Single crystal structure analysis of sea urchin spine calcites: Systematic investigations of the Ca/Mg distribution as a function of habitat of the sea urchin and the sample location in the spine

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.