Monohydrocalcite in Comparison with Hydrated Amorphous Calcium

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Monohydrocalcite in Comparison with Hydrated Amorphous Calcium Carbonate: Precipitation Condition and Thermal Behavior Tomoyasu Kimura and Nobuyoshi Koga* Chemistry Laboratory, Department of Science Education, Graduate School of Education, Hiroshima University, 1-1-1 Kagamiyama, Higashi-Hiroshima 739-8524, Japan

bS Supporting Information ABSTRACT: Monohydrocalcite (MHC) and hydrated amorphous calcium carbonate (ACC) indicate many similarities in chemical composition, precipitation condition, and thermal behavior, from which the comparable thermodynamic state of these hydrated calcium carbonates can be deduced. In a reaction system of a mixed aqueous solution of Ca2+ and Mg2+ ions with CO32 ion, the MHC single phase is precipitated in a very restricted condition of the molar fraction of Mg2+ from 15 to 25% and in the temperature region from 288 to 303 K, which is surrounded by the precipitation conditions of calcite, hydrated ACC, and hydromagnesite. Two MHC samples with different crystallinities and morphologies are obtained within the present precipitation condition of MHC single phase. Although these MHC samples indicate apparently different thermal behaviors, those behaviors of thermal dehydration and subsequent transformations of as-produced anhydrous calcium carbonate can be correlated to those reported for hydrated ACC. The similarities in the precipitation conditions and thermal behaviors of MHC and hydrated ACC indicate the close relevancy of these compounds in the biomineraliztion and biomimetic mineralization processes of calcium carbonate polymorphs.

’ INTRODUCTION Mineralization of calcium carbonate polymorphs (CCPs) has been studied as one of the model processes that produces different polymorphs with various specialized morphologies.1 14 In such biomineralization of CCPs, hydrated amorphous calcium carbonate (ACC) has been focused as a key intermediate compound that transforms to different CCPs with various morphological and functional characteristics.5 The stabilization mechanism of the ACC, which is unstable thermodynamically in comparison with the CCPs of crystalline forms,15,16 and the transformation mechanisms to selected CCPs in ACC substrate are the research topics to be clarified for successful biomimetic synthesis of variously orientated functional materials of CCPs through ACC precursor roots. In additive-free systems, hydrated ACC is stabilized in ethanol solutions,17 19 and the stability of ACC changes depending on the pH of the mother solution.20 Confinement of ACC nanoparticles by water molecules was proposed as the possible stabilization mechanism.21,22 It is known that Mg2+ ion coexisting with Ca2+ ion in the mother solution largely influences the polymorphs selection and morphologies of precipitated CCPs.23 25 It was reported that the first precipitated ACC and the subsequently crystallized aragonite are stabilized in a mother solution with a molar ratio n(Mg2+)/n(Ca2+) g 4 by the inhabitation of calcite nucleation.6,23 The role of Mg2+ ion in the stabilization of biogenic ACC was explained recently by the distortion of the structure around the incorporated Mg2+ ion in ACC.26 Learning after the stabilization of biogenic ACC induced by specialized protein and by confinement in organic membranes,6,11,27 biomimetic r 2011 American Chemical Society

synthesis of additive-stabilized ACC was succeeded using various organic macromolecules3,5 and inorganic polymerizing species such as silicate.28,29 Monohydrocalcite (CaCO3 3 H2O: MHC), which is the target compound of the present study, has been found as sediments and a principle component of beachrock in several saline lakes,30 34 as moonmilk in limestone-dolomite caves,35 37 and as a decomposition product of ikaite (CaCO3 3 6H2O) from submarine.38 40 Biogenetic MHC has also been found in some living organisms such as in the otoliths of a tiger shark,41 in the gall bladder of a guinea pig,42 in the calcareous corpuscles of parasitic worm,43 as a trace in the bacterial mineralization,44 46 and in the dung of domestic animals.47 MHC was also reported as an intermediate compound of the carbonation reaction of calcium hydroxide.48 50 Laboratory synthesis of MHC has also been succeeded from artificial seawater51 56 or from a mixed solution of calcium chloride and magnesium chloride,57 where Mg2+ ion in the mother solution plays a predominant role for the selective precipitation of MHC in the mother solution. The crystal structure of MHC has been determined by single crystal diffraction as P31 with a = b = 10.5536 Å and c = 7.5446 Å,58 which was reexamined recently by neutron and X-ray powder diffractions making refinements on the hydrogen bonding structure and orientations of carbonate groups.34 Two different morphologies of MHC crystalline Received: April 2, 2011 Revised: June 26, 2011 Published: July 12, 2011 3877

