, India) used contained 1.25 M CaCl2 and 0.85 M NaCl. The reaction vessel of 1 L capacity was kept on a stirring hot plate (Schott Gera¨te GmbH, Germany) for temperature control and stirred using a Teflon coated magnetic bar at 150 rpm. After the completion of reaction time, the product was filtered out of the solution using vacuum (10-2 mmHg), washed with distilled water till free from chloride ions as tested by silver nitrate solution, and dried overnight in an air oven at 105 °C. In addition to characterization of selected samples by scanning electron microscope (LeO, VP1430), the samples were analyzed by powder X-ray diffraction (XRD) using Cu KR radiation (1.540 56 Å) in the 2θ range from 20° to 40° (Philips, X’pert MPD PW3123 diffractometer) to obtain the content of different polymorphs. The content of the polymorph has been calculated from the calcium carbonate precipitates by measuring the intensity ratio between the most intense peaks of aragonite (2θ ) 26.32°, {h k l} ) {1 1 1}) and calcite (2θ ) 29.50°, {h k l} ) {1 0 4}), as per the following equation.

(6)

% aragonite ) [Iarg/(I arg + I cal)] x 100 and % calcite ) [I cal/(I arg + I cal)] x 100 (7)

The sodium chloride solution thus obtained can be recycled in the soda ash manufacturing process according to eq 3.

where Iarg ) intensity (peak height, counts/s) of peak at 2θ )

(CaCl2 + NaCl) + Na2CO3 f CaCO3 + 3NaCl

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Figure 4. Segmented FTIR spectra (400-1700 cm-1) of precipitated calcium carbonate prepared by (a) usual, (b) reverse, and (c) simultaneous modes of mixing the reactants.

26.32°, {h k l} ) {1 1 1}, for aragonite and Ical ) intensity (peak height, counts/s) of peak at 2θ ) 29.50°, {h k l} ) {1 0 4}, for calcite. The particle size distribution (PSD) of the samples was measured as dry powder using laser diffraction technique (Malvern Instruments, Mastersizer 2000). Fourier transform infrared (FTIR) spectra of the samples were recorded as KBr pellets (Perkin-Elmer, Spectrum GX spectrophotometer).

3. Results and Discussion 3.1. Effect of Variation in Concentrations of Both the Reactants on the Polymorphs and the Particle Size Distribution of Calcium Carbonate. It is observed that, in these experiments, only aragonite and calcite precipitate and vaterite do not appear. When the precipitation was carried out by addition of sodium carbonate solution within 60 min at 95 °C to Still effluent (i.e., CaCl2 + NaCl solution), the aragonite and

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Figure 5. Effect of modes of mixing the reactants on particle size distribution of calcium carbonate: (a) usual ) sodium carbonate solution is added to Still effluent (i.e., CaCl2 + NaCl solution); (b) reverse ) Still effluent (i.e., CaCl2 + NaCl solution) added to sodium carbonate solution; and (c) simultaneous ) both reactants added simultaneously to 100 mL of water.

calcite content in the precipitates changed with the concentration, as shown in Figure 1. It is observed from Figure 1 that, with an increase in reactant concentration, crystallization of calcite is favored. It is also noticed that, even at the highest reactant concentration studied (1.25 M), 10% aragonite is crystallized. This may be attributed to the higher reaction temperature, which is reported to favor aragonite crystallization.14 However, the crystallization behavior of aragonite is very characteristic; by increasing reactant concentration 2-fold in the lower concentration range from 0.16 to 0.31 M, a sharp decrease in aragonite content in precipitates is observed. Aragonite content remains almost the same beyond 0.94 M reactant concentration. As can be seen from Figure 2, the average particle size obtained from volume-based size distribution increased in the lower reactant concentration range up to 0.31 M and decreased as the reactant concentration increased up to 0.94 M. At the highest concentration studied (1.25 M), it again showed an increase. At the lowest reactant concentration (0.16 M), the

