Phase transitions of natural corals monitored by ESR spectroscopy

NIM B Beam Interactions with Materials & Atoms

Nuclear Instruments and Methods in Physics Research B 243 (2006) 167–173 www.elsevier.com/locate/nimb

Phase transitions of natural corals monitored by ESR spectroscopy V. Vongsavat, P. Winotai, S. Meejoo

*

Department of Chemistry, Faculty of Science, Mahidol University, Rama VI Road, Rajathevi, Bangkok 10400, Thailand Received 25 February 2005; received in revised form 30 June 2005 Available online 25 August 2005

Abstract The main purpose of this work is to present a systematic study of structure of marine exoskeletons, Acropora coral and its structural transformation upon heat treatments. The coralline sample was ground and characterized as powder throughout this work. Structural identifications of all samples have been confirmed using X-ray diffraction and IR spectroscopy. It was clearly found that the fresh specimen is made of aragonite, a common phase of the mineral CaCO3. Thermal analyses, DSC and TGA were used to monitor structural and thermal decompositions and an irreversible solid-state phase transition from aragonite to calcite of the marine carbonate. Next, the coral powder was annealed at specific temperatures over the range 350–900 °C, and the effects of heat treatment on the structure of coralline samples were carefully studied by Rietveld refinement method. In addition, we have examined Mn2+ paramagnetic ions and free radicals present in the coral and changes of those upon heating by using ESR spectroscopy. The local environments of Mn2+ ions were verified from the calculated ESR spectra using appropriate spin Hamiltonian parameters, i.e. gyromagnetic tensor g, zero-field splitting D and hyperfine tensor A. This work reported structures and compositions as well as physical, chemical and thermal properties of the coralline material upon heat treatments qualitatively and quantitatively. Ó 2005 Elsevier B.V. All rights reserved. PACS: 61.50.Ks; 64.70.Kb; 87.64.Hd; 87.64.Je Keywords: Aragonite; Calcite; Coral; XRD; Rietveld refinement; IR; Thermal analyses and ESR

1. Introduction Several hundred different corals are produced naturally in seawater on Earth and they are one of the most diversity of sea animals in the world. By comparing the variety of marine exoskeletons with respect to recent research work, there have been very few publications. As reported earlier elsewhere [1,2], marine skeletons may be composed of aragonite or calcite or a mixture of both structures. Aragonite with an orthorhombic structure is less common and less stable than calcite, while calcite with trigonal symmetry is the most thermodynamically stable form of pure CaCO3

*

Corresponding author. Tel.: +66 22015164; fax: +66 23547151. E-mail address: [email protected] (S. Meejoo).

0168-583X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.07.197

at room temperature and atmospheric pressure [3]. However, aragonite still exists since the metamorphosis aragonite to calcite is kinetically unfavorable. Besides these two forms of CaCO3, there is another structural form so called vaterite, which is reported to be a metastable phase found in an extreme environment [4]. Furthermore, the exoskeletons of marine animals may contain various amounts of trace metals. However, manganese has been of a great interest constituent even though it is often found in the range of 1–3000 ppm [5]. This is because the isomorphism of MnCO3 and one form of CaCO3 leads to easy substitutions of Mn2+ for Ca2+, without significant crystallographic distortion [6]. In addition, since the ionic radius ˚ ) is very close to that of Ca2+ (1.00 A ˚ ), of Mn2+ (0.99 A Mn deposition on the calcified skeleton is plausible. There may be a variation of radical species according to ions

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present in the marine exoskeleton, such as CO
2 , CO3 , SO2
and SO3 [7]. Such radicals and other defects are mainly produced by natural radiations. Electron spin resonance (ESR) spectroscopy is a very suitable tool to study paramagnetic ions and free radicals. It has been reported [8] that heat treatment procedures often enhance ESR signals. Thus, effects of thermal treatments on the structure of coral must be clarified. Since radicals, trace elements and impurities in the skeletons directly depend on their environmental habitats. Consequently, it has been found that they can be used as a biomonitor of heavy metal pollution in coastal areas [9]. Corals have been mostly found as decorations or jewelry products. However, it has been also reported elsewhere that corals are used sometimes as part of tradition medicine and cosmetic [10]. Most recently, there are reports that corals can be used to prepare biomaterials for efficient bone implants [11]. These involve using coral as raw material concerning human health; therefore a careful study should be made in order to acquire the property of this material as much as possible. In our work, the coral specimens of Acropora genus were collected from a very clean eastern coast of our country so that our results should be considered as from non-polluted corals.

