Nucleation of amorphous calcium phosphate (ACP) and its phase transformation with a decrease in solution pH were investigated at a constant temperature of 32 degrees C. A solution containing a mixture of CaCl2 and KH2PO4 was prepared (initial pH=7.7), and a drop was sampled at a constant interval to observe the morphological evolution of the precipitates that formed in the solution. A gel-like solution structure formed immediately after mixing and contained a small amount of sea-urchin-like ACP spherulites (3-20 mu m in size). These spherulites consisted of 1.5-10-mu m-long flexible needles that formed simultaneously with numerous ACP spherical particles. They first transformed into beta-tri calcium phosphate-like material (called "pseudo beta-TCP") and then into single crystals of octacalcium phosphate (OCP) without dissolution. The flexible needles in the spherulites changed into blade springs, then into flexible plates, and finally into rigid plates during the transformation. The OCP structure appeared in the pseudo beta-TCP plates and gradually substituted for the beta-TCP structure over time. The macroscopic spherulite morphology of the initial ACP remained unchanged during the phase transformation, suggesting that OCP is a pseudomorph of ACP. This feature was observed only when the ACP spherulites formed in the initial solution. Fiber-like aggregates consisting of beta-TCP single crystals nucleated around the ACP spherical particles and grew over time. They survived until the final stage of the reaction, and OCP polycrystals formed in the mixture of beta-TCP and ACP spheres. The OCP polycrystals gradually substituted for the ACP spheres without phase transformation of beta-TCP into OCP. (C) 2011 Elsevier B.V. All rights reserved.