Aragonite Crystal Growth and Solid-State AragoniteâCalcite

Article pubs.acs.org/crystal

Aragonite Crystal Growth and Solid-State Aragonite−Calcite Transformation: A Physico−Geometrical Relationship via Thermal Dehydration of Included Water Nobuyoshi Koga,* Daisuke Kasahara, and Tomoyasu Kimura Department of Science Education, Graduate School of Education, Hiroshima University, 1-1-1 Kagamiyama, Higashi-Hiroshima 739-8524, Japan ABSTRACT: A relationship between the physico−geometrical mechanisms of aragonite crystal growth and the thermally induced aragonite−calcite transformation was revealed by focusing on the morphological changes during these processes. Thermal dehydration of the included water during the aragonite−calcite transformation was investigated to characterize the relationship. The trapping of water molecules at the twin boundaries is expected from the aragonite crystal growth mechanism of the twinning of poorly crystalline needle-like crystals to form pseudohexagonal columnar crystals. Heating the aragonite gives the two-step thermal dehydration of the included water (total mass loss due to the dehydration is less than 2% of original sample mass), in which the second dehydration process with rapid water vapor release simultaneously occurs with the aragonite−calcite transformation. During the transformation, the morphology of the aragonite crystal dramatically changes to form dumbbell-like crystal with cauliflower-like structures at each end. The splitting of the aragonite crystal is initiated at both ends of the columnar crystals and propagates to the column center along the twin boundaries. The kinetic behavior of the thermal dehydration during the aragonite−calcite transformation describes the physico−geometrical mechanism of the aragonite−calcite transformation well, and this is closely related to the crystal morphology and the crystallographic characteristics of the synthetic aragonite.

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INTRODUCTION The mineralization of calcium carbonate has long been studied as a model process for the formation of polymorphs with various specialized morphologies.1 Recently, the mineralization of calcium carbonate polymorphs in living organisms has been extensively studied,2,3 because biogenetic calcium carbonate gives highly functional properties that are realized by very precisely controlled morphologies and architectures. Learning from biomineralization processes, knowledge for controlling the crystalline phases, morphology, and architecture using biomimetic processes is developing.4−6 Understanding the transformations of the as-produced calcium carbonate is another important task for evaluating the stability of the calcium carbonate and is the fundamental information for material processing by structural phase transformation and thermal decomposition of the calcium carbonate. We have previously reported the reaction mechanisms and kinetics of the thermally induced transformations of hydrated amorphous calcium carbonate (ACC),7,8 monohydrocalcite,9,10 and vaterite.11 Aragonite is a typical biomineralized calcium carbonate that is widely found because it is a building block for shell exoskeletons, coral skeletons, and so on.2,3 Thermodynamically, aragonite (CaCO3, Pmcn, a = 4.96183, b = 7.969914, c = 5.74285)12,13 is a metastable phase at room temperature and atmospheric pressure. It is generally understood that the © 2013 American Chemical Society

structural transformation of aragonite to stable calcite (CaCO3, R3̅c, a = 4.9910, b = 4.9910, c = 17.0620)14 is an endothermic process controlled by kinetics15 and is mediated by a solvent in a suspension16 or induced by heat in the solid-state. The kinetics of the aragonite−calcite phase transformation in the solid state have been studied using thermoanalytical methods such as differential thermal analysis (DTA) and differential scanning calorimetry (DSC),17−19 X-ray diffraction (XRD),20 and spectroscopic measurements.21 The overall kinetics has been explained by a nucleation and growth type physico− geometrical reaction model.22−24 At the atomic level, GomezVillalba et al.25 recently reported that the nucleation and growth of the calcite phase occur as a result of the rearrangement of CO32− anions in areas characterized by the disorder resulting from aragonite lattice dislocations. However, at the macroscopic level, the reported transformation enthalpy and the kinetic parameters of the transformation largely vary depending on aragonite sample tested and are influenced by impurities.19,26−28 Fujinuki and Igarashi29 first reported that the thermally induced transformation of coral aragonite to calcite is accompanied by the thermal dehydration of included water. Received: March 7, 2013 Revised: April 4, 2013 Published: April 5, 2013 2238

