The effect of impurity on the two-dimensional (2D) nucleation rate, J, was investigated by estimating the edge free energy, gamma, on the (0001) face of a hydroxyapatite crystal in the presence of magnesium and zinc as impurities. It was found that edge free energies, gamma, at 0.06 mM magnesium and 0.75 muM zinc concentrations were constant acid almost the same as that in the absence of impurities in the supersaturation range of sigma = 9.8-22.0. This indicates that magnesium and zinc did not affect the energy barrier for 2D nucleation at least up to these concentrations, although these impurities inhibited the growth rate of the (0001) face by adsorbing at kink sites of 2D islands. Taking into account the effect of impurities on J, the adsorption constants K of the Langmuir kink model were calculated as K-Mg = (1.30 +/- 0.2) x 10(4) L/mol and K-Zn = (1.23 +/- 0.1) x 10(6) L/mol at sigma = 22.0 and 25 degreesC.