CRYSTAL GROWTH & DESIGN
Special Vaterite Found in Freshwater Lackluster Pearls
2007 VOL. 7, NO. 2 275-279
Li Qiao, Qing-Ling Feng,* and Zhuo Li Laboratory of AdVanced Materials, Department of Materials Science and Engineering, Tsinghua UniVersity, Beijing 100084, People’s Republic of China ReceiVed May 26, 2006; ReVised Manuscript ReceiVed NoVember 21, 2006
ABSTRACT: In recent years, scientists have special interests in biovaterite found in freshwater cultured lackluster pearls in south China. Vaterite is an unstable crystalline form of calcium carbonate and rare in nature. In this work, we first reported that vaterite crystals in freshwater lackluster pearls have a similar hiberarchy to that of nacre with the “brick and mortar” structure. The dimension of a single vaterite tablet is approximate 8 × 2 × 0.4 µm3. In half-lackluster pearls, the aragonite-vaterite interface is abrupt, and the boundary is sequential; two different kinds of contact modes between aragonite and vaterite tablets were observed. Furthermore, crystallographic orientation regulation of vaterite tablets was analyzed for the first time. The results show that vaterite tablets are arrayed with the [010] direction perpendicular to the tablet plane; strong [010] texture and relatively weak [101] and [102] texture were obtained in a region from several neighboring tablets to micrometer scale. In addition, the misorientation angles between adjacent tablets mainly concentrate on small angles lower than 10° and have another gather in 60°. 1. Introduction In south China, freshwater cultured pearls are generally gestated in Hyriopsis cumingii Lea. Vaterite is found in these freshwater cultured lackluster pearls.1 This kind of vaterite generally coexists in a half-lackluster pearl with aragonite, the main component of the lustrous part of the pearl, or presents individually in a complete lackluster pearl. As we know, calcium carbonate has three major modifications, calcite, aragonite, and vaterite, of which vaterite acts as a precursor in the formation of aragonite or calcite and is a very unstable phase of calcium carbonate rarely occurring in nature. Biovaterite was first reported to occur in the repair tissues of shells of certain gastropods.2 In biological systems, vaterite exists in a variety of organisms controlled by gene or environment such as gallstone in humans3 or otolith in diversified freshwater fish.4,5 But vaterite synthesized normally under control of a gene in nature just exists in two organisms: a member of the class Ascidiacea, a marine creature,6 and the snail, Pomacea paludosa.7 Different from normal biomineralization, it is difficult to obtain simple and pure vaterite phase in abnormal mineralization, often presented with calcite or aragonite. Although vaterite in lackluster pearl is deposited abnormally under the control of environment, this deposit is often pure, and its dimension can extend to the macroscopic scale. Its aggregation is also one of the biggest minerals precipitated with vaterite completely in a biological system. Normal pearl is formed by nacre, which has attracted much attention because of its complex architectures,8 superior mechanism properties,9,10 and applications in materials design.11,12 Nacre inspires bright luster due to its regular structure layers of uniformly thick tablets of aragonite and high mechanical performance thanks to the organic matrix lying between neighboring tablets and lamellae to form a “brick and mortar” structure. Much research has revealed that the aragonite tablets in nacre have a strong texture basically with their c-axis perpendicular to the tablet plane,13 and mineral bridges keep neighboring crystals maintaining the same crystal orientation in three dimensions as a domain structure.14,15 In recent years, some research results seem to be different from classical * Corresponding author. Tel: +86-10-62782770. Fax: +86-10-62771160. E-mail address:
[email protected].