2. Materials and methods The tree coral, Acropora was obtained from the eastern coastal area near the Gulf of Thailand. The marine specimens were cleaned by soaking in dilute NaOH solution for several hours, followed by washing thoroughly with distilled water. Such procedures were needed to get rid of macroscopic dirt, including stains and other impurities attached on their surface. The cleaned sample was ground into fine powder and denoted as AC then used for characterizations. Next, seven powder coral samples were separately sintered at specific temperatures between 300 and 900 °C in a Lenton model EHF 18/5, furnace. The heating rate was limited to 4 °C/min until it reached the required temperatures and maintained at those temperatures for 2 h and then cooled down at 5 °C/min back to room temperature. The annealed samples are denoted as AC-300, AC-350, AC-450, AC-500, AC-550 and AC-900 where the labeling numbers indicate the annealing temperatures. Powder diffraction data were recorded at room temperature using a Bruker AXS D8 Advance powder diffractometer [CuKa (Ni-filtered) with scintillation detector; 2h range, 20–70°; step size 0.0194°; data collection time 12 h]. Changes of crystal structure and unit cell parameters upon heat treatment were also determined by Rietveld refinements [12], which were carried out by using the GSAS program [13]. High resolution IR spectra were obtained on a Perkin Elmer model PE 2000 FT-IR spectrometer in the wave number region 400–4000 cm
1. All IR measurements were carried out at room temperature using the KBr pellet technique.

Thermogravimetric analyses (TGA) were carried out in the temperature range 25–1200 °C at heating rate 10 °C/ min on a TA instruments TA-SDT 2960 instrument under flowing nitrogen. The differential scanning calorimetry (DSC) was performed in the temperature range 25– 1200 °C with a heating rate of 10 °C/min on a Perkin Elmer DSC Series-7 instrument under flowing nitrogen gas. All ESR spectra of the powder samples were measured on a JEOL ESR spectrometer operating at X-band microwave frequency using 0.5 mT field (100 KHz) modulation amplitude with a time constant of 0.03 s. In order to study local environments of Mn2+ ions in the exoskeletons, theoretical resonance peaks are evaluated and compared with each of those in the experimental spectra by using the Math-Lab and Easyspins programs [14]. 3. Results and discussion 3.1. X-ray diffraction The powder X-ray diffraction pattern of AC in Fig. 1 clearly shows that the fresh coral specimen was made of a pure aragonite phase, a common form of CaCO3 mineral. The diffractograms indicate that aragonite transforms to calcite to some extent upon heating the coral sample at 300, 350 and 400 °C. We can conclude that AC-300 and AC-350 are composed of aragonite with a small fraction of calcite whereas AC-400 is clearly a mixture of calcite and aragonite. Furthermore, AC-450, AC-500 and AC550 are of a pure calcite phase only, suggesting that the phase transition is complete. By sintering the coral sample at 900 °C, the fluffy, very white powder, pure phase of CaO

Fig. 1. Powder XRD patterns of coral samples annealed at specific temperatures.

V. Vongsavat et al. / Nucl. Instr. and Meth. in Phys. Res. B 243 (2006) 167–173

was formed, which resulted from the well known decomposition of carbonate at high temperature. This was further confirmed by XRD pattern. In our study, the Rietveld refinement was used as the simplest tool to verify exact structure of CaCO3 and to quantify the phase transformation from aragonite to calcite. Using this method, we analyzed the structural compositions of coral samples qualitatively and quantitatively by fitting the experimental powder XRD profiles with respect to corresponding structural parameters (i.e. lattice parameters, atomic coordinates) and instrumental parameters (i.e. zero-point and profile parameters). By taking the structure information established from the single-crystal X-ray diffraction [15,16], the crystal structure of aragonite is orthorhombic with space group Pmcm whereas that of calcite is trigonal with space group R 3c respectively. In addition, the Rietveld refinement on AC-900 sample was also carried out, while the crystal structure of the CaO lime is cubic with space group Fm3m. A summary of the refined lattice parameters and phase fractions of the coral sample upon heat treatment are reported in Table 1. Fig. 2 illustrates the final Rietveld refinement of powder data for the fresh coral sample. As seen in Table 1, it should be pointed out that the refinement shows that the structural

