title: Characteristics of biogenic calcite in the prismatic layer of a pearl oyster, Pinctada fucata.
abstract: The fine structure of the calcite prism in the outer layer of a pearl oyster, Pinctada fucata, has been investigated using various electron beam techniques, in order to understand its characteristics and growth mechanism including the role of intracrystalline organic substances. As the calcite prismatic layer grows thicker, sinuous boundaries develop to divide the prism into a number of domains. The crystal misorientation between the adjacent domains is several to more than ten degrees. The component of the misorientation is mainly the rotation about the c-axis. There is no continuous organic membrane at the boundaries. Furthermore, the crystal orientation inside the domains changes gradually, as indicated by the electron back-scattered diffraction (EBSD) in a scanning electron microscope (SEM). Transmission electron microscopy (TEM) examination revealed that the domain consists of sub-grains of a few hundred nanometers divided by small-angle grain boundaries, which are probably the origin of the gradual change of the crystal orientation inside the domains. Spherular Fresnel contrasts were often observed at the small-angle grain boundaries, in defocused TEM images. Electron energy-loss spectroscopy (EELS) indicated the spherules are organic macromolecules, suggesting that incorporation of organic macromolecules during the crystal growth forms the sub-grain structure of the calcite prism.
authors.affiliation: Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, Bunkyo-ku, Tokyo, Japan. okumura@eps.s.u-tokyo.ac.jp
authors.forename: Taiga
authors.initials: T
authors.lastname: Okumura
authors.forename: Michio
authors.initials: M
authors.lastname: Suzuki
authors.forename: Hiromichi
authors.initials: H
authors.lastname: Nagasawa
authors.forename: Toshihiro
authors.initials: T
authors.lastname: Kogure
mesh_terms: D000818:Animals; D002119:Calcium Carbonate / Q000737:chemistry*; D046529:Microscopy, Electron, Transmission; D049911:Pinctada / Q000737:chemistry* / Q000648:ultrastructure*; D046590:Spectroscopy, Electron Energy-Loss
chemical_list: D002119:Calcium Carbonate