theories, for instance, independint nucleation sites for each crystal tablet, nanostructure in single aragonite tablets,17 and a continuous layer of disordered amorphous CaCO3 covering aragonite platelets.18 Therefore, understanding how nacre is formed has represented a major challenge in the field of nacre biomineralization. The existence of the vaterite is one of the key factors influencing the quality of freshwater cultured pearls. Compared with a number of elegant studies on the calcite-aragonite switch, little is known about the aragonite-vaterite interface in biomineralization. In this paper, we studied the phase, morphology, structure, and orientation of vaterite crystals deposited in lackluster pearls, in order to provide evidence for the study of rare biovaterite and control the quality of freshwater pearls. To the best of our knowledge, this is the first observation about the hiberarchy, the aragonite-vaterite interface, and the crystal orientation regulation of vaterite tablets in lackluster pearls. 2. Experimental Section Materials. Pearls gestated in Hyriopsis cumingii Lea were collected from Zhuji, south China. The lackluster and half-lackluster pearls were prepared for the experiment. Methods. X-ray Diffraction Analysis (XRD). Powder X-ray diffractometry of lackluster and half-lackluster pearls was carried out using a Rigaku X-ray diffractometer at 40 kV with a copper anticathode. Rigaku R-AXIS SPIDER diffractometer was used for tiny area analysis of the pearl surface at 18 kV with a molybdenum anticathode. The minimum analysis area of R-AXIS SPIDER is 100 × 100 µm2; here we selected the research area of 800 × 800 µm2. Scanning Electron Microscopy (SEM). We selected fragments of lackluster and half-lackluster pearls to observe the morphology of the surface and section parts of pearls. Fragments selected were cleaned in 5% NaOH solution for 10 min to eliminate the organic proportion. Then samples were etched in 10 wt % sodium ethylenediaminetetraacetic acid (EDTA-2Na) solution for 30 s. We washed the etched surfaces in deionized water and dried them in air. Samples with carbon coating were observed in a JEOL-1530 SEM at 10 kV. Transmission Electron Microscopy (TEM). A sample was obtained from the lackluster pearl by mechanical grinding parallel to the surface. Then it was milled by the double-side ion millling technique with a Gatan-600 miller. TEM observations together with selected area electron diffraction (SAED) were performed using a JEOL-2011 electron microscope at 200 kV.
10.1021/cg060309f CCC: $37.00 © 2007 American Chemical Society Published on Web 01/10/2007
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Figure 1. Powder XRD pattern of a lackluster pearl. The figure shows that the only phase is vaterite.
Electron Backscatter Diffraction (EBSD). The EBSD measurement was made using a TESCAN-5136XM SEM with an electron backscattered diffraction analysis system working at 20 kV. For the misorientation determination, an area of up to 50 × 30 µm2 was selected using a step size of 0.3 µm. The result was analyzed by the Channel 5 commercial software of HKL Technology.
3. Results and Discussion Phase Analysis. Figure 1 shows XRD pattern of lackluster pearl powder, which is very similar to the patterns of vaterite samples referring to the standard PDF card 72-0506, without any peak of other calcium carbonate polymorphs. The only mineral phase in a complete lackluster pearl is vaterite. Powder XRD patterns of the half-lackluster pearls revealed that they are composed of both aragonite and vaterite. Morphology. Figure 2 shows the microarchitecture of lackluster pearls after 10 wt % EDTA-2Na treatment, which reveals that the deposition of vaterite is in the form of a “brick
Qiao et al.
and mortar” hiberarchy similar to nacre. Figure 2a shows that the vaterite tablet is an elongated rectangle with dimensions of approximately 8 × 2 µm2; they align tightly with parallel lengths in two dimensions and assemble layer by layer. The side elevation of vaterite tablets in Figure 2b shows that the vaterite layer is a lamella of 0.4-µm-thick tablets, thinner and more irregular than nacre, which may result in the lackluster characteristic in the pearls. Figure 2c shows that another section showing width and thickness is a rhombus. Interestingly, the crystals between successive sheets appear to nucleate close to the center of pre-existing tablets located in the layer below, as shown in Figure 2c, which is a remarkable “stack-of-coins” structure of gastropod shells and quite different from that observed in nacre. Here we give a three-dimensional structure sketch of both vateite tablets and layers in lackluster pearls, as shown in Figure 2d. Planes x, y, and z stand for the observed surfaces in Figure 2a,b,c, respectively. In the next three paragraphs, we discussed the possible formation mechanism of tier 1, tier 2, and tier 3 of the hierarchy. It is well-known that protein can modulate calcium carbonates not only on polymorphs but also on morphology. The battenlike vaterite tablet found in lackluster pearls is a new kind of morphology, the reason for which may be considered to contribute to changeable face-specific growth rates under biological control. Protein modifies the surface energies via preferential adsorption on crystal surfaces; the final form of crystals would show the slow-growing surfaces. On the other hand, another function of a matrix is a structurematching template. The matrix acts as an organic template to only align crystals perpendicular to the matrix or array crystal iso-oriented with three-dimension crystallographic alignment.9 Due to the uniform arrangement in two dimensions, vaterite tablets should be an iso-oriented arrayed. Therefore, the structure of vaterite crystals in lackluster pearls performs a superior biomineralization process and may undergo a biologic control.