dx.doi.org/10.1021/cg400350w | Cryst. Growth Des. 2013, 13, 2238−2246

Crystal Growth & Design

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Characterization. The samples were characterized by powder X-ray diffractometry (XRD) and Fourier transform infrared spectroscopy (FT-IR). The powder XRD patterns of the samples were obtained using a diffractometer (RINT2200 V, Rigaku Co.) with monochrome Cu−Kα radiation (40 kV, 20 mA). FT-IR spectra were obtained using a spectrometer (FTIR8400S, Shimadzu Co.) by the diffuse reflectance method after diluting the samples with KBr. The morphology of the samples was examined using a scanning electron microscope (SEM: JSM-6510, Jeol) after sputter coating the samples with platinum. Thermal Behavior. TG−DTA measurements were carried out using a top-loading type instrument (DTG-50, Shimadzu Co.). Each sample (m0 = 10.0 mg) was weighed in a platinum cell (6 mm ϕ and 2.5 mm high) and linearly heated at a rate, β, of 10 K min−1 in flowing air (80 cm3 min−1). The evolved gas during heating the samples was analyzed using a quadrupole mass-spectrometer (MS, M-200QA, Anelva Co.), which was connected to a TG−DTA Instruments (TG8120, Rigaku Co.) using a silica capillary tube (0.075 mm internal diameter) heated at 500 K. TG/DTA−MS measurements were carried out on samples of approximately 5.0 mg or 10 mg weighed in platinum cells (5 mm ϕ and 2.5 mm high) by heating at β = 10 K min−1 in flowing He or mixed gas of He−O2 (30% O2) at a rate of 200 cm3 min−1. The mass spectra of the evolved gases were recorded over 10−50 amu or 10−100 amu (EMSN: 1.0A; SEM: 1.0 kV). Changes in the crystalline phases while the samples were heated were traced using the diffractometer described above, which was equipped with a programmable heating chamber (PTC-20A, Rigaku Co.). The samples were press fitted onto a platinum plate and heated at β = 10 K min−1 in flowing N2 (100 cm3 min−1), and the diffraction measurements were carried out at various temperatures, keeping the sample temperature constant for 15 min during each diffraction measurement. FT-IR measurements and SEM observations were performed on samples that had been heated to different temperatures using the DTG-50 instrument under the same conditions as the TG−DTA measurements and immediately cooled to room temperature. For the kinetic analysis of the mass-loss process during sample heating, the mass-change traces during the heating of a selected sample (Ara24h) were recorded using a hanging-type TG (TGA-50, Shimadzu Co.) by heating the sample (m0 = 10.0 mg, in a 6 mm ϕ and 2.5 mm high platinum cell) at different β, 2 ≤ β ≤ 10 K min−1, in flowing N2 (80 cm3 min−1). Blank measurements were carried out under the same conditions and used to baseline correct the mass-change traces. The corrected mass-change traces were further separated into two different mass-loss steps using a mathematical procedure based on a kinetic equation for a partially overlapping two-step reaction. The extracted mass-change traces that accompanied with the aragonite−calcite transformations were subjected to kinetic analysis.

