Sea urchin tooth design: An "all-calcite" polycrystalline reinforced fiber composite for grinding rocks

A study was conducted to investigate how different kinds of calcite crystals in the sea urchin tooth work together as an effective grinding tool. The study showed that the polycrystalline matrix has a higher elastic modulus and hardness than the single crystalline needles and plates, and that the working surface is smooth. It ascribed the unique properties of the matrix to a combination of a very high Mg content, the lack of orientation of the nanocrystals, and their very small size. The single crystals contain relatively high concentrations of Mg, and presumably like other echinoderm crystals, occluded proteins that reduce the brittleness of the calcite and allow it to deform in a plastic manner and fracture with conchoidal cleavage. The study proposed that it is the unusually high hardness and modulus of the Mg-enriched matrix that is mainly responsible for the ability to grind the rocky substrate, whereas the arrays of calcite needles and plates act as a support framework for the polycrystalline matrix.