On the Characteristics and Growth of Biominerals

Minerals formed by organisms may either be simply a side-product of the metabolism or they carry a function, for which their properties and morphologies are controlled by the organism. The rst case, bioinduced mineralisation, was encountered in this work in the case of the precipitation of the mineral schwertmannite (Fe8O8(OH)6SO4) by the bacterial strain Leptospirillum ferrooxidans. In the literature the characteristic morphology of the mineral had been viewed as bio-speci c. However, in this work, in could be produced in abiotic syntheses with appropriate conditions. Products of biocontrolled mineralisation investigated in this work were brachiopod shells, sea urchin spines and sea urchin teeth. These products are hybrid composite materials consisting of soft organic and hard mineral components. For their complex, highly characteristic, and purpose-oriented microstructures an inorganic mechanism can not be envisaged. The formation of the minerals and their properties has been investigated with scanning electron microscopy, electron backscatter di raction, transmission electron microscopy, x-ray di raction, Vickers microhardness indentation and nanoindentation. The use of a low accelerating voltage allowed the increase of the lateral resolution of electron backscatter di raction. The angular resolution of electron backscatter di raction could be increased by a statistical approach. Comparative biotic and abiotic synthesis experiments identi ed schwertmannite precipitation in cultures of Leptospirillum ferrooxidans as a process of bioinduced mineralisation. Di erent abiotic synthesis started from ferric or ferrous-solutions and used various methods of oxidation and/or precipitation to produce schwertmannite. The samples prepared by these methods led to a variety of morphologies of the mineral, including the "hedgehog" morphology which had been adressed as cell overgrown with schwertmannite needles in the literature. Rietveld re nements of the di raction pro le of the amorphous-to-nanocrystalline schwertmannite show that the crystallite size is anisotropic. It varies depending on the growth conditions and is between 2-2.5 nm perpendicular and 5-11 nm parallel to channels created by the network of [FeO6] 3 -octahedra in the structure. The analyses of calcitic brachiopod shells show that, depending on the species, a distinction of up to three microstructures are present: columnar layer, brous layer and primary layer. The microstructure and texture of the columnar layer of the brachiopod shell can be explained by a process of competitive growth, which can also be found in inorganic systems. However, the formation of the brous and the primary layer can not be described by processes known from inorganic systems. The microstructure of the primary layer, which was correctly resolved in this thesis for