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Single crystal structure analysis of sea urchin spine calcites

Institut GMG, Lehrstuhl für Kristallographie, Ruhr-Universität Bochum, Universitätsstr. 150, D-44780 Bochum, Germany

^* Corresponding author, e-mail: uta.magdans{at}ruhr-uni-bochum.de

The hard skeleton of sea urchin spines consists of a monolithic single crystalline magnesian calcite with typical contents of MgCO₃ ranging from 2 to 12 mol% and about 0.1 weight% of occluded organic macromolecules. These macromolecules take effect on the calcite crystal growth, forming the spine shape in a porous spongy structure crossed by filled wedges of calcite radially arranged about the long axis of the spine. The single crystal structure analysis and polarization microscopy confirm the perfect alignment of the crystallites, thus the spine is a single crystal of calcite with the spine axis along the crystallographic c-axis, in agreement with earlier experiments.

The magnesium content of the skeletal parts of the sea urchin is approximately linearly related with the water temperature of the sea urchin habitat. Elemental and chemical analysis showed an inhomogeneous Mg distribution in single spines: The Mg content increases from the spine-tip to the base by about 2 mol% at average. This structural inhomogeneity leads to a solid solution hardening of the material by the statistical incorporation of Mg²⁺ ions in a crystal structure. The size difference between Ca²⁺ and Mg²⁺ ions impedes the propagation of cracks, thus affecting the fracture behavior of the biogenic calcite, which is conchoidal in contrast to the perfect {10.4} cleavage of geological calcite. The accumulation of Mg in the spine-base most likely serves to enhance this effect and thus strengthens the spine close to the shell.

Key-words: biomineralization, sea urchin spine, magnesian calcite, single crystal.

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