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Merkel, Casjen (2009): Nanoskalige Untersuchungen zur Calcitmineralisation in Gegenwart von Kieselsäuren und Nanohärte-Messungen von Brachiopodenschalen. Dissertation, LMU München: Fakultät für Geowissenschaften



In the present work three topics are outlined which are connected via biomineralisation. The investigation of the mechanical response of brachiopod shell material, the macroscopic calcite crystal growth, and the microscopic calcite crystal growth are discussed. Brachiopod shells have been investigated with nanoindentation and scanning electron microscopy to determine the material properties and the effects of the microstructure on the material properties. Brachiopod shells are highly optimised fibre composite materials. The material properties are adapted to the local material demands. The calcitic brachiopod \emph{Megerlia truncata} exhibits a hard primary layer and a softer secondary layer with a tendency to higher hardness at the inner shell margin. At the hinge region the hardness is considerably higher and reflects the possibly high abrasive wear. The phosphatic brachiopod shells of \emph{Lingula anatina} and \emph{Discradisca stella} have laminated architecture with alternating hard and soft layers. They are harder in the center and have softer, more flexible regions at the margins. The macroscopic crystal growth of calcite was performed in gels. The obtained crystals were investigated with scanning electron microscopy. With inert pore solutions the $\left\{104\right\}$ rhombohedron is the dominating habit. Succinic acid and aspartic acid inhibit the acute steps of the calcite $\left\{104\right\}$ surface. In combination with $Mg^{2+}$ and $Sr^{2+}$ one side divergant forms are obtained. Pore solutions containing silicic acids produce catastrophic nucleation. Crystal aggregates from calcite, aragonite, and vaterite develop. The aggregates have morphological similarities with calcites from travertines. $Mg^{2+}$ and $Sr^{2+}$ counteract the effect of silicic acids. The microscopic crystal growth of the calcite $\left\{104\right\}$ surface was investigated with the atomic force microscope in the presence of silicic acids. The two dimensional nucleation is enhanced. The form of the hillocks change from the additive free rhomb to lenses with increasing silicic acids concentrations. The step velocities increase from 0 ppm to 20 ppm silicic acids and decrease after 20 ppm. Therefore silicic acids possess a promoter and a inhibitor effect on calcite crystal growth. The inhibitory effect can be fitted by a Langmuir isotherm. Both effects can be fitted together with a polynom of rank three. For supersaturation $\beta = 30$ the step velocities are constant. This means that the process follows the step pinning model.