Analogous thermal dehydration behavior in biomineralized aragonite has been reported by a number of researchers.30−32 Yoshioka and Kitano30 and Peric et al.33 observed thermal dehydration during the aragonite−calcite transformation in synthetic and mineral aragonite samples. The existence of included water is not mentioned in many reports of aragonite synthesis, but some of the reported thermogravimetry (TG)− DTA curves indicate mass loss of several percent in the temperature range of the endothermic DTA peak that is attributed to the aragonite−calcite transformation.34 In some reported cases, aragonite was crystallized in aqueous ethanol via hydrated calcium carbonate (such as ACC),35−37 and the crystals grew by the twinning of individual crystallites.38−40 It is thus deduced that the incorporation of water molecules in the aragonite crystal lattice is closely related to the crystal growth mechanism. Contributions from dynamic actions on the aragonite−calcite transformation caused by the included water should be considered if the aragonite crystals are heated in a dry atmosphere. Parker et al.41 reported anomalous thermal behavior in the aragonite lattice expansion parameter and attributed it to the effect of increasing internal pressure caused by gas inclusions trapped within the structure. Therefore, included water in the aragonite lattice is a key for revealing the physico−geometrical mechanism of the aragonite−calcite transformation and its relationship with the aragonite crystal growth mechanism. In this study, we synthesized aragonite with different rod sizes and investigated the variability in its included water content. The mass lost during the thermal dehydration of the included water and its relationship with thermal effects in the aragonite-calcite transformation temperature range were investigated by focusing on the evolved gas during the transformation. We selected a sample and analyzed the kinetics of the thermal dehydration behavior in the thermally induced aragonite−calcite transformation temperature range. The kinetic results were interpreted by correlating with the morphological change in the sample caused by the thermal treatment. Our findings allow us to discuss the physico− geometrical mechanism of the aragonite−calcite transformation with reference to the aragonite crystal growth mechanism and the resulting morphological and crystallographic characteristics of aragonite.

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EXPERIMENTAL SECTION

Sample Preparation. Aragonite crystals with systematically controlled sizes were synthesized according to a previously reported method34,35 involving a hydrothermal urea decomposition from a water/ethanol solution of calcium acetate at 363 K. Stock solutions of Ca(CH3COO)2 aq (0.50 mol dm−3) and urea aq (1.50 mol dm−3) were prepared by dissolving Ca(CH3COO)2·H2O (Special grade >99.0%, Sigma-Aldrich Japan) and urea (Special grade >99.0%, Sigma-Aldrich Japan) into deionized−distilled water. Each stock solution (20 cm3) and 10 cm3 of ethanol (Special grade >99.5%, Nacalai Tesque) were mixed in a Teflon hydrothermal reactor (100 cm3), which was sealed and heated to 363 K in an electric oven for 6, 12, 24, 48, or 96 h. The precipitates obtained were isolated by filtration and repeatedly rinsed with deionized−distilled water. The isolated precipitates were dried in a vacuum desiccator for 24 h and then stored in a refrigerator at 278 K. The samples were labeled by the duration of the hydrothermal treatment (Ara6h, Ara12h, Ara24h, Ara48h, and Ara96h).

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RESULTS AND DISCUSSION Crystal Growth of Aragonite in Hydrothermal Treatment. Figure 1 shows the XRD patterns and the FT-IR spectra of the as-prepared aragonite samples. The Ara6h sample, which required the shortest hydrothermal treatment duration, has characteristic XRD patterns indicating poor crystallinity and a small diffraction peaks corresponding to aragonite.12,13 All diffraction peaks attributed to aragonite gradually grow with increasing hydrothermal treatment duration. Ara6h has IR 2239



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Figure 1. (a) XRD patterns and (b) FT-IR spectra of Ara6h−Ara96h.