transformation aragonite to calcite occurred approximately 50%, 75% and eventually 100% in the AC-350, AC-400 and AC-450 samples respectively. 3.2. IR spectroscopy and thermal analyses By analyzing IR spectra, it was found that, in spite of identical chemical formula, aragonite polymorph gives specific IR frequencies different from those of calcite. Fig. 3 displays IR spectra of the AC, AC-350 and AC-450 coral samples which represent three observed cases; pure aragonite (a), mixture of calcite and aragonite (b), and pure calcite (c) respectively. The IR signal positions and their assignments for CO2
ions of aragonite and calcite are 3 given in Table 2. In Fig. 3(a), IR signals at about 1083 cm
1 (t1) and 857 cm
1 (t2) are specific and can be observed only for aragonite, as they are infrared inactive for carbonate ions in calcite structure. The broadband center at 1498 cm
1 (t3) is assigned to asymmetric stretching mode of CO2
3 , whereas the absorption at about 713 cm
1 (t4) is of in-plane bending mode which due to a change in local symmetry of the carbonate ions. At above transition temperature (aragonite ! calcite), the molecular vibration of CO2
ions gives frequency shift to about 3

Table 1 Relevant crystallographic data for coral samples, established from powder X-ray diffraction refinements ˚) Sample Phase Phase fraction Cell parameters (A

AC AC-350 AC-400 AC-450 AC-550 AC-900

Aragonite Aragonite Calcite Aragonite Calcite Calcite Calcite Lime

1.00 0.44 0.56 0.24 0.76 1.00 1.00 1.00

169

˚ 3) V (A

a

b

c

4.963(1) 4.977(4) 4.951(3) 4.937(2) 4.978(5) 4.993(1) 4.938(8) 4.799(1)

7.970(7) 7.942(6) 4.972(6) 7.939(3) 4.978(5) 4.993(1) 4.938(8) 4.799(1)

5.748(5) 5.728(7) 16.856(2) 5.720(8) 17.061(8) 17.102(2) 16.909(3) 4.799(1)

227.4 366.0 226.1 224.2 366.2 369.2 357.2 110.5

Fig. 2. Experimental (+ marks), calculated (solid line) and difference (bottom) powder diffraction profiles for the Rietveld refinement of fresh coral sample (denoted AC). Reflection positions are marked. The refinement had converged to final residual factors of Rwp = 9.3%, Rp = 6.75% and v2 = 0.294.

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Fig. 3. IR spectra of coralline specimens indicating: (a) pure aragonite, (b) coexistence of calcite and aragonite and (c) pure calcite. Note that a = aragonite and c = calcite.

Table 2 IR band positions and assignment for coralline carbonate Crystal structure

Aragonite Calcite a

Frequency (cm
1) t1

t2

t3a

t4

1082.9 –

856.8 874.8

1400–1500 1400–1500

712.8, 699.6 712.6

Indicates very broad band.

875 cm
1 as shown in Fig. 3(c). The shift corresponds to the change of CO2
structure coordinations, leading the 3 change of carbonate from orthorhombic to trigonal symmetry. It should be noted here that at a moderate sintering temperature (ca. 350 °C), the transformation has not yet completed, yielding a more complex IR spectrum corresponding to the mixture of two CaCO3 polymorphs as displayed in Fig. 3(b). Besides the combination frequencies between 1780–1800 cm
1 (t1 + t4) and 2510–2520 cm
1 (t1 + t3), the IR band at 3373 cm
1 corresponded to the presence of water was observed only for the fresh coral as shown Fig. 3(a). More importantly, we have observed extra IR frequencies corresponding to another chemical species in addition to carbonate ions. The IR band observed at about 2918 cm
1 is attributed to C–H stretching mode. Since we do not observe any evidence to confirm the presence of hydrocarbon in the coral, we purposed that this IR signal may arise from a chemical having formate type structure. The formate ion was generally reported as