Figure 2. SEM images of vaterite tablets and layers in lackluster pearls after 10 wt % EDTA-2Na treatment: (a) etched surface of lackluster pearl (scale of amplificatory image, 1 µm); (b) etched section of length by thickness of vaterite tablets; (c) etched section of width by thickness of vaterite tablets; (d) three-dimensional structure sketch of vateite crystal in lackluster pearls.
Special Vaterite in Freshwater Lackluster Pearls
This “brick and mortar” structure reveals that the growth mechanism of vaterite crystals in lackluster pearls is the same as that of nacre. About the growth mechanism of nacre, mineral bridges for the gastropod shell and template theory for the bivalve shell are the most important hypotheses to explain “brick and mortar” structure and uniform crystal orientation in nacre. For nacre of bivalve shells and pearl, each successive layer of crystals is offset, and the lateral growth of the aragonite tablets occurs to a much greater extent before the next sheet is added. At present, nanostructured and amorphous CaCO3 are believed to play an important role in the forming of nacre,17,18 which seems to contradict the epitaxial match between the structural organic matrix and the formed mineral. The growth mechanism of vaterite crystals in lackluster pearls is in demand for further study. Aragonite-Vaterite Interface. In half-lackluster pearls, aragonite and vaterite regions in one pearl are separated, each of which emerges with freedom; we cannot describe their sites and dimensions exactly. Figure 3a shows the aragonite-vaterite interface in half-lackluster pearls. Two kinds of contact modes between aragonite and vaterite tablets were found: one is sideside contact, in which vaterite or aragonite tablets can be formed in one layer and two different kinds tablets developed separately and stopped growing as soon as meeting each other, as shown in Figure 3b; another is front-back contact, in which neonatal aragonite layers were deposited on formed vaterite layers, or contrarily, as shown in Figure 3c. Anyhow, the transition from aragonite to vaterite is abrupt, and the boundary is sequential. With regard to the remarkable calcite-aragonite switching in shell, Mann represented that this phenomenon is controlled by a layer of closely packed cells called the outer epithelium that is separated from the inner shell surface by a space filled with an aqueous solution.9 Belcher suggested that soluble polyanionic proteins alone are sufficient to control the crystal phase without the need for deposition of an intervening protein sheet.19 In this article, the aragonite-vaterite interface is aroused abnormally by the environment, and this transition is random. So we consider that some special soluble proteins are the key factor that controls the aragonite-vaterite switch in halflackluster pearls. However, the control function of the soluble organic matrix on the polymorph switch is affirmed, but the mechanism of it is still in discussion. Crystallography. To investigate the crystal orientation relationships between neighboring tablets, the sample was prepared by the ion-milling technique and observed in TEM coupled with SAED. Figure 4 is the TEM micrograph of two neighboring tablets in a vaterite layer, with the respective SAED patterns inset. In this micrograph, the parallel vaterite tablets have the same diffraction patterns strictly, with the [010] direction perpendicular to the tablet plane. It is well-known that the c-axis in aragonite tablets of nacre is perpendicular to the layer directly and a- and b-axes of successive tablets have a common orientation in a domain consisting of 1-5 crystals.15 In this study, the vaterite layer arranged an oriented assembly with vaterite units along the b-axis, although the orientation of the a- and c-axes in a layer remains unclear. Because of the spherical shape of pearls, it is hard to obtain the preferred orientations of vaterite layers extended to macroscopic scale using general XRD analysis. Electron backscatter diffraction (EBSD) is a simple and effective approach to study the crystal orientation of vaterite layers. In this experiment, we selected an analysis area of 50 × 30 µm2. Its inverse pole figure (IPF) illuminates that the vaterite tablets had a very strong texture near to the [010] direction, as shown in Figure 5, which
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Figure 3. SEM images of the aragonite-vaterite boundary in lackluster pearls after 10 wt % EDTA-2Na treatment: (a) the boundary between aragonite and vaterite tablets (arrow); (b) side-side contact between aragonite and vaterite tablets (scale of amplificatory image, 1 µm); (c) front-back contact between aragonite and vaterite tablets (arrow).
is accordant with the result of TEM with SAED. The preferred orientation of the [010] direction is observed ranging from several neighboring tablets to micrometer scale. Leeuw described that the (010) carbonate plane of the vaterite crystal is the dominant surface, both in the dry and in the hydrated form, with the lowest surface and attachment energies via an atomistic approach.20 Meanwhile, the orientation relationship of vaterite tablets in the region mentioned above was measured and
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Figure 4. TEM image of two neighboring tablets in one vaterite layer and corresponding diffraction patterns. Figure 7. XRD pattern of the surface of lackluster pearl. The analysis area is 800 × 800 µm2.