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Crystal Growth & Design particles can be found in the literature, one is spherulite51 53 and the other is an aggregate of platelets,55 which indicate the different reaction behaviors during the thermal dehydration of crystalline water. Although it was clearly shown by X-ray absorption spectoroscopy55 that the local structure of MHC is different from that of hydrated ACC,59 the compositions of these compounds are very close. Thermodynamically, both of the compounds are metastable with respect to crystalline anhydrous CCPs.15,16,20,60 It was also reported that the transformation of MHC to aragonite takes place by aging in mother solution.57 It is expected that the comparative studies on the physicochemical properties and reaction behaviors of MHC and hydrated ACC provide the fundamental information required for discussing the possible participation of MHC in biomineralization of CCPs and the possible utilization of MHC for biomimetic mineralization of CCPs as a substitute for hydrated ACC. In the present work, phase variations of CCPs, including MHC and hydrated ACC, and magnesium compounds precipitated by the reactions of mixed aqueous solutions of calcium and magnesium chlorides with CO32 ion were investigated by changing systematically the molar fractions of Ca2+ and Mg2+ ions and the temperature of the mother solution. From the positional relations of the respective precipitation conditions for different CCPs with respect to the axis of molar fraction of Mg2+ and of the temperature of the mother solution, the region of selective precipitation of MHC is estimated. In addition, the possible morphological change of MHC crystal particles depending on the precipitation conditions is also examined. As-precipitated MHCs with different particle morphologies are subjected to a comparative thermoanalytical study to reveal the thermal stability and reaction pathway of the thermally induced transformations with reference to those of hydrated ACC reported previously.18 20

’ EXPERIMENTAL SECTION Sample Preparation and Characterization. The samples were synthesized using previously reported method57 by systematically changing the preparation conditions. Stock solutions of CaCl2(aq) (0.06 mol dm 3) and MgCl2(aq) (0.06 mol dm 3) were prepared by dissolving the respective regents of CaCl2 3 2H2O (special grade 99.0>, SAJ) and MgCl2 (special grade 99.0>, SAJ) into deionized distilled water. The stock solutions were mixed in various volume ratios to give the mother solutions of 200 cm3. The mother solutions in 200 cm3 beaker were stirred mechanically with a temperature control by keeping it at 298 K using a temperature programmable stirrer (CSB-900, As-One Co.). Anhydrous Na2CO3 (special grade 99.5>, SAJ) of 1.7 g was dispersed in the mother solution. The resultant solution was stirred mechanically for 48 h by keeping it at 298 K. Precipitates obtained were filtered and washed with deionized distilled water. The precipitates filtered were dried in a vacuum desiccator for 24 h and stored in a refrigerator at 278 K. The samples obtained from different mother solutions were labeled using the mixed volume % of the MgCl2(aq) (Mg) for preparing the mother solution and the preparation temperature (T) such as Mg20T298 for the sample obtained at 298 K from the mother solution with 80% CaCl2(aq) and 20% MgCl2(aq). Using the mother solution prepared in a mixed ratio of 80% CaCl2(aq) and 20% MgCl2(aq), alternative series of samples were precipitated at different temperatures from 278 to 308 K and otherwise identical procedures and conditions with the above. The samples were characterized by the measurements of powder X-ray diffraction (XRD) and Fourier transfer infrared spectroscopy (FT-IR). The powder XRD patterns of the samples were collected using a diffractometer (RINT2200 V, Rigaku Co.) with monochrome Cu KR radiation (40 kV, 20 mA). The IR spectra of the samples were recorded