aragonite content is 80% and calcite is 20% in precipitated calcium carbonate. Because of the low reactant concentration (0.16 M) and the high reaction temperature (95 °C), there may be formation of a large number of nuclei with less crystal growth of the aragonite crystals, which might have resulted in an average size of 6 µm. The average particle size increased up to 7.3 µm at 0.31 M reactant concentration, and the aragonite content decreased drastically up to 37%. In contrast, calcite content increased up to 63%. This can be attributed to the formation of a large number of nuclei and the higher crystal growth of calcite crystals. Afterward, the average particle size decreased continuously up to 4.2 µm at the reactant concentration 0.94 M with continuous increase in calcite content up to 84%. The average particle size of this sample might have decreased because of the formation of a very large number of calcite particles with less crystal growth. At 1.25 M reactant concentration, the crystal growth of calcite particles may be more as compared to that of 0.94 M reactant concentration and

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Figure 7. Effect of sodium carbonate concentration on the content of CaCO3 polymorph.

Figure 6. Variation in (a) calcite and (b) aragonite content of precipitated calcium carbonate as a function of reaction temperature.

the calcite content is 87.5% in the precipitates. As a result, the average particle size increased up to 5.3 µm in the higher reactant concentration range. The formation of aragonite at lower reactant concentration (0.16 M) is observed from SEM (Figure 10), whereas formation of calcite at higher reactant concentrations (0.94 and 1.25 M) is also observed from SEM (Figures 11 and 12). 3.2. Effect of Mode of Mixing the Reactants on Polymorphs and Average Particle Size of Calcium Carbonate. In this set of experiments, usual mode of mixing means that the 1.25 M sodium carbonate solution is added to Still effluent (i.e., 1.25 M CaCl2 + 0.85 M NaCl solution) at 95 °C. The reverse mode of mixing means that the Still effluent (i.e., 1.25 M CaCl2 + 0.85 M NaCl solution) is added to the 1.25 M sodium carbonate solution at 95 °C. The simultaneous mixing means that both reacting solutions were added at the same addition rate in to 100 mL of distilled water maintained at 95 °C. The reaction temperature was maintained at 95 °C throughout the addition and the reaction time was 60 min with the stirring speed of 150 rpm for all the three experiments. The effect of mode of mixing on the type of polymorph formed is shown in Figure 3. It is observed that, when carbonate solution is added to Still effluent (i.e., CaCl2 + NaCl solution), mainly calcite is obtained. When Still effluent (i.e., CaCl2 + NaCl solution) is added to carbonate solution (i.e., reverse mode), a mixture containing aragonite and calcite is obtained. Aragonite appeared as a major phase (∼69%) when the reactants are added simultaneously to water. This was further confirmed by obtaining FTIR spectra of these three samples as KBr pellets. Aragonite can be distinguished

