Synchrotron x-ray microbeam diffraction from abalone shell
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M. Sarikaya Materials Science and Engineering, University of Washington, Seattle, Washington 98195 (Received 10 October 2003; accepted 6 February 2004)
Microstructured biomaterials such as mollusk shells receive much attention at present, due to the promise that advanced materials can be designed and synthesized with biomimetic techniques that take advantage of self-assembly and aqueous, ambient processing conditions. A satisfactory understanding of this process requires characterization of the microstructure not only in the mature biomaterial, but at the growth fronts where the control over crystal morphology and orientation is enacted. In this paper, we present synchrotron microbeam x-ray diffraction (XRD) and electron microscopy observations near the nacre–prismatic interface of red abalone shell. The relative orientations of calcite and aragonite grains exhibit some differences from the idealizations reported previously. Long calcite grains impinge the nacre–prismatic boundary at 45° angles, suggestive of nucleation on (104) planes followed by growth along the c axis. In the region within 100 m of the boundary, calcite and aragonite crystals lose their bulk orientational order, but we found no evidence for qualitative changes in long-range order such as ideal powder texture or an amorphous structure factor. XRD rocking curves determined the mosaic of calcite crystals in the prismatic region to be no broader than the 0.3° resolution limit of the beamline’s capillary optics, comparable to what can be measured on geological calcite single crystals.
I. INTRODUCTION
Molluscan mother-of-pearl, or nacre, exemplifies the capability of living organisms to produce high-strength and high-toughness mineralized tissues through biomolecular control of mineral crystals combined with an organic matrix. Through biomineralization processes, brittle materials such as calcium carbonates are incorporated into high-durability organic/inorganic laminates.1,2 Nacre is a calcium carbonate laminate material, and although the microstructure differs between species, structural studies remain of great interest in trying to further understand the growth processes.3–7 In the case of the red abalone, Haliotis rufescens, structural studies have been accompanied by experiments designed to elucidate the effects of proteins and biomolecules upon crystal growth and morphology.8–10 Despite these diverse efforts, the growth fronts and interfaces of the mineralized regions have yet to be characterized as completely as the mature material. For this reason, questions remain about the genesis of the material. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0196 J. Mater. Res., Vol. 19, No. 5, May 2004
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There are two distinct mineralized regions in the abalone shell, illustrated schematically in Fig. 1(a), which depicts a cross section near the growth front at the edge of the shell. The outer layer is calcite, the stable CaCO3 polymorph, and referred t
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