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Figure 1. XRD patterns of the samples obtained by changing systematically the molar fraction of Mg2+ in the mother solution at 298 K. (a) Mg0T298, (b) Mg10T298, (c) Mg15T298, (d) Mg20T298, (e) Mg25T298, (f) Mg30T298, (g) Mg40T298, (h) Mg50T298, (i) Mg60T298, (j) Mg70T298, (k) Mg80T298, (l) Mg90T298, and (m) Mg100T298. by the diffuse reflectance method after diluting the samples with KBr using a FT-IR spectrometer (FTIR8400S, Shimadzu Co.). Using a scanning electron microscope (SEM: JSM-6510, Jeol), SEM images of the samples were examined after coating the samples with Pt by sputtering. Characterization of Thermal Behaviors. Using an instrument (TGD-9600, ULVAC), thermogravimetry differential thermal analysis (TG-DTA) measurements were carried out for the 10.0 mg samples weighed into a platinum cell (5 mm ϕ and 5 mm in height) at a heating rate β = 10 K min 1 in flowing N2 (100 cm3 min 1). A 10.0 mg amount of alumina weighed into the twin cell was used for the DTA reference. Some selected samples were subjected to the measurements of TGDTA mass spectroscopy (MS) using an instrument constructed by coupling a TG-DTA (TG8120, Rigaku) with a quadrupole mass spectrometer (M-200Q, Anelva), where the samples of 5.0 mg weighed into a platinum cell (5 mm ϕ and 2.5 mm in height) were heated at β = 10 K min 1 in flowing He (200 cm3 min 1). The mass spectra of the evolved gases were monitored continuously during the TG-DTA measurements (mass range, 10 50 amu; EMSN, 1.0 A; SEM, 1000 V). Phase changes during heating of the selected samples were followed by powder XRD measurements using the above diffractometer by equipping it with a programmable heating chamber (PTC-20A, Rigaku). By heating the samples press-fitted on a platinum plate at β = 10 K min 1 in flowing N2 (100 cm3 min 1), the diffraction measurements were started at various temperatures, where the sample temperature was kept constant during the measurements.

’ RESULTS AND DISCUSSIONS Influence of Mg2+ in the Mother Solution on the Precipitate. Figure 1 compares XRD patterns of the samples obtained

by changing systematically the molar fraction of Mg2+ in the mother solution at 298 K. A typical XRD pattern of calcite61 is observed for Mg0T298 and Mg10T298. For the samples

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Figure 2. FT-IR spectra of the samples obtained by changing systematically the molar fraction of Mg2+ in the mother solution at 298 K. (a) Mg0T298, (b) Mg10T298, (c) Mg15T298, (d) Mg20T298, (e) Mg25T298, (f) Mg30T298, (g) Mg40T298, (h) Mg50T298, (i) Mg60T298, (j) Mg70T298, (k) Mg80T298, (l) Mg90T298, and (m) Mg100T298.

precipitated from the mother solutions with the molar fraction of Mg2+ more than 15%, no distinguished diffraction peaks of calcite are found, indicating the inhibitation effect of Mg2+ on the nucleation of calcite as has been reported.6,23 25 The XRD patterns of the samples from Mg15T298 to Mg25T298 indicate exclusively those corresponding to MHC.58,62 The diffraction peaks of hydromagnesite (HM: 4MgCO3 3 Mg(OH)2 3 4H2O)63 appear in the samples precipitated from the mother solutions with a molar fraction of Mg2+ more than 30%, together with those of MHC. With an increase in the molar fraction of Mg2+, the diffraction peaks of HM grow accompanied by the attenuation of those of MHC. The sample of Mg100T298 indicates absolutely the XRD pattern of HM.63 The same trend can be followed by the FT-IR spectra shown in Figure 2. The samples of Mg0T298 and Mg10T298 indicated the characteristic IR absorption of calcite, that is, at 876 and 712 cm 1 due to v2 and v4 modes of CO32 .64 For the samples characterized as MHC single phase by XRD, that is, from Mg15T298 to Mg25T298, the existence of crystal water can be confirmed by an O H stretching band and H2O deformation at 3236 and 1701 cm 1, respectively. All of the reported vibration bands of CO32 of MHC58,65 are also observed for these samples. Two split absorption peaks at 1416 and 1491 cm 1 are due to the v3 mode of CO32 . The absorption peaks due to v1, v2, and v4 modes of CO32 appear at 1071, 874, and 700 cm 1, respectively. For the samples from Mg30T298 to Mg90T298, those absorption peaks of MHC attenuate gradually with an increase in the molar fraction of Mg2+ in the mother solutions, because of the change in the ratio of MHC and HM. The growing absorption peak at 3647 cm 1 is due to O H stretching band of hydroxide ion in HM.66 68 Figure 3 compares TG and derivative TG (DTG) curves for some selected samples. Well-distinguished single step decomposition

