A calcium phosphate amorphous to x-ray diffraction (ACP) exists in bone mineral in addition to the main bone apatite component, such as hydroxyapatite (HA). Experimental studies found that ACP has definite local atomic order and contains microcrystallites about 9.5 Angstrom in extent rather than a random network structure. Experimental evidence indicates that Posner's cluster (PC), Ca-9(PO4)(6), could be the basic component of ACP. In addition, it is present in various simulated body fluids and could be the growth unit of HA. In the transformation from ACP to HA, ACP need only dissociate into the clusters rather than undergo complete ionic solvation. Although PC could bridge the biologically important gap between ACP and HA, the form it is likely to take in body fluids is not known. In this study, we have performed ab initio density functional calculations to investigate the structure and stability of PC alone in vacuum and in the presence of H+, OH-, Na+, and Cl- ions mimicing the interaction with water and other constituents of body fluids. We find that the cluster with C-1 symmetry is the most stable isomer in vacuum. The interaction of PC with sodium ions and especially with protons leads to a great increase in its stability and surprisingly, the cluster with six protons and six OH- recovers the C-3 symmetry and similar atomic arrangement it has as a structural unit in HA crystal. This may be a key factor in the transformation from ACP to HA crystal. (C) 2003 American Institute of Physics.