Transition Bars during Transformation of an Amorphous Calcium Carbonate Precursor

A variety of calcium carbonate (CaCO 3 ) biominerals are now thought to be formed via an amorphous precursor pathway. Our group has been using in vitro model systems to examine potential crystallochemical reactions that might occur during biomineralization and, in particular, has proposed that a polymer-induced liquid-precursor (PILP) mineralization process may play a fundamental role in biomineralization. The PILP process is induced with polyanionic polypeptides (containing aspartic acid and phosphoserine) to produce colloids of an amorphous mineral that are so highly hydrated that they coalesce into smooth mineral films. In situ observations, however, show that these films are not smooth throughout the reaction because transition bars form ridges in the tablets/films as the amorphous phase crystallizes. Using fluorescently labeled polymer, it was determined that the transition bars are due to exclusion of the polymeric impurity into diffusion limited zones as crystallization proceeds across the amorphous phase. There is also evidence to suggest that the excluded polymer may stimulate a secondary deposition of PILP phase that fills in the valleys and ridges to produce a smooth mineral film. These transition bars, which result in anisotropic exclusion of polymer along well-defined crystallographic zones, may be relevant toward understanding the anisotropy of crystal texture in biominerals with occluded proteins.