absorption peaks characteristic of the carbonate ion at 877 cm−1 (v2 mode) and 711 cm−1 (v3 mode), which almost corresponded to those of calcite or ACC42 rather than aragonite. The IR absorption peaks for the carbonate ion disappear in samples prepared with longer hydrothermal treatment, accompanied by the appearance and growth of IR absorption peaks for carbonate ions in the aragonite structure, i.e., split peaks at 700 and 713 cm−1 (ν4 mode), 852 cm−1 (ν2 mode), and 1082 cm−1 (ν1 mode).43−45 All of the samples have O−H stretching absorption between 2600−3600 cm−1. The above observations indicate that the initially precipitated amorphous or poorly crystalline phase transforms into aragonite during the hydrothermal treatment, where the existence of adsorbed or included water is expected in all samples from the IR absorption attributed to the O−H stretching. Figure 2 shows typical SEM images of the as-prepared aragonite. Ara6h contains bundles of needle-like crystals growing in the same direction, with sharp growth points (Figure 2a). With the longer hydrothermal treatment, the needle-like crystals change to columnar crystals, and the sharp growth points gradually change to pseudohexagonal six-sided planes (Figure 2b−d) via a half-pseudohexagonal shape in Ara24h (Figure 2c). Pseudohexagonal columnar crystals with diameters 2−5 μm are seen in Ara48h and Ara96h (Figure 2d,e). The textural changes in the aragonite crystals with the hydrothermal treatment duration led us to expect that the initially formed needle-like crystals twin during the crystal growth. Formations of pseudohexagonal aragonite having rod and tabular shapes have been reported from an electrochemical deposition from an artificial seawater electrolyte on a titanium foil and by precipitation−crystallization in a stirred solution of mixed Ca- and Mg-dodecyl sulfates and urea, respectively.38,39 Traces of multiple twinning processes, forming pseudohexagonal prism, were seen in both cases. The aragonite twinning mechanism has recently been discussed by Sand et al.40 Crystal defects and dislocations might be inserted in the twin boundaries because the contact angles between two aragonite crystals on the {110} symmetry planes and intergrowth contact on the ∼{130} faces are different from that of the ideal angles for the formation of a hexagonal prism. In addition, because the individual needle-like crystals that twin to form the pseudohexagonal rods are poorly crystalline hydrated calcium

Figure 2. SEM images of the aragonite samples: (a) Ara6h, (b) Ara12h, (c) Ara24h, (d) Ara48h, and (e) Ara96h.

carbonate as was deduced from the poorly crystalline nature and the existence of water molecules in Ara6h, the trapping of water molecules at the twin boundary during crystal growth is very probable.46 The twin boundaries are located along the lengths of the aragonite rods obtained in this study. Thermal Behavior of Synthetic Aragonite. Figure 3 shows the TG−DTA curves and mass chromatograms of the

Figure 3. TG−DTA curves and mass chromatograms of evolved gases recorded during heating Ara24h (m0 = 5.0 mg) at β = 10 K min−1 in flowing He (200 cm3 min−1). (a) TG−DTA curves and (b) mass chromatograms of m/z 18 and m/z 44.

evolved gases recorded by heating Ara24h. A DTA anomaly with a detectable mass loss is observed at 750−775 K before the thermal decomposition of CaCO3 at higher than 825 K (Figure 3a). The mass loss observed, less than 2%, starts at approximately 600 K, and a rapid mass loss appears from 750 to 2240



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temperature range of the rapid mass-loss step (Figure 5a). The endothermic effect is further enhanced during the rapid massloss step in flowing He, but an exothermic effect appears in flowing mixed He−O2. A detailed analysis of the mass chromatograms of gas evolved during the thermal dehydration in flowing He indicates that a small amount of CO (m/z 28) is evolved along with the water vapor (Figure 5b). The oxidation of CO is the most probable cause for the exothermic effect seen during the rapid mass-loss step in flowing He−O2. The source of CO is likely to be CO2 and carbonate ions trapped in the crystal lattice with the water molecules. Figure 6 shows the changes in the XRD pattern and FT-IR spectrum when Ara24h was heated to different temperatures.

