High-Mg Calcite Nanoparticles Within a Low-Mg Calcite Matrix via Spinodal Decomposition: A Widespread Phenomenon in Biomineralization

During the process of biomineralization, organisms utilize various biostrategies to enhance the mechanical durability of their skeletons. In this work, we establish that the presence of high-Mg nanoparticles embedded within lower Mg calcite matrices is a widespread strategy utilized by various organisms from different kingdoms and phyla to improve the mechanical properties of their high Mg calcite skeletons. We show that such phase separation and the formation of high-Mg nanoparticles are achieved through spinodal decomposition of an amorphous Mg calcite precursor. Such decomposition is independent of the biological characteristics of the studied organisms belonging to different phyla and even kingdoms, but rather originates from their similar chemical composition and a specific Mg content within their skeletons, which generally ranges from 14 to 48 mol percent of Mg. We show evidence of high Mg calcite nanoparticles in the cases of 6 biologically different organisms all demonstrating more than 14 mol percent Mg calcite, and consider it likely that this phenomenon is immeasurably more prevalent in nature. We also establish the absence of these high Mg nanoparticles in organisms whose Mg content is lower than 14 mol percent, providing further evidence that whether or not spinodal decomposition of an amorphous Mg calcite precursor takes place is determined by the amount of Mg it contains. The valuable knowledge gained from this biostrategy significantly impacts the understanding of how biominerals, though comprised of intrinsically brittle materials, can effectively resist fracture.