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.

Fracturas “Fósiles” en Esqueletos De Equinoideos Del Cretácico Superior - “Fossil” Fractures in Upper-Cretaceous Equinoids Tests

“…En primer lugar, las placas y las espinas de ejemplares recientes ("frescos") de erizos de mar actuales, a pesar de poseer un esqueleto esponjoso de calcita monocristalina, rompen frágilmente tanto a escala macroscópica (rotura brusca con muy poco gasto de energía y tras muy pequeña deformación, sin apenas deformación plástica), como a escala microscópica (la rotura es también frágil a la escala de los ligamentos microscópicos de la estructura celular), pero no por los planos esperables de clivaje. Por ejemplo, a escala macroscópica, espinas cuya orientación cristalina axial coincide con el eje c, ensayadas a flexión, rompen frágilmente por un plano macroscópico perpendicular a su eje, que no es un plano de clivaje [18,23,28]. A escala microscópica, la superficie de fractura de los ligamentos del esqueleto de las espinas o del test es predominantemente concoidea [ 14,21,23].…”

“…Incluso, en ausencia de una oclusión total de la porosidad, para que se diera la facies de rotura de clivaje observada bastaría que la porosidad abierta (continua) hubiera pasado a cerrada (discontinua) por el recrecimiento del esqueleto esponjoso inicial; la transición ocurre para un volumen relativo de calcita de aprox. 70% [28].…”

Constructional design of echinoid endoskeleton: main structural components and their potential for biomimetic applications

“…Recently, Lauer et al (2020) demonstrated that unlike mechanical properties of other biogenic ceramic composite materials, such as nacre, the combination of high Mg-calcite with ACC and organic phases have little effect on macromechanical properties of the Heterocentrotus mamillatus spines [110]. Thus, although the micromechanical properties of the echinoid skeleton are governed by the interplay of ACC, organic phase and Mg calcite [96,99,111], the macromechanical properties seem mainly governed by the porous stereom structure and architecture resulting in a remarkable damage tolerance [110].…”

Heat-Mediated Micro- and Nano-pore Evolution in Sea Urchin Biominerals

“…In order to further investigate the role of porosity in biomineralization processes, we study here the nanometer and micrometer structural reorganization occurring during the heat-induced crystallization of biogenic ACC, initially trapped within Paracentrotus lividus sea urchin skeletal elements that present different levels of macroporosity. We propose to do so by taking advantage of the unique characteristic of sea urchin biominerals, i.e., the presence of remnant anhydrous ACC in their mineralized structures. − Indeed, adult sea urchin skeletal elements, in particular the spines, were shown to form via hydrated ACC nanoparticles that later crystallize into calcite through anhydrous ACC as first evidenced in sea urchin larval spicule. , In addition, organic macromolecules are entrapped within the calcite lattice during crystal growth. ,,− Consequently, nanoinclusions of ACC and/or macromolecules ranging in size from 5 to 400 nm were observed in the spines of different sea urchin species (Anthocidaris crassispina, Holopneustes porossimus, and Heterocentrotus trigonarius). ,, …”