775 K. Water vapor is the major evolved gas during the massloss process (Figure 3b), although the evolution of CO2 from the thermal decomposition of CaCO3 already starts in the end part of the rapid mass-loss process. It should be noted from the m/z 18 mass chromatogram that the water vapor release process consists of two overlapped processes, basal dehydration from 600 to 900 K and a rapid-release process at around 750 K. Similar water vapor release behavior before the thermal decomposition of CaCO3 has been reported for corals, other minerals, and some synthetic aragonites.29−33 Figure 4 shows the TG−DTA curves in the temperature range of thermal dehydration for the as-prepared samples

Figure 4. TG−DTA curves of the aragonite samples (m0 = 10.0 mg) recorded at β = 10 K min−1 in flowing air (80 cm3 min−1).

recorded in flowing air. The mass-loss percentage during the two-step dehydration process systematically decreases with the duration time of the hydrothermal treatment. The initial gradual mass-loss step accompanies an endothermic effect, whereas there is an exothermic effect during the rapid mass-loss step. Figure 5 shows the TG/DTA−MS results for Ara24h in flowing He and He−O2 (30% O2) in the temperature range of the thermal dehydration process. Comparable TG curves are found in both gaseous atmospheres (Figure 5a). A distinguishable difference is, however, observed in the DTA curves in the

Figure 6. Changes of (a) XRD pattern and (b) FT-IR spectrum of Ara24h when heated to different temperatures.

No distinguishable changes in the XRD pattern are seen while it was heated from room temperature to 723 K, where all the diffraction peaks are attributed to aragonite (Figure 6a). The aragonite−calcite transformation occurs between 723 and 773 K. The IR absorption peaks attributed to the carbonate ion in the aragonite structure grow in the temperature range of the gradual dehydration from 600 K (Figure 6b), and the growth of aragonite crystallites accompanied by thermal dehydration is expected in this temperature range. The aragonite−calcite transformation is also confirmed by changes in the FT-IR spectrum between 723 and 803 K. The transformation temperature range observed by XRD and FT-IR corresponds with the rapid release process of water vapor; thus, the aragonite−calcite transformation and the rapid release of water vapor can be assumed to be simultaneous processes. Figure 7 shows typical SEM images of the Ara24h heated to different temperatures. No distinguishable differences in the morphology of the aragonite crystals can be seen between the samples at room temperature and those heated to 673 K (Figure 7a,b). A dumbbell shape with a cauliflower-like structure at each end starts to appear at 723 K (Figure 7c,d), and at 803 K, all of the crystals finely split along the direction of the column and form dumbbell shapes with a cauliflower-like structure at each end (Figure 7e,f). The temperature range for this dramatic change in crystal morphology corresponds with

Figure 5. (a) TG−DTA curves for Ara24h (m0 = 5.0 mg) at β = 5 K min−1 in flowing He and He−O2 (30% O2) and (b) mass chromatograms of m/z 18(H2O+) and m/z28(CO+) in flowing He (200 cm3 min−1). 2241



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aragonite−calcite transformation could be analyzed by focusing on the dehydration process. Figure 8 shows the TG and

Figure 8. TG−DTG curves for Ara24h (m0 = 10.0 mg) around the temperature range of the aragonite−calcite transformation recorded at different β in flowing N2 (80 cm3 min−1).

derivative TG (DTG) curves for Ara24h in the temperature range of the aragonite−calcite transformation, recorded at different heating rates β. Assuming that the gradual dehydration process starting at 500 K and the rapid dehydration process at around 750 K are independent kinetic processes, the overall thermal dehydration rate, (dα/dt), is the sum of the kinetic equations for the different processes.47,48 dα = dt

Figure 7. SEM images of Ara24h heated to different temperatures: (a) the original sample, (b) 673 K, (c, d) 723 K, and (e, f) 803 K.

