  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.