Figure 5. Inverse pole figure of the surface of lackluster pearl.
evidence of preferred orientation, as indicated by broad (101) arcs and sharp (102) spots. Each reflection arc or spot is due to very good alignment of many crystals within individual bundles. Therefore, in addition to the (010) texture, (101) and (102) texture also occurred in vaterite tablets, but weaker than (010) texture. The (101) and (102) surfaces have been shown in Figure 4, which are vertical to (010). Extended to the area of 800 × 800 µm2, vaterite tablets still represent the preferred orientation, and this preferred orientation indicates a precise relevance with the result of electron diffraction analysis. Based on the results of SAED, EBSD, and XRD, vaterite tablets show a high degree of alignment ranging from several neighboring tablets to macroscopic scale. Two possible mechanisms were considered to explain the oriented growth of nacre: mineral bridges and the heteroepitaxial model. In the model of mineral bridges,8 the sheets have a central layer of core fibers, covered by protein layer, and are pierced by small pores that allow materials to be supplied to the growing aragonite crystals, as well as keeping the layers crystallogrphically aligned via mineral bridges. In the heteroepitaxial model,22 distances between regularly spaced binding sites on the surface of the orgainc matrix are commensurate only with certain lattice spacings in particular crystal faces. Nudelman and Weiner’s results support this mechanism.16 A new mechanism to explain the alignment of a- and b-axes of nacre crystals is by competition between units within a single lamella. The effect of this competitive mechanism was extended in three dimensions since there is epitaxial growth of nacre crystals from one lamella on the previous one.23 These growth mechanisms of nacre are the same as those of vaterite crystals in lackluster pearls.
Figure 6. The distribution of misorientation angles between adjacent tablets.
4. Conclusion
analyzed using Channel 5 software. The distribution of misorientation angles between adjacent tablets in Figure 6 shows that most of the misorientation angles concentrate on small angles lower than 10° and there is another gather at 60°; the most intensive angle is 3°. This result reveals that the tablets oriented with low misorientation of their neighbors existed in a domain. About a small quantity of misorientation angles near to 60°, this phenomenon occurred in nacreous layer in mussel; two possible modes of structural correspondence between protein sheets and aragonite lattice were assumed to interpret the orientation preference.21 An X-ray-diffraction pattern was collected by R-AXIS SPIDER at 18 kv and a molybdenum anticathode, as shown in Figure 7. The diffraction patterns are typical vaterite. This X-ray diffraction pattern of the surface of lackluster pearl shows some
A new morphology of vaterite in lackluster pearl was found in this paper. The investigation provided a similar hierarchy to nacre. It is more worth regarding that two kinds of contact modes between aragonite and vaterite tablets were observed, which reveals the coexisting state of aragonite and vaterite in one pearl. It is first reported that strong texture of [010], [101], and [102] was found in different scale with various methods. Vaterite tablets have a high degree of oriented arrangement in three dimensions from several neighboring tablets to macroscopic scale. The distribution of misorientation angles showed the domain structure and the cluster character in vaterite tablets. In conclusion, the forming of vaterite crystal in lackluster pearls has typical biomineralization characteristics: (1) the size and morphology of inorganic crystals are regular; (2) the crystals are oriented in arrays; (3) the transition from vaterite to aragonite
Special Vaterite in Freshwater Lackluster Pearls
is abrupt. Thus both aragonite and vaterite in pearls have the same growth mechanism according to the semblable morphology and structure. The study of the aragonite-vaterite switch is noteworthy as the remarkable calcite-aragonite switch in shell. The (010) plane in vaterite layers is a significant crystalline surface, just as the (001) plane in aragonite tablets of nacre. Vaterite tablets in lackluster pearls would be investigated to expand the research on biovaterite for the future. Acknowledgment. We thank Prof. Gangsheng Zhang and Dr. Wentao Hou for experimental discussion. This work is supported by the National Natural Science Foundation of China (Grant No. 50672044) and the Foundation of Analysis and Testing in Tsinghua University. Supporting Information Available: A note regarding the prevalence of this phenomenon. This material is available free of charge via the Internet at http://pubs.acs.org.