N

⎛ Ea, i ⎞ ⎟ f (α ) ⎝ RT ⎠ i i

∑ ciAi exp⎜− i=1 N

N

with ∑ ci = 1 and

that of the aragonite−calcite transformation. Because of the growth mechanism of aragonite rod seen in Figure 2, we suppose that the splitting of the aragonite crystals during the aragonite−calcite transformation occurs along the twin boundaries with crystal defects and dislocations introduced during the aragonite crystal growth, triggered by mechanical stresses because of the phase transition. We also suppose that, because of the production of cauliflower-like structures, the aragonite−calcite transformation initiates at both ends of the aragonite rods. On the above assumptions, the rapid release of water vapor accompanying the aragonite−calcite transformation and the splitting of the crystals can be explained by the evolution of water vapor trapped in the twin boundaries of the aragonite rods. The complex thermal effects observed during the aragonite− calcite transformation appear to be resulting from the contributions of the CaCO3 structural phase transition, the evolution of water vapor by the thermal dehydration of included water, and, in an oxidative atmosphere, the oxidation of evolved CO in an oxidative atmosphere. The direct DSC measurement of the aragonite−calcite transformation enthalpy, ΔtransH, when gases are evolved is difficult because it is relatively small in comparison with the other simultaneous processes. The kinetic analysis of the aragonite−calcite transformation using DSC and DTA curves is also difficult for the same reason. Thermal Dehydration of Included Water. From the observations described above, the rapid release process of water vapor appears to be induced by the aragonite−calcite transformation and the splitting of the aragonite crystals. It was therefore expected that the kinetic behavior of the

i=1

∑ ciαi = α i=1

(1)

where αi, ci, Ai, and Ea,i are the fractional reaction, the mass-loss fraction, the pre-exponential factor, and the apparent activation energy for the reaction process i, respectively. An empirical Šesták−Berggren model function,49 SB(m, n, p), was used as the kinetic model function f i(αi) for the respective reaction processes because there is enough flexibility in the kinetic model function to fit the different reaction mechanism types and their deviations.50−53 SB(m , n , p): f (α) = α m(1 − α)n [−ln(1 − α)]p

(2)

The most appropriate parameters, ci, Ai, Ea,i, mi, ni, and pi, for the respective reaction steps were simultaneously optimized using nonlinear least-squares analysis to minimize the sum of the squares of residues when fitting the calculated curve, (dα/ dt)cal, versus time to the experimental curve, (dα/dt)exp, versus time. 2 ⎡⎛ ⎞ ⎛ dα ⎞ ⎤ dα ⎟ ⎢ ⎥ ⎜ ⎜ ⎟ − F=∑ ⎢⎝ dt ⎠exp , j ⎝ dt ⎠cal, j ⎥⎦ j=1 ⎣ M

(3)

A total of 12 parameters were optimized assuming that there were two independent thermal dehydration processes, i = 1 or 2. In this type of parameter optimization using nonlinear leastsquares analysis, it is necessary to set appropriate initial values for all the parameters to avoid obtaining apparent solutions for a local minimum F value. In this study, the default values for ci, Ai, and Ea,i were determined by empirical peak deconvolution of the derivative mass-loss curves using a statistical function and the subsequent formal kinetic analysis of the deconvoluted 2242



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curves.48,54−56 For the kinetic model function f i(αi), the first order reaction, that is, SB(0,1,0), was selected as the initial setting for both of the reaction steps. The results of the optimization run using eq 1 for the overall thermal dehydration process of Ara24h are shown in Figure 9.

Figure 9. Extraction of the kinetic rate data for the rapid mass-loss process accompanied with the aragonite−calcite transformation from the overall mass-loss data of the thermal dehydration. (a) A typical result of kinetic deconvolution for the kinetic rate data at β = 10 K min−1 and (b) extracted kinetic rate data for the rapid mass-loss process, i = 2, at different β.

Figure 10. Kinetic results for the extracted kinetic rate data for the rapid mass-loss process accompanying the aragonite−calcite transformation. (a) Friedman plots at different α2 from 0.1 to 0.9 in steps of 0.1, (b) Ea,2 values at different α2, and (c) the experimental master plot and the curves fitted using the SB(m, n, p) and JMA(m) models.