from calcite and vaterite on the basis of its characteristic vibrational (FTIR) spectra.15 Aragonite displays a characteristic symmetric carbonate stretching vibration (V1) at 1083 cm-1 and a carbonate out-of plane bending vibration (V2) at 854 cm-1 in its FTIR spectrum. The symmetric stretching vibration (V1) is infrared active only for aragonite and not for calcite. Thus, the peak at ∼1083 cm-1 is used to identify and sometimes to quantify aragonite from a mixture of aragonite and calcite. On the other hand, the out-of-plane bending (V2) vibrational mode is infrared active for all three polymorphs of calcium carbonate, but there are some characteristic shifts among these. For calcite and vaterite, the positions of the corresponding (V2) vibrations are very similar and appear at ∼875 cm-1, whereas for aragonite, this vibration appears at ∼855 cm-1. Moreover, the asymmetric carbonate stretching vibration (V3) appears at ∼1480 cm-1, and a carbonate “in-plane” bending vibration (V4) appears at 700 and 713 cm-1. The segmented FTIR spectra (400-1700 cm-1) of all three samples described in this section are displayed in Figure 4 parts a-c. The strongest peak in each spectrum is the (V3) asymmetric stretch of CO32- ion at ∼1480 cm-1. A peak at 1083 cm-1 is characteristic of aragonite and is present in all three of our samples. However, the intensity of this peak is very weak in the case of the sample prepared by the usual mode of mixing. In addition, the (V2) vibration is also observed at 855 cm-1 in all three samples. It can be noted that, in the case of both usual and reverse modes of mixing, the (V2) band split, indicating the presence of both calcite and aragonite. However, it is observed that peak intensity of the peak at 874 cm-1 is stronger than that of the one at 855 cm-1 in the case of the usual mode of mixing (Figure 4a), whereas the intensity of the peak at 874 cm-1 is weaker than that at 855 cm-1 in the case of the reverse mode of mixing (Figure 4b). In the case of simultaneous mixing, there is no splitting of the peak (Figure 4c). This indicated that the usual mode of mixing has produced more calcite than aragonite and the reverse mode of mixing has produced more aragonite than calcite in precipitated calcium carbonate. For aragonite, the site symmetry of the CO32- ion causes the two doubly degenerate (V4) “in-plane” bending modes to split into a pair of nondegenerate modes.16 The splitting of the (V4) band appeared at 700 and 713 cm-1 in one of our samples, prepared by simultaneous mixing mode, and we ascertained that this sample is mainly aragonite. The variation in particle size distribution when changing the mode of mixing the reactants is depicted in Figure 5 parts a-c. It is observed that the highest average particle size, 4.90 µm, is obtained when reactants are mixed in reverse mode, i.e., Still effluent (i.e., CaCl2 + NaCl solution) is added to carbonate solution. The minimum average particle size, 3.45 µm, is obtained upon mixing the reactants simultaneously. These results

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Figure 8. XRD patterns of precipitated calcium carbonate (A ) aragonite, C ) calcite) prepared at different sodium carbonate concentration.

show that the mode of mixing the reactants has an influence on the crystallization behavior of the polymorphs besides that on the average particle size. The possible explanations for the observed influences together with an evaluation of the results are discussed below. The supersaturation ratio as the driving force of the nucleation of the calcium carbonate polymorph is expressed as

S ) [Ca2+] [CO32-]/[sp]

(8)

where [Ca2+] and [CO32-] are the concentrations of calcium and carbonate ions, respectively, and [sp] is the solubility product of calcium and carbonate ions. In the case of the usual mode of addition, the CO32-/Ca2+ ratio is 1 throughout the reaction. The carbonate ion is available from sodium carbonate solution, and calcium ion along with sodium ion is fed dropwise. As a result, the calcium ion concentration in the solution may be effectively diluted by the large volume of the solution. Because of this, the supersaturation ratio may

