Polycrystalline echinoderm calcite and its fracture mechanics.

Polycrystalline calcite was revealed by scanning electron microscopy of fractured skeletal ossicles of the sea star Echinaster spinulosus (Echinodermata, Asteroidea). Whisker-like calcite crystals were observed in specimens that were loaded in stress relaxation before being fractured; rapidly broken surfaces were smooth and glassy. The crystallites were 1300 angstroms wide and at least 3600 angstroms long and were packed together in lamellae. The lamellae were wound into spirals that formed the trabecular bars. All the crystallites in an ossicle appear to be aligned in the same direction. Geometric considerations indicate that the requirement for packing the crystallites smoothly may explain the high magnesium ion concentration of echinoderm calcite.