⎛ Ea,2 ⎞ ⎛ dα ⎞ dα 2 = ⎜ 2 ⎟ exp⎜ ⎟ ⎝ dt ⎠ ⎝ RT ⎠ dθ2

The simulated reaction rate curve is a nearly perfect fit for the experimental curve (Figure 9a). The average kinetic parameters optimized for the rapid water vapor release process at different β were Ea,2 = 226.7 ± 7.7 kJ mol−1, A2 = (4.36 ± 3.52) × 1013 s−1, and f 2(α2) = SB(0.34, 1.22, 0.24). The kinetic rate data extracted for the rapid mass-loss step systematically shift to higher temperature with increasing β (Figure 9b). The Ea,2 values at different α2 were recalculated by the Friedman method57 using kinetic rate data for the rapid mass-loss step during the aragonite−calcite transformation at different β. Ea,2 ⎛ dα ⎞ ln⎜ 2 ⎟ = ln[A 2 f2 (α2)] − ⎝ dt ⎠ RT

Figure 10c shows the experimental master plot (dα2/dθ2) vs α2, which can be related to the A2 value and f 2(α2) using eq 7.60−62 dα 2 = A 2 f2 (α2) dθ2

(4)

Figure 10a shows the plots of ln(dα2/dt) versus T at various α2. The Friedman plots indicate the linear correlations regardless of the selected α2. The Ea,2 values at different α2, calculated from the slope of the Friedman plot, are shown in Figure 10b. The Ea,2 values are practically constant over a wide α2 range, 0.1 ≤ α2 ≤ 0.9, with an average value of 249.5 ± 5.5 kJ mol−1. This value is close to that reported by Peric et al. for a mineral aragonite, 234.5 ± 5.6 kJ mol−1,33 and for synthetic aragonite, approximately 270 and 290 kJ mol−1.19 Knowing the Ea value for the process, an experimental master plot is drawn by introducing Ozawa’s generalized time θ.58,59

∫0

t

⎛ E ⎞ exp⎜ − a ⎟dt ⎝ RT ⎠

(7)

Because the experimental master plot shown in Figure 10c indicates the characteristic shape of nucleation and growth type transformation with the peak maximum at α2 = 0.31, one of the most widely used kinetic model functions for the nucleation and growth type transformation known as the Johnson−Mehl− Avrami−Erofeyev−Kolgomorov model,63−66 JMA(m), with the kinetic exponent m was used for analyzing the experimental master plot.

−1

θ=

(6)

JMA(m): f (α) = m(1 − α)[− ln(1 − α)]1 − 1/ m

(8)

The best m and A2 values were simultaneously optimized through nonlinear regression analysis using the Levenberg− Marquardt optimization algorithm. The experimental master plot was fitted best (R2 = 0.9722) by f 2(α2) = JMA(1.46 ± 0.02) and A2 = (4.63 ± 0.04) × 1014 s−1. The JMA(1.46) curve describes the overall behavior of the transformation to an acceptable degree compared with the curve drawn using the empirical model function, f 2(α2) = SB(0.51, 1.17, 0.01) and A2 = (1.01 ± 0.01) × 1015 s−1. Lech and Slezak21 reported that the kinetic exponents in the JMA model were m = 1.6 and 2.0 for the aragonite−calcite transformation of two different mineral aragonite samples. Although one of those m values agrees with our results, it is

(5)

The experimental master plot of (dα2/dθ2) against α2 is drawn by calculating the (dα2/dθ2) value at different α2 according to60−62 2243



