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 unders aturated 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 hydr oxyapatite grows by step flow 0.8 or 1.6 nm in height and that the pro bability of incorporation of the growth unit into the crystal is extre mely low, as revealed previously. We propose a cluster growth model wh ere hydroxyapatite grows by selective hexagonal packing of left- and r ight-handed chiral Ca-9-(PO4)(6) clusters 0.8 nm in size. Theoreticall y, stacking faults of clusters create a reflection-twin crystal, edge dislocations with a Burgers vector of C/2 and screw dislocations. An e xample of the reflection-twin is the merohedry twin which is frequentl y found in cadmium chlorapatite. An atomic image corresponding to the edge dislocations with a Burgers vector of C/2 was actually obtained o n the surface of synthetic single-crystal hydroxyapatite.