Some new aspects of surface nucleation applied to the growth and dissolution of fluorapatite and hydroxyapatite

The polynuclear mechanism of crystal growth and dissolution is revised using calcium concentrations instead of mean ion activities in the expression for the lateral growth of surface nuclei. The rate-limiting process for the lateral growth of surface nuclei for the growth and dissolution of fluorapatite, FAP, and for dissolution of hydroxyapatite, HAP, is suggested to be calcium ions performing a diffusion jump into a kink and simultaneously being partly dehydrated. The experimental values of nu(in) 10(5) s(-1), for these processes hereby agree with the theoretical value for this process, except for growth of FAP at pH 5.0. A new mechanism, mononuclear in the direction of the minor axis and polynuclear in the direction of the major axis, is introduced to explain the rate of growth of FAP at pH 5.0. The rates of growth of HAP lead to low values of nu(in) about 10(3) s(-1). It is suggested that the building of hydroxyl ions into the crystal lattice in this case limits the rate. It is further illustrated that empirical rate expressions in general cannot be used to distinguish between rate-controlling processes.