Lead is a toxic heavy metal that is commonly used to manufacture products such as storage batteries and electroceramics. In an effort to minimize the health risks to humans, the low-cost stabilization of lead via its incorporation into the apatite crystal structure has been explored. Therefore, apatite formation in the quaternary system CaO-PbO-P2O5-H2O at room temperature (23 degrees +/- 1 degrees C) and atmospheric pressure front PbO, CaHPO4, Ca-4(PO4)(2)O, and H3PO4 was studied. Apatites were prepared via precipitation from aqueous-based mixtures with a constant (Ca + Pb):P molar ratio of 1.67. Aqueous mixtures that contained up to 1 mol of PbO (massicot) per mole of apatite (Ca10-xPbx(PO4)(6)(OH)(2)) resulted in the precipitation of weakly crystalline Ca-10(PO4)(6)(OH)(2) as the predominant phase. Highly crystalline Pb-10(PO4)(6)(OH)(2) was formed from mixtures that contained 8-9 mol of PbO per mole of apatite, Intermediate proportions of lead oxide, which entailed 4.5-7 mol of PbO per mole of apatite, produced a stable quaternary apatite phase of moderate crystallinity, with an approximate composition of Ca5Pb5(PO4)(6)(OH)(2). The lattice parameters of this quaternary apatite were calculated using a least-squares refinement program and other existing X-ray diffraction software. Experimental observation, along with computer modeling of the solution chemistry, enabled the mechanism of apatite formation to be established. Dissolution of lead oxide and calcium-bearing minerals, such as monetite (CaHPO4), with the aid of H3PO4, resulted in the precipitation of lead-rich apatites, This phenomenon was followed by their slow conversion to the stable quaternary apatite (Ca5Pb5(PO4)(6)(OH)(2)).