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Figure 3. TG-DTG curves of some selected samples obtained by changing systematically the molar fraction of Mg2+ in the mother solution at 298 K.

with the mass-loss value of 42.6% is observed for Mg0T298 at higher than 850 K, corresponding to the value calculated for the reaction: CaCO3 f CaO + CO2. As is typically seen for Mg20T298, the samples from Mg15T298 to Mg25T298 were decomposed in two steps with the mass-loss values of 17.1 ( 0.3 and 34.1 ( 1.0% for the first and second mass-loss steps, respectively. These values are nearly equivalent to those calculated by assuming the following reactions: CaCO3 3 H2O f CaCO3 + H2O and CaCO3 f CaO + CO2, respectively. Although the XRD pattern of MHC was observed for these samples, the first mass-loss step of thermal dehydration, which initiates from room temperature, is apparently different from the reported dehydration behavior of MHC55 in which MHC is stable up to ca. 480 K, and the thermal dehydration takes place in the narrow temperature region around 480 520 K. The difference may be due to the poorly crystalline characteristics of the samples from Mg15T298 to Mg25T298. In addition, it is expected that hydrated ACC exists possibly as the secondary phase, because hydrated ACC dehydrates gradually from room temperature by producing anhydrous ACC.18 20 Four distinguished mass-loss steps are observed for the samples from Mg30T298 to Mg90T298, as is seen for the TG curve of Mg60T298. Mg100T298 decomposes in three mass-loss steps, which is in good agreement with the thermal decomposition of HM reported previously.67 The three mass-loss steps are ascribed by the thermal dehydration of crystalline water, thermal decomposition of hydroxide ion, and thermal decomposition of carbonate ion, respectively, that is, 4MgCO3 3 Mg(OH)2 3 4H2O f 4MgCO3 3 Mg(OH)2 + 4H2O, 4MgCO3 3 Mg(OH)2 f 4MgCO3 + MgO + H2O, and 4MgCO3 f 4MgO + 4CO2.66 Because the samples from Mg30T298 to Mg90T298 were identified as the mixtures of MHC and HM, the four decomposition steps can be explained as the thermal dehydration of HM, overlapped process of the thermal dehydration of MHC and the thermal dehydroxylation of dehydrated HM, thermal decomposition of MgCO3, and thermal decomposition of CaCO3, 3879



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Figure 4. Dependence of the molar fractions of MHC and HM phases in the precipitates on the molar fraction of Mg2+ in the mother solution.

respectively. The mass-loss values of the third step, that is, the thermal decomposition of MgCO3, increased systematically from Mg30T298 to Mg100T298, whereas those of the fourth step decreased, due to the variation of mixed ratios of MHC and HM depending on the molar fraction of Mg2+ in the mother solution. Figure 4 shows changes of the molar fractions of MHC and HM phases in the precipitates evaluated from the mass-loss values of the respective mass-loss steps of thermal decompositions of CaCO3 and MgCO3. Typical SEM images of some selected samples in the series from Mg0T298 to Mg100T288 are shown in Figure S1 in the Supporting Information. Influence of Temperature on the Precipitate. From the above, the region of molar fraction of Mg2+ in the mother solutions from 15 to 25% is favored for the selective nucleation of MHC phase in the present synthetic condition. However, the MHC prepared at 298 K indicates a poorly crystalline characteristic by the thermal dehydration in a wide temperature region from room temperature, which is apparently different from those reported for MHC. Control of nucleation and growth rates of MHC crystals seems to be one of the possible ways to obtain a well-crystalline MHC. In this purpose, the influence of preparation temperature was investigated systematically by selecting the mother solution containing 20% of molar fraction of Mg2+. Figure 5 shows XRD patterns of the samples precipitated at different temperatures from the mother solution containing 20% of molar fraction of Mg2+. At the temperature lower than 283 K, the precipitated phases indicate no distinguished diffraction peaks, indicating the precipitation of amorphous phase. XRD patterns of the samples prepared at the temperature higher than 288 K indicate MHC single phase, where the diffraction intensities of all of the peaks decrease with an increase in the preparation temperature. At 308 K, the XRD pattern of HM appears as the minor phase in addition to the major MHC phase. The trend was also followed by FT-IR spectra and TG-DTG curves of the samples as shown in Figures S2 and S3 in the Supporting Information. Different particle morphologies of MHC particles precipitated at different temperatures are worthy of special mention. Figure 6 shows SEM images of MHC particles precipitated at 288 and 303 K. The particles of Mg20T288, characterized as the most well crystalline MHC in view of XRD, are spherical and spherical polygons in the size from 1 2 μm in diameter (Figure 6a). The other single MHC phase precipitated at the higher temperatures, that is, from Mg20T293 to Mg20T303, consists of the aggregates in the shape of twisted spindle (Figure 6b). The size of the aggregates tends to increase with precipitation temperature.

