On the Relation of Amorphous Calcium Carbonate and the Macromechanical Properties of Sea Urchin Spines

Abstract

Amorphous calcium carbonate (ACC) plays a crucial role in the formation of biogenic carbonates. It is widely accepted that ACC and organic macromolecules alter the fracture properties of Echinoderm calcite from the well‐defined cleavage planes of the raw material to conchoidal. However, the influence of ACC on the outstanding macromechanical properties of Echinoderm calcite is unknown. To address this question, full‐grown spines of the slate pencil urchin are shortly heated to 250 °C. Differential scanning calorimetry indicates that all ACC is crystallized at this temperature. Heated spines are compared with an untreated control group and no significant differences in compressive strength, bending strength, damage tolerance, and Young's modulus are detected. This highlights the weak influence of ≈6 wt% ACC on the macromechanical properties of Echinoderm calcite, which are likely established by its intricate and damage tolerant microstructure. When heating Echinoderm calcite, organics decompose, Mg calcite transforms, water is lost, and cracks and micropores form. All these processes are analyzed to exclude their influence on the mechanical properties, and it is imperative to consider them all. Only this way meaningful results can be achieved as these processes are temperature and dwelling time dependent and may even occur below 250 °C.