Nonclassical Crystallization of Amorphous Calcium Carbonate in the Presence of Phosphate Ions

Abstract

Phosphate ions are commonly found in amorphous biominerals and play critical roles in controlling the stability and crystallization pathways of amorphous calcium carbonate (ACC). However, the role of phosphate ions during the transformation process of ACC remains elusive. Here, we show that while the onset of ACC transformation in solution could not be effectively delayed by addition of phosphate ions up to 5% (molar fraction in the initial carbonate solution), interestingly, the transformation process can be significantly prolonged. This is ascribed to the formation of a more stable phosphate-enriched amorphous phase during transformation and the inhibition of vaterite dissolution and calcite crystallization. Phosphate ions can also promote the nonclassical crystallization of calcite, where calcite crystals grow via oriented attachment of nanoparticles. In addition, nanoporous calcite single crystals can be obtained through crystallization of ACC after being embedded in the crystals. Our results are of significance to understand the crystallization mechanisms of amorphous materials and biomineralization processes.