Figure 5. XRD patterns of the samples precipitated at different temperatures from the mother solution containing 20% of molar fraction of Mg2+. (a) Mg20T278, (b) Mg20T283, (c) Mg20T288, (d) Mg20T293, (e) Mg20T298, (f) Mg20T303, and (g) Mg20T308.

Figure 6. SEM images of MHC particles with different morphologies precipitated at different temperatures. (a) Mg20T288 and (b) Mg20T303.

Precipitation Condition of MHC. Figure 7 summarizes the precipitated phases in the conditions examined in the present study. As is generally observed for the mineralization of CCPs, a gel-like amorphous phase seems to be precipitated first in the mother solution.5,22 Nucleation of the most stable phase of calcite is inhibited when containing Mg2+ more than 15% in molar fraction with respect to the total amount of Ca2+ and Mg2+. The initial gel-like precipitates of hydrated ACC are stabilized at least for 48 h in the mother solution at the temperature lower 3880



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Figure 7. Summary of the precipitated phases in the conditions examined presently.

than 283 K. Nucleation of MHC phase takes place in the mother solutions containing more than 15% of molar fraction of Mg2+ and at the temperatures higher than 288 K. When the mother solution contains Mg2+ more than 30% in molar fraction at 298 K or the precipitation temperature is higher than 308 K in the mother solution containing 20% of the molar fraction of Mg2+, the secondary HM phase cannot be avoided. Accordingly, in the present preparation method, the selective precipitation of MHC single phase is restricted in the conditions of the molar fraction of Mg2+ of the mother solution from 15 to 25% and of the preparation temperature from 288 to 303 K. The region of the precipitation conditions corresponds to those adopted in the previous works of MHC precipitation,57 where the incorporation of about 5 mol % Mg2+ ions in the precipitated MHC phase was reported. Furthermore, MHC particles with two different textures are obtained in the restricted region of precipitation conditions. At the lowest temperature in the restricted precipitation conditions, MHC particles are spherical and spherical polygons of 1 2 μm, whereas the MHC particles obtained at the higher temperatures are the aggregates in the shape of twisted spindle. The higher crystallinity of the spherical MHC particles was confirmed by the larger diffraction intensities in XRD, the higher thermal stability, and the thermal dehydration in the very restricted temperature region (see Figure 5 and Figure S3 in the Supporting Information). Thermal Decomposition Pathway of Synthetic MHC. The MHC phases of two different textures precipitated were subjected to the investigation on the thermal decomposition pathways by selecting Mg20T288 and Mg20T303, respectively, as the typical samples of spherical and/or spherical polygons and an aggregate in the shape of twisted spindle. Figure 8 compares TGDTA curves of these samples, together with mass chromatograms of m/z 18 and m/z 44 of the evolved gases. Changes of XRD patterns during heating MG20T288 (a) and MG20T303 (b) from room temperature to 823 K are shown in Figure 9. In the first mass-loss steps of the respective samples, only water vapor is the gaseous product, indicating the thermal dehydration process as assumed above. The higher thermal stability of Mg20T288 is confirmed in views of TG/DTA-MS and XRD. The thermal dehydration of Mg20T288 is characterized by the distinguished sharp peak of the mass chromatogram of m/z 18 observed in the range from 460 to 485 K. By the thermal dehydration, Mg20T288 produces preferentially calcite phase, but detectable diffraction peaks of vaterite are also observed. The characteristics of the thermal stability and dehydration behavior seem to be interpreted as regulating by the nucleation of calcite for the

Figure 8. TG-DTA curves of Mg20T288 (a) and Mg20T303 (b) together with the mass chromatograms of m/z 18 and m/z 44 for the evolved gases during the thermal decompositions.

