Evaluating the single crystallinity of sea urchin calcite

Recent advancements in electron backscatter diffraction (EBSD) data evaluation enable the determination of misorientation between crystals below 0.1°, while with conventional EBSD data evaluation, the smallest misorientation precision between crystals scatters between 0.5°-1°. Sea urchin tests and spines are lightweight biomaterials with a serrated microstructure comprising interlinked calcite crystals. We investigated the microstructure and crystallographic texture of Cidaris cidaris and Paracentrotus lividus test and spine calcite with advanced EBSD measurement and data evaluation. In particular, we re-evaluated the widely accepted single-crystallinity of sea urchin calcite. We found that the test and the spines comprise calcite crystals with different fabrics and a significant variation in crystal co-orientation strength. Even the highly co-oriented calcite of C. cidaris and P. lividus is not perfectly single-crystalline. We found test and spine portions that feature significant internal misorientations (1-3°). Test c-axis orientation in C. cidaris is tangential to the outer test surface, while in the spines, it is parallel to the morphological axis of the spine. Primary and secondary spines feature a bimodal crystal texture comprising co-oriented calcite surrounded by a cortex of misoriented crystals. Crystal misorientation in the spine cortex seems to result mainly from competitive growth determinants. Deciphering the degree of crystallinity and mode of crystal organisation of biological hard tissues is vital for understanding their exceptional control of structure, material architecture and material properties.

Statement of significance

Echinoids form lightweight biomineralised skeletal elements with outstanding material properties and a complex microstructure formed of interlinked calcite crystals. The degree of crystallinity and the crystallographic organisation of the calcitic tests and spines are still under debate. We investigate and discuss the crystallinity, microstructure, and texture of Cidaris cidaris and Paracentrotus lividus test and spine crystals. Unprecedented and not yet used for biomineralised carbonate tissues, we apply electron backscatter diffraction pattern matching data evaluation, enabling detection of misorientation precision below 0.1°, relative to 0.5°-1° misorientation precision obtained from conventional EBSD data evaluation. We demonstrate that sea urchin test plates and spines are not single crystals. They feature internal small-angle misorientations and poorly co-oriented, polycrystalline regions with intricate microstructures.