References (1) Ma, H. Y.; Dai, T. G. The First Discovery of Vaterite in Lusterless Fresh Water Pearls of Leidian, Zhejiang. Acta Mineral. Sin. 2001, 21, 153-157. (2) Mayer, F. K.; Weineck, E. Die Verbreitung des Kalziumkarbonates im Tierreich unter besonderer Berucksichtigung der Wirbellosen. Jena. Z. Naturwiss. 1932, 66, 199-220. (3) Palchik, N. A.; Moroz, T. N. Polymorph modifications of calcium carbonate in gallstones. J. Cryst. Growth 2005, 283, 450-456. (4) Oliveira, A. M.; Farina, M. Vaterite, Calcite, and Aragonite in the Otoliths of Three Species of Piranha. Naturwissenschaften 1996, 83, 133-135. (5) Tomas, J.; Geffen, A. J.; Allen, I. S.; Berges, J. Analysis of the soluble matrix of vaterite otoliths of juvenile herring (Clupea harengus): do crystalline otoliths have less protein. Comp. Biochem. Physiol. 2004, 139, 301-308. (6) Lowenstam, H. A. Vaterite: Amineralization Product of the Hard Tissues of a Marine Organism (Ascdiacea). Science 1975, 188, 363365. (7) Meenakshi, V. R.; Blackwelder, P. L.; Watabe, N. Studies on the formation of calcified egg-capsules of ampullarid snails. I. Vaterite crystals in the reproductive system and the egg capsules of Pomacea paludosa. Calcif. Tissue Res. 1974, 16, 283-291.
Crystal Growth & Design, Vol. 7, No. 2, 2007 279 (8) Addadi, L.; Weiner, S. A pavement of pearl. Nature 1997, 389, 912914. (9) Mann, S. Biomineralization Principles and Concepts in Bioinorganic Materials Chemistry; Oxford University Press: New York, 2001. (10) Ji, B. H.; Gao, H. J. Mechanical properties of nanostructure of biological materials. J. Mech. Phys. Solids 2004, 52, 1963-1990. (11) Deville, S.; Saiz, E.; Nalla, R. K.; Tomsia, A. P. Freeing as a Path to Build Comlpex Composites. Science 2006, 311, 515-518. (12) Oaki, Y.; Imai, H. The Hierarchical Architecture of Nacre and Its Mimetic Material. Angew. Chem., Int. Ed. 2005, 44, 6571-6575. (13) Mann, S. Biomineralization: Chemical and Biological PerspectiVes; VCH: Weinheim, Germany, 1989. (14) Feng, Q. L.; Su, X. W.; Cui, F. Z.; Li, H. D. Crystallographic Orientation Domains of Flat Tablets in Nacre. Biomimetics 1995, 3, 159-169. (15) Feng, Q. L.; Li, H. B.; Cui, F. Z.; Li, H. D. Crystal orientation domains found in the single lamina in nacre of the mytilus edulis shell. J. Mater. Sci. Lett. 1999, 18, 1547-1549. (16) Nudelman, F.; Gotliv, B. A.; Addadi, L.; Weiner, S. Mollusk shell formation: Mapping the distribution of organic matrix components underlying a single aragonitic tablet in nacre. J. Struct. Biol. 2006, 153, 176-187. (17) Rousseau, M.; Lopez, E.; Stempfle, P. Multiscale structure of sheet nacre. Biomaterials 2005, 26, 6254-6262. (18) Nassif, N.; Pinna, N.; Gehrke, N.; Antonietti, M. Amorphous layer around aragonite platelets in nacre. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 12653-12655. (19) Belcher, A. M.; Wu, X. H.; Christensen, R. J.; Hansma, P. K. Control of crystal phase switching and orientation by soluble mollusc-shell proteins. Nature 1996, 381, 56-58. (20) de leeuw, N. H.; Parker, S. C. Surface Structure and Morphology of Calcium Carbonate Polymorphs Calcite, Aragonite, and Vaterite: An Atomistic Approach. J. Phys. Chem. B 1998, 102, 2914-2922. (21) Hou, W. T.; Feng, Q. L. Crystal orientation preference and formation mechanism of nacreous layer in mussel. J. Cryst. Growth 2003, 258, 402-408. (22) Mann, S. Molecular recognition in biomineralization. Nature 1988, 332, 119-124. (23) Checa, A. G.; Rodriguez-Navarro, A. B. Self-organisation of nacre in the shells of Pterioida (Bivalvia: Mollusca). Biomaterials 2005, 26, 1071-1079.
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