a product of thermal reactions on surfaces containing C=O chemical species where protons are available [17]. The thermal process in the temperature range 25– 1200 °C was also investigated by means of differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Fig. 4 shows that the coral is stable up to 300 °C and, then, decomposes in two steps. Firstly, the 1.52% mass loss and endothermic behavior were observed within the temperature range from 300 to 340 °C. This could result from the removal of the water molecules from carbonate lattice as well as the phase transformation from aragonite to calcite. It has to be noted that diffraction data give an

Fig. 4. DSC (solid line) and TGA (dash line) plots of coral sample.

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evidence of the phase transformation in good agreement with the temperature range observed by thermal analyses. The transition temperature in this work is also consistent with that reported previously [18] for that the temperature of aragonite–calcite phase transition in coral is much lower than that of the phase transition in the aragonite of mineral origin. Secondly, the mass loss of 41.80% and the endothermic behaviour from 600 °C to 800 °C indicate a significant decomposition of CaCO3 to CaO. Then, the sample remains unchanged up to the measurement limit. It should be pointed out here that the percentage of mass loss estimated for the releasing of CO2 gas upon carbonate decomposition is in good agreement with TGA result as well as the XRD data discussed earlier. 3.3. ESR spectroscopy The ESR spectrum of the fresh coral powder (AC) is shown in Fig. 5(a). The spectrum exhibits broad resonance peaks corresponding to a trace of the Mn2+ ions present in the carbonate of aragonite structure and a single sharp peak at g = 1.988, line width DH = 1.20 mT. The sharp signal at g  2 is a typical resonance peak attributed to free radicals [19], which often exist in natural carbonates especially of marine origin. The possible radical species can be carbonate, formate and other organic matrices or those trapped from the metal coordination and from water adsorbed within the lattice. The ESR signal of free radicals is very intense such that it obscures resonance peaks corresponding to Mn2+ paramagnetic ions. Furthermore, the fresh coral possibly contains more Mn3+ than Mn2+ ions. The Mn3+ ions (S = 0), are ESR inactive, therefore yield no ESR signals. The ESR spectra of the coralline samples annealed in the temperature range of 350–550 and 900 °C are shown in Figs. 5 and 6. The ESR spectra of annealed samples at the temperature less than 900 °C in Figs. 5(b)–(d) and 6(a) and (b) clearly confirmed the Mn2+ ion substitution at the calcium sites of carbonate structure. Each spectrum consists mainly of a group of six peaks, which attributed to j
1/2i M j + 1/2i sextet fine structure transition (electron spin S = 5/2 and nuclear spin I = 5/2) of Mn2+ ions. The weaker pairs of peaks between the main peaks are so-called the forbidden transitions in which both electron and nuclear spin state change, i.e. DmS = 1, DmI = 1. As shown in Figs. 5(b)–(d) and 6(a) and (b), each Mn2+ peak splits into a doublet, which results from the formation of calcite. There are two magnetically inequivalent Mn2+ paramagnetic centers due to two different Ca2+ sites occupied by Mn2+ ions in trigonal symmetry, arising from the distortion of oxygen octahedrons of nearest neighbors. However, it was found that, after heat treatment at 900 °C, the sixdoublet peaks form of Mn2+ ions becomes the six-singlet peaks form, as shown Fig. 6(c), because the coral sample is no longer calcite, but the cubic structure of CaO [20]. Similarly, a single peak observed at the center of each spectrum, Figs. 5(a)–(d) and 6(a) and (b), also corresponds to free radicals existed in the samples over the annealing tem-

Fig. 5. X-band ESR spectra of: (a) fresh coral and coral sample annealed at (b) 350 °C, (c) 400 °C and (d) 450 °C.

