Crystal growth and the role of the organic network in eggshell biomineralization

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Abstract
A model based on geometrical crystal growth considerations is proposed for the deposition of the crocodilian, testudinian and avian eggshells. In each shell column, crystal deposition is initiated at a single location, from which growth fans out at all angles to the shell normal. In both calcitic and aragonitic shells, growth is in the [001] direction, resulting in an increase in the degree of (001) preferred orientation with distance from nucleation. Where there is unhindered crystal growth, the shells show a crystalline fracture morphology, and the degree of texture that develops is a simple function of the column radius. This type of growth makes up the whole of the testudinian shell, the inner 0.3-0.4 (30-40%) of the thick ratite shells and the cone layer of the other avian shells. At the start of the palisade layer of the avian shell, the onset of deposition of the organic component coincides with a hindrance to texture development, which thereafter proceeds at a lower rate. A further hindrance occurs about halfway through the shell, probably caused by a change in the physical characteristics of the organic network. The degree of texture that develops in the avian shell is a function of the column radius and the degree of physical hindrance presented by the organic network. The palisade layer of the avian shell has a composite fracture morphology resulting from the intermingling of the network with the inorganic phase. The organic component does not appear to control crystal growth, as previously believed, but instead acts as a reinforcing fibrous network.