Stabilization of amorphous calcium carbonate by specialized macromolecules in biological and synthetic precipitates

Communication: Composite structures in which amorphous and crystalline phases coexist are formed by some organisms. The spicules of the sponge Clathrina are shown to be composed of a crystalline calcitic core and an outer layer of amorphous CaCO3 (see figure). The latter material, normally a highly unstable mineral, is stabilized in vitro by macromolecules extracted from the outer sponge spicule layer. The combination of crystalline and stabilized amorphous phases may have important implications for materials applications.