perature range 350–550 °C. Note that IR spectra have shown that the annealed coral samples have no water embedded within; therefore the contribution of radicals from water or OH group is not the case. The intensity of the central signal increased with increasing the sintering temperature from 350 to 550 °C. On the other hand, AC900 has no radical left, Fig. 6(c), because carbonate decomposed at 900 °C, releasing CO2 and eliminating all radicals including those of carbonate and formate ions. The observed ESR spectrum of Mn2+ ions in CaCO3 can b of the form [21], be explained by the spin Hamiltonian H b ¼ bS  g  H þ S  D  S þ S  A  I; H

ð1Þ

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V. Vongsavat et al. / Nucl. Instr. and Meth. in Phys. Res. B 243 (2006) 167–173 Table 3 Spin Hamiltonian parameters of Mn2+ ions used to simulate ESR spectra corresponding to calcite and CaO in coral samples Phase

T (°C)

Spin Hamiltonian parameters g

CaCO3 (calcite)

A (MHz) D (MHz) E (MHz)

350–550 gx = 1.990 Ax = 273 gy = 1.990 Ay = 297 270 gz = 1.995 Az = 278

CaO

Line width (mT)

900

gx = 1.997 Ax = 273 gy = 1.997 Ay = 273 70 gz = 1.997 Az = 273

70

0.4

20

1.1

Fig. 6. X-band ESR spectra of coral sample annealed at: (a) 500 °C, (b) 550 °C and (c) 900 °C.

where the three terms represent the electron-Zeeman effect, zero-field contribution hyperfine interaction and the core polarization respectively. The spin Hamiltonian parameters of paramagnetic Mn2+ ions present in the carbonate structure can be evaluated by fitting of theoretically line positions to those of the measured spectra. Simulated ESR powder spectra for Mn2+ in the coral samples before and after heat treatments are calculated by using the parameter sets in Table 3. Note that the spin Hamiltonian parameters are g-tensor; hyperfine coupling constant A; the common parameters D and E are related to zero fields splitting parameter (D). The relations between to zero fields splitting parameter (D) and D, E are shown in Eq. (2). 0 1 Dxx 0 0 D 1B 3 Dxx
Dyy C ¼ @ 0 Dyy 0 A; D ¼ Dzz ; E ¼ : h h 2 2 0 0 Dzz ð2Þ Fig. 7 illustrates the comparison between experimental and simulated spectra for the coral samples annealed at 400 °C.

Fig. 7. Experimental and simulated ESR spectra corresponding to the coral sample annealed at 400 °C.

4. Conclusions The fresh Acropora coral consists mainly of CaCO3, aragonite structure, crystal water, free radicals, Mn2+ and possibly Mn3+ ions in the lattice. The heat treatment applied on the sample has induced the solid-state phase transformation aragonite to calcite. The transformation of this coralline aragonite begins at approximately 300 °C and is completed at about 450 °C. The phase transformation occurs as well as dehydration process. After the heat treatment at 300 °C, a minor phase of calcite formed and this irreversible phase transformation of aragonite to calcite is expedited by increasing the annealing temperature. Rietveld analysis

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allows us to evaluate the percentage aragonite transformation to calcite at different annealing temperatures. The ESR spectra indicate that Mn2+ replaced some Ca2+ ions within calcite structure resulting in 2 sextets due to site symmetry. Free radicals related to carbonate and formate ions were substantially accumulated with increasing annealing temperatures over the range 350–550 °C, where decomposition processes occurred. After annealing at 900 °C, the coral sample is no longer calcium carbonate but a stable form of free-of-radical calcium oxide. The simulation of ESR spectra for the annealed samples has ensured the changes of local environment of Mn2+ ions and the stability of radical species present in the coral. Acknowledgements All authors wish to thank the Postgraduate Education and Research Program in Chemistry (PERCH-ADB) for financial support to make this research possible. S.M. is thankful to Thailand Research Fund (Grant #MRG4780077) for their funding. References [1] K.M. Wilbur, in: K.M. Wilber, C.M. Yong (Eds.), Physiology of Mollusca, vol. 2, Academic Press, NY, USA, 1964, p. 243. [2] V. Nothig-Laslo, L. Brecevic, Phys. Chem. Chem. Phys 15 (1999) 3697.

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