Published papers

We clarified the specific micrometric arrangement and nanometric structure of the radiolarian crystalline spines that are not a simple single crystal. A body of the celestite (SrSO4) skeleton of acantharian Acanthometra cf. multispina (Acanthometridae) composed of 20 radial spines having four blades was characterized using microfocus X-ray computed tomography. The regular arrangement of three types of spines was clarified with the connection of the blades around the root of each spine. The surface of the spines was covered with a chitin-based organic membrane to prevent from dissolution in seawater. In the nanometric scale, the mesocrystalline structure that consists of nanoscale grains having distorted single-crystal nature was revealed using scanning- and transmission electron microscopies, electron diffraction, and Raman spectroscopy. The acantharian skeletons have a crystallographically controlled architecture that is covered with a protective organic membrane. These facts are important for penetrating the nature of biogenic minerals.

Pure and carbonated hydroxyapatite (HA) nanocrystals are regarded as capital biocompatible materials. However, the formation process for faceted nanorods of HA has not been sufficiently clarified in supersaturated aqueous solution systems. Here, we monitored morphological evolution of carbonated HA to faceted crystals on the conversion of a parent calcite nanocrystal under hydrothermal conditions. In the initial stage, low-crystalline carbonated HA nanosheets ∼5 nm thick are formed as metastable intermediates. In the progressive stage, faceted nanorods ∼50 nm wide and ∼200 nm long grow as a relatively stable form of HA through the intermediates. The morphological evolution is ascribed to a stabilizing process of HA for improvement of the crystallinity and a decrease in the specific surface area and carbonate content.

Calcareous skeletons of stony corals are composed of a low-crystalline core (center of calcification: COC) and surrounding fibrous crystals. We characterized the overall framework of juvenile Acropora digitifera corals to clarify the entire calcic architecture and the contribution of abiotic processes. Skeletal and tubular frameworks contain string and sheet COCs, respectively, consisting of strained aragonite nanograins. Parallel and curved structures of fibrous aragonite develop around the COC cores through geometric selection and a gradual change in the growth direction of the c-direction-elongated nanorods. Based on a study of the nanoscale morphology of the coral frameworks by comparing them to fibrous crystals grown in artificial systems, it appears that subsequent precipitation of the fibrous aragonite on the COC is primarily regulated by environmental abiotic parameters. The architecture of the COC network is deduced to be designed biologically and then thickened by the fibrous crystals that are produced with morphology formation through physicochemical processes although the vital activity controls the total mineralization.