thermal dehydration. Figure 10 shows a typical record of the isothermal mass-loss trace for the initial part of the thermal dehydration of Mg20T288 at 432 K, where a distinguished induction period is observed. Because, in general, the formations of nuclei and/or possible nucleus forming sites are accepted as the physicochemical event taking place during the induction period,69,70 the existence of the induction period supports the interpretation of the characteristics of the thermal stability and dehydration behavior as a nucleation controlled phenomena. The intensities of the diffraction peaks of as-produced calcite phase and minor phase of vaterite remain unchanged during further heating up to 673 K. In the XRD pattern at 723 K corresponding to the DTA exothermic peak observed in Figure 8a, the minor vaterite phase disappears and the calcite crystals start to grow. As a probable mechanism, inhibition of crystal growth of calcite by the miner phase of vaterite existing in the boundaries of calcite grains is proposed,19,71 where the crystal growth of calcite resumes triggered by the phase transition of the interfacial vaterite to calcite. A similar behavior of calcite crystal growth triggered by the vaterite calcite phase transition has been observed for the calcite phase produced by the thermal dehydration and subsequent crystallization of hydrated ACC prepared from ethanol solution.18 3881



Figure 9. Changes of XRD patterns during heating Mg20T288 (a) and Mg20T303 (b) from room temperature to 823 K.

Figure 10. Isothermal mass-loss trace for the initial part of the thermal dehydration of Mg20T288 at 432 K.

Mg20T303 shows a largely different pathway during the thermal dehydration and formation of calcite from those of Mg20T288. Two separated peaks of the mass chromatogram of m/z 18 are observed for the thermal dehydration of Mg20T303 from near room temperature, where the XRD peaks of MHC attenuate gradually as the reaction advances. In comparison with that of Mg20T288, the higher temperature portion of the thermal dehydration of Mg20T303 is taking place in the wider temperature range, that is, 400 560 K. During the thermal dehydration, no alternative diffraction peak appears up to 523 K. Thus, anhydrous ACC is produced as the primary product of the thermal dehydration. As one of the possible mechanisms of the amorphous product formation, the selfinduced sol gel process72 75 can be recalled, where a wet gel phase is produced at the reaction interface during the thermal dehydration, followed by formation of amorphous phase due to diffusional removal of water. Just after completing the thermal

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dehydration, a DTA exothermic peak is observed at around 570 610 K (Figure 8b), and XRD peaks of calcite grow abruptly (Figure 9b). The similar behavior has been reported for the thermal dehydration of hydrated ACC,18 20 where the exothermic peak has been identified as the crystallization of in situproduced anhydrous ACC. The growth behavior of as-crystallized calcite phase on further heating is also different from that of Mg20T288. In contrast to the inhibitation of crystal growth of calcite in the temperature region up to the structural phase transition of vaterite to calcite observed for Mg20T288, the crystal growth of calcite takes place, continuously accompanying the transformation of vaterite to calcite, resulting in the pure calcite phase at 773 K. The overall behavior of the thermally induced transformation of Mg20T303 resembles that of a hydrated ACC prepared from aqueous solution.18 After the thermal dehydration, both of the samples are stable up to about 800 K in view of the mass change. The second massloss steps of the samples are apparently the thermal decomposition of calcite as evidenced by the quantitative mass loss, the singly evolution of CO2 detected by the mass chromatogram of m/z 44, and the change of the XRD pattern from that of calcite to CaO. In addition to the chemical compositions and the precipitation conditions, thermal behaviors of MHC and hydrated ACC show the large similarities. These indicate that, in spite of the different local structures of these two compounds,55,59 MHC and hydrated ACC are on the vicinity state of thermodynamics. It has been recognized that hydrated ACC can have the different thermodynamic state with different degrees of atomic ordering.16,20 As has been seen above, MHC phases precipitated in the different conditions also represent different thermal behaviors due to different morphologies and crystallinities. Detailed structural characterization and kinetics of the thermal dehydration of these two MHC will be reported soon in a separated paper, in comparison with those of hydrated ACC.

