Nanoskalige Untersuchungen zur Calcitmineralisation in Gegenwart von Kieselsäuren und Nanohärte-Messungen von Brachiopodenschalen

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.