A mechanism for the formation of hydroxyapatite (HAp) surface layers compositionally different from the bulk compositions is described. Surface layer formation is a consequence of HAp being a compound of variable composition: Ca-10(PO4)(6)(OH)(2) to similar to Ca9HPO4(PO4)(5)OH, existing over Ca/P ratio from 1.67 for stoichiometric to similar to 1.5 for fully calcium-deficient HAp. One consequence of this compositional variability is that only one HAp composition (Ca/P ratio) dissolves congruently (the solution and solid have the same Ca/P ratio). HAp compositions having Ca/P ratios different from the congruently dissolving composition will form surface layers. Equilibration of stoichiometric HAp with water will result in the formation of a surface layer having a Ca/P ratio of less than 1.67. In general, for solid HAp compositions having Ca/P ratios greater than that of the congruently dissolving composition, a nonstoichiometric surface layer will form and the Ca/P ratio in solution will exceed that of the surface layer. For solid compositions having lower Ca/P ratios than the congruently dissolving composition, the opposite will be true.