ON THE NANOSTRUCTURE OF BIOGENIC AND BIO-INSPIRED CALCIUM CARBONATE As studied by electron microscopy techniques

Most biominerals in nature are formed from both organic and inorganic (mineral) compounds, and are thus by definition a composite material. They are hierarchically ordered from the nanoscale and often have superior mechanical properties compared to synthetic ceramics [1].
This study focusses on the structural characterisation of aragonite and calcite biominerals, combined with the investigation of formation mechanisms through the synthesis of bio-inspired or biomimetic crystals. To this end a multi-length scale study of aragonite and calcite based minerals is presented, based primarily on electron microscopy techniques and supported by Raman spectroscopy and chemical analysis of the organic compounds.
Aragonite skeletal material from corals is studied in detail from the nano-to microscale. This is compared to calcium carbonate crystals precipitated in the presence of organic molecules with hydroxyl-groups, namely ethanol. Secondly, we look at the calcite based system of coccolithophores (marine algae) which precipitate their exoskeleton intracellulary. Such crystals formed in confinement are compared to the structure of synthetically produced calcite nanowires, grown in track-etch membranes.
The coral’s spherulites (roughly 10-20 µm in size) were found to consist of three distinct crystalline phases. This microstructural sequence could for the first time be directly correlated to diurnal growth bands observed in optical transmission images and are linked to a light enhanced calcification process. The synthetic CaCO3 precipitation experiments showed that increasing ratios of ethanol resulted in a shift of crystal phase and morphology from single crystal rhombohedral calcite to branched polycrystalline aragonite, the latter being similar to the coral.
The calcite coccoliths of Rhabdosphaera clavigera exhibit centrally positioned, several micrometre long five-fold symmetric spines. The spines are made up of spiral staircase arrangements of {104} single crystal calcite rhombohedra. However the rim of the coccolith has complex shaped, kinked, crystal elements. It was found that such unconventional crystal shapes can be promoted by external anisotropic surface stresses as was seen for the calcite nanowires investigated in this study.