’ CONCLUSIONS In the system of reacting a mixed aqueous solution of Ca2+ and Mg2+ with the molar fraction, 0 < n(Mg2+)/[n(Ca2+) + n(Mg2+)] < 1, and CO32 ion, the MHC single phase is precipitated in a very restricted region of the molar fraction of Mg2+ and temperature of mother solution, that is, in 15 25% of molar fraction of Mg2+ and at 288 303 K. In the lower molar fraction of Mg2+ at 298 K, the calcite phase is predominantly precipitated. At the lower temperature in 20% of molar fraction of Mg2+, no crystalline phase can be found precipitating hydrated ACC. At the higher temperature in 20% of the molar fraction of Mg2+ and in the higher molar fraction of Mg2+ at 298 K, HM is coprecipitated with MHC. Accordingly, the precipitation condition of MHC single phase in the present reaction system is surrounded by those of calcite, hydrated ACC, and HM. Within the reaction conditions of the precipitating MHC single phase, two typical MHC particles with different crystallinities and morphologies are obtained. One is well-crystalline spherical particles precipitated at the restricted conditions in 20% of molar fraction of Mg2+ at 288 K. Another is poorly crystalline aggregates of platelets in the shape of twisted spindle precipitated in the other region of reaction conditions of MHC precipitation. These two MHC samples indicate apparently different reaction behaviors of thermal dehydration and subsequent transformations of anhydrous CaCO3. The former indicates the thermal 3882


Crystal Growth & Design stability higher than that of the latter. The thermal dehydration takes place at a temperature higher than 423 K with a distinguished sharp endothermic effect, where calcite is the predominant phase of the solid product. The thermal dehydration of the latter MHC sample initiates from room temperature. During the thermal dehydration, no crystalline phase of solid product appears, producing anhydrous ACC as a transient phase. After completing the thermal dehydration, the calcite phase crystallizes with an exothermic effect. The thermal behavior of the latter MHC sample resembles that of the synthetic hydrated ACC. As has been demonstrated in the present study, MHC and hydrated ACC indicated many similarities in chemical composition, precipitation condition, and thermal behavior, from which the comparable thermodynamic states of the overall phases of MHC and hydrated ACC are expected qualitatively. These indicate the possible participation of MHC in biomineralization of CCPs and the possible utilization of MHC for biomimetic mineralization of CCPs as a substitute for hydrated ACC.

’ ASSOCIATED CONTENT

bS

Supporting Information. SEM images of the samples precipitated from the mother solutions containing different molar fractions of Mg2+ at 298 K. FT-IR and TG-DTG curves of the samples precipitated at different temperatures from the mother solution containing 20% of the molar fraction of Mg2+. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The present work was supported partially by a grant-in-aid for scientific research (B) (21360340 and 22300272) from Japan Society for the Promotion of Science. ’ REFERENCES (1) Mann, S. Biomineralization: Principles and Concepts in Bioinorganic Materials Chemistry; Oxford University Press: Oxford, 2001. (2) Baeuerlein, E. Biomineralization: Progress in Biology, Molecular Biology and Application, 2nd ed.; Wiley-VHC: Weinheim, 2005. (3) Meldrum, F. C.; Colfen, H. Chem. Rev. 2008, 108, 4332–4432. (4) Sommerdijk, N. A. J. M.; de With, G. Chem. Rev. 2008, 108, 4499–4550. (5) Gower, L. B. Chem. Rev. 2008, 108, 4551–4627. (6) Raz, S.; Weiner, S.; Addadi, L. Adv. Mater. 2000, 12, 38–42. (7) Levi-Kalisman, Y.; Raz, S.; Weiner, S.; Addadi, L.; Sagi, I. Adv. Funct. Mater. 2002, 12, 43–48. (8) Aizenberg, J.; Lambert, G.; Weiner, S.; Addadi, L. J. Am. Chem. Soc. 2002, 124, 32–39. (9) Raz, S.; Testeniere, O.; Hecker, A.; Weiner, S.; Luquet, G. Biol. Bull. 2002, 203, 269–274. (10) Weiss, I. M.; Tuross, N.; Addadi, L.; Weiner, S. J. Exp. Zool. 2002, 293, 478–491. (11) Raz, S.; Hamilton, P. C.; Wilt, F. H.; Weiner, S.; Addadi, L. Adv. Funct. Mater. 2003, 13, 480–486. (12) Addadi, L.; Raz, S.; Weiner, S. Adv. Mater. 2003, 15, 959–970. (13) Politi, Y.; Arad, T.; Klein, E.; Weiner, S.; Addadi, L. Science 2004, 306, 1161–1164.

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