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become low, even though the carbonate concentration is high. In such a case, the low local supersaturation favors the crystallization of aragonite. However, nearly an equal amount of calcite and aragonite in calcium carbonate precipitates is obtained in the reverse mode of mixing. This could be attributed to the transformation from aragonite to calcite via a mechanism considered as “solution mediated”,18,19 i.e., the aragonite crystals dissolve and the calcite crystals nucleate and grow during the course of reaction. In the case of the simultaneous mode of mixing, the CO32-/Ca2+ ratio remained equal or near to one during the reaction. The calcium ion solution and carbonate ion solution having the same concentration are added to the water at 95 °C. Therefore, the dilution of both calcium and carbonate ions takes place, and at such a low concentration and high reaction temperature, the crystallization of aragonite is favored. In this case, the transformation of aragonite to calcite is limited. This can be attributed to the favorable experimental conditions of concentration, temperature, and time. In the case of the usual mode of mixing the reactants, because of the high local supersaturation, the carbonate ion may be consumed mainly by the vigorous nucleation of calcite and the growth may be limited. This might have resulted in calcite particles of average particle size of 6.0 µm. In the case of the reverse mode of mixing the reactants, because of the low local supersaturation, a mixture of aragonite and calcite is obtained. The experimental conditions favored the crystallization of aragonite, which later on transformed to calcite. Aragonite crystals are of needle shape and large in size, and the calcite particles are rhombohedral and fine because they are formed by the dissolution of aragonite crystals and are grown as calcite particles. The higher content of aragonite than that of usual mode of mixing might have contributed to an increase in the average particle size to 6.7 µm. The conditions of simultaneous mixing of the reactants have favored the formation of aragonite, and there may be vigorous nucleations of aragonite and the growth is limited. Therefore, the average particle size is decreased to 5.2 µm. 3.3. Effect of Reaction Temperature on the Polymorph of Calcium Carbonate. As can be seen from Figure 6a, the reaction temperature has no significant effect on the type of polymorph formed, as it is mostly (>95%) calcite. However, a decrease in aragonite content from 4% to 0% is observed with an increase in reaction temperature, as can be seen from Figure 6b. It is worth noting that this trend is uncommon compared to the conventionally observed relation of an increase in aragonite content with increasing reaction temperature.14 This uncommon trend can be attributed to the high concentration (1.25 M) of reacting solutions, the presence of sodium ions (0.85 M), and the reaction time of 60 min. 3.4. Effect of Carbonate Concentration on Polymorph of Calcium Carbonate. When precipitation of calcium carbonate is carried out by reacting Still effluent (i.e., 1.25 M CaCl2 + 0.85 M NaCl solution) with sodium carbonate solution having different concentrations between 0.4 and 3.0 M, mostly calcite is formed. However, as can be seen from Figure 7, at a lower sodium carbonate concentration, more aragonite is formed compared to that obtained at a higher concentration. This can be also evidenced by the XRD patterns compared in Figure 8. The dependence of average particle size on sodium carbonate concentration is shown in Figure 9. As can be seen from Figure 9, the average particle size increases with increasing carbonate concentration. At a carbonate concentration of 3.0 M, it increased sharply, reaching the average particle size of 6.36 µm. At lower carbonate concentration, the carbonate ion

Figure 9. Dependence of average particle size on carbonate concentration.

Figure 10. SEM images of aragonite type calcium carbonate obtained at 0.16 M reactant concentration, 95 °C, and 60 min.

may be consumed mainly by the vigorous nucleation of the calcite. It is considered that, after the initial nucleation, a large part of the carbonate ion added continuously may be consumed by the growth of the calcite crystals at higher carbonate concentration. It can be seen from Figure 10 that needlelike aragonite crystals are formed at 0.16 M reactant concentration, 95 °C reaction temperature, and 60 min reaction time. Figure 11 shows the formation of rhombohedral crystals of calcite at 0.94 M reactant concentration, 95 °C reaction temperature, and 60 min reaction time. Figure 12 also indicates the formation of rhombohedral crystals of calcite at 1.25 M reactant concentration, 95 °C reaction temperature, and 60 min reaction time. 4. Conclusions We have demonstrated that calcium carbonate with mainly two types of polymorphs (calcite and aragonite) and different particle sizes can be manufactured by controlling the reactant concentration, the reaction temperature, and the mode of mixing the reactants. Still effluent of a soda ash manufacturing (Solvay) process, which is a mixture of 1.25 M CaCl2 + 0.85 M NaCl, can be used as a source of calcium ions to produce precipitated calcium carbonate by reacting with sodium carbonate solution. The byproduct of this process is NaCl solution, which can be mixed with the starting material and, thus, recycled in the soda

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Acknowledgment The authors thank Dr. P. K. Ghosh, Director, Central Salt & Marine Chemicals Research Institute, Bhavnagar, for providing the facilities. Literature Cited

Figure 11. SEM images of calcite type calcium carbonate obtained at 0.94 M reactant concentration, 95 °C, and 60 min.

Figure 12. SEM images of calcite type calcium carbonate obtained at 1.25 M reactant concentration, 95 °C, and 60 min.

ash manufacturing process. The dilute reactant solutions are advantageous for the formation of aragonite. The content of calcite increased with increasing the carbonate concentration. The method of the selective crystallization of the polymorphs (calcite, aragonite, and their mixtures) by changing the mode of mixing the reactants and other controlling factors was also indicated. The variation in particle size is correlated with the nucleation and crystal growth as well as polymorph/phase transformation. Various morphologies of calcium carbonate, such as rhombohedral and needlelike, were produced depending on the controlling factors.

