Structure of Clusters and Formation of Amorphous Calcium Phosphate and Hydroxyapatite: From the Perspective of Coordination Chemistry

Abstract

Amorphous calcium phosphate often forms as a precursor phase in a solution at sufficiently high supersaturation and pH, and then transforms to the thermodynamically stable hydroxyapatite. The chemical composition, structure, and property of the amorphous phase are dependent on the structure of its composing clusters. Based on the results from the measurements of in situ Ca K-edge X-ray near-edge structure and ex situ X-ray diffraction, as well as the concomitant pH change in the reaction process, here we propose an improved model for the structure of “Posner’s cluster” and identify the three types of reactions that lead to the formation of amorphous calcium phosphate and its subsequent transition to crystalline hydroxyapatite.