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clear that the m value is different for different samples. The JMA model is theoretically derived from nucleation kinetics, linear growth kinetics, and growth dimension.63−66 The m value for the aragonite−calcite transformation is, therefore, influenced by both the crystal morphology and crystal defects and dislocations in the aragonite sample. The kinetic exponent in the JMA model found in this study, m ≈ 1.5, can be interpreted as indicating random nucleation and subsequent one-dimensional growth controlled by diffusion. As was confirmed by the SEM observations, the appearance of the dumbbell-like calcite crystals is a kinetic event that occurs over a specific temperature range. The contribution of the nucleation process to the overall kinetic behavior is, therefore, expected from both the morphological observations and the kinetic analysis of the dehydration of the included water. In addition, the onedimensional advancement of the reaction front is explained by initial nucleation at both ends of the pseudohexagonal column and the crystal splitting along the column direction toward the middle of the crystal (deduced from the morphological changes during the transformation), where the twin boundaries generated during the aragonite crystal growth plays a predominant role in determining the nucleation sites and the direction of the reaction advancement. It seems that the atomic displacements during the aragonite−calcite transformation and the removal of included water are controlled by diffusion kinetics. Physico−Geometrical Relationship. The observations described above clearly indicate that the aragonite−calcite transformation is a physico−geometrical process that depends on the crystallographic and morphological characteristics of the aragonite sample. The geometry of the splitting of the crystals during the aragonite−calcite transformation and the accompanied evolution of the included water, and their kinetics, are interpreted as being closely related to the aragonite crystal growth mechanism and the generation of twin boundaries with crystal defects and dislocations. The physico−geometrical relationships between the aragonite crystal growth in the hydrothermal decomposition of urea and the thermally induced aragonite−calcite transformation of the as-prepared aragonite that can be expected for the aragonite samples synthesized in this study are described below. During the growth of aragonite crystals, it is probable that poorly crystalline calcium carbonate forms on the surface of the needle-like aragonite crystals that are initially formed and mediates the subsequent growth to form pseudohexagonal columnar aragonite through the crystal twinning of needle-like crystals and outward growth. The hydrated calcium carbonate at the twin boundaries crystallizes to form aragonite as the crystal growth advances, but some water molecules are trapped at the twin boundary as included water. The amount of included water decreases as the crystal growth advances. Heating the as-prepared pseudohexagonal columnar aragonite in a dry atmosphere leads to the included water trapped in the twin boundaries being evolved at approximately 500 K. The twin boundaries that are exposed to the aragonite crystal surfaces are possible sites for the early stage thermal dehydration. Both ends of the pseudohexagonal columnar crystal are the most reactive faces because they have differently orientated twin boundaries. The evolution of water vapor indicates the dynamic action of water molecules in the crystal lattice and the lattice deformation by the loss of the water molecules and other incorporated minor chemical species such as CO. Calcite nucleation randomly occurs at both ends of the

columnar crystals affected by the dynamic lattice changes at the twin boundaries. The calcite phase propagates one dimensionally toward the center of the column along the twin boundaries. The pseudohexagonal columnar aragonite crystals split into smaller columnar crystals of calcite from both ends of the aragonite crystal, and the calcite crystals “bloom,” forming a cauliflower-like structure. Crystal splitting along the twin boundaries enhances the evolution of water vapor trapped at the twin boundaries, and, therefore, the kinetic behavior of the rapid water vapor release process during the aragonite−calcite transformation is described by the physico−geometrical mechanism in accordance with the morphological changes.

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CONCLUSIONS The physico−geometrical characteristics of the thermally induced aragonite−calcite transformation are closely related to the crystal growth mechanism of the particular aragonite sample and its crystallographic characteristics (crystal defects and dislocations). The phenomenological findings, revealed by the morphological studies of aragonite crystal growth and the thermally induced aragonite−calcite transformation and the kinetic study of the thermal dehydration of the included water in the aragonite crystals, provide an insight into the dynamic processes involved in the polymorphous transformation and indicate the possibility to control the morphology and functional characteristics of calcite via the controlled mineralization of metastable calcium carbonate polymorphs and thermally induced polymorph transitions.

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AUTHOR INFORMATION

Corresponding Author

*Tel/fax: +81-82-424-7092. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work was partially supported by a grant-in-aid for scientific research (A) (25242015), (B) (22300272), and (C) (25350202) from the Japan Society for the Promotion of Science.

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