(1) Stumm, W.; Morgan, J. J. Aquatic Chemistry; Wiley: New York, 1996. (2) Chakrabarty, D.; Mahapatra, S. Aragonite crystals with unconventional morphologies. J. Mater. Chem. 1999, 9, 2953. (3) Lippmann, F. Sedimentary Carbonate Minerals; Springer: Berlin, 1973. (4) Weiner, S.; Addadi, L. Design strategies in mineralized biological materials. J. Mater. Chem. 1997, 7, 689. (5) Reeder, R. J. Crystal chemistry of the rhombohedral carbonates. In Carbonates: Mineralogy and Chemistry; Reeder, R. J., Ed.; ReV. Miner. 1983, 11, 1. (6) Morse, J. W.; Mackenzie, F. T. Geochemistry of Sedimentary Carbonates; Elsevier: Amsterdam, 1990. (7) Stipp, S. L.; Hochella, M. F.; Parks, G. A.; Lackie, J. O. Cd2+ uptake by calcite, solid-state diffusion and the formation of solid-solution: Interface processes observed with near-surface sensitive techniques (XPS, LEED and AES). Geochim. Cosmochim. Acta 1992, 56, 1941. (8) Richeter, A.; Petzold, D.; Hofman, H.; Ulrich, B. CHEMTECH 1995, 6, 306. (9) Williams, R. J. P. In Biominerallization. Chemical and Biochemical PerspectiVes; Mann, S., Webb, J., Williams, J. P., Eds.; VCH Verlagsgesellschaf: Weinheim, Germany, 1989; p 1. (10) Parsiegla, K. I.; Katz, J. L. Calcite growth inhibition by copper(II). II. Effect of solution composition. J. Cryst. Growth 2000, 213, 368. (11) Tracy, S. L.; Williams, D. A.; Jenning, H. M. The growth of calcite spherullites from solution. II. Kinetics of formation. J. Cryst. Growth, 1998, 193, 382. (12) Jiaguo, Y.; Ming, L.; Bei, C. Facile preparation of monodispersed calcium carbonate spherical particles via a simple precipitation reaction. Mater. Chem. Phys. 2004, 88, 1. (13) Gehrke, N.; Co¨lfen, H.; Pinna, N.; Antonietti, M. Superstructures of Calcium Carbonate Crystals by Oriented Attachment. Cryst. Growth Des. 2005, 5, 1317. (14) Tai, C. Y.; Chen, F. B. Polymorphism of CaCO3, precipitated in a constant-composition environment. AIChE J. 1998, 44, 1790. (15) Behrens, G.; Kuhn, L. T.; Ubic, R.; Heuer, A. H. Raman spectra of Vaterite calcium carbonate. Spectrosc. Lett. 1995, 28, 983. (16) Anderson, A. Group theoretical analysis of the V1 (CO32-) vibration in crystalline calcium carbonate. Spectrosc. Lett. 1996, 29, 819. (17) Kitamura, M.; Konno, H.; Yasui, A.; Masuoka, H. Controlling factors and mechanism of reactive crystallization of calcium carbonate polymorphs from calcium hydroxide suspensions. J. Cryst. Growth 2002, 236, 323. (18) Kitamura, M. Crystallization behaviour and transformation kinetics of L-Histidine Polymorphs J. Chem. Eng. Jpn. 1993, 26, 303. (19) Kitamura, M. Crystallization and Transformation Mechanism of Calcium Carbonate Polymorphs and the Effect of Magnesium Ion. J. Colloid Interface Sci. 2001, 236, 318.

ReceiVed for reView December 1, 2005 ReVised manuscript receiVed April 3, 2006 Accepted April 13, 2006 IE0513447