Cluster Growth Model for Hydroxyapatite

An intensity-enhanced dynamic light scattering technique revealed the presence of calcium phosphate clusters from 0.7 to 1.0 nm in size in a simulated body fluid. The clusters were also present in fluids undersaturated with respect to octacalcium and amorphous calcium phosphates and supersaturated with respect only to hydroxyapatite. These clusters are the growth unit of hydroxyapatite judging from the fact that hydroxyapatite grows by step flow 0.8 or 1.6 nm in height and that the probability of incorporation of the growth unit into the crystal is extremely low, as revealed previously. We propose a cluster growth model where hydroxyapatite grows by selective hexagonal packing of left- and right-handed chiral Ca9(PO4)6 clusters 0.8 nm in size. Theoretically, stacking faults of clusters create a reflection-twin crystal, edge dislocations with a Burgers vector of C/2 and screw dislocations. An example of the reflection-twin is the merohedry twin which is frequently found in cadmium chlorapatite. An atomic image corresponding to the edge dislocations with a Burgers vector of C/2 was actually obtained on the surface of synthetic single-crystal hydroxyapatite.