The crystallization of fluoroapatite dumbbells from supersaturated aqueous solution

The crystallization of fluoroapatite is of importance because a number of biological systems makes use of this mineral in teeth (sharks) or for protection (molluscs). The crystallization of fluoroapatite was performed on titanium metal, on polymer (poly(ethylene-co-1,4-cyclohexane-dimethylene-terephthalate), PETG), and in a tubular reactor made of polytetrafluoroethylene (PTFE). A special morphogenesis leading to a distinct dumbbell-like morphology was observed. The morphology was clearly dependent on the external parameters pH and supersaturation. Secondary parameters were the crystallization time, the ionic strength, and the stoichiometric ratios of calcium, phosphate, and fluoride. Dumbbells were only observed in the pH range of 5-7. Two different growth mechanisms for fluoroapatite at a pH of 5-6 and a pH of 6-7 are proposed, based on branching hexagonal prisms and on branching spikes, respectively. The presence of charged additives (cationic and anionic surfactants, poly(aspartic acid), polylysine) led to inhibition of the crystal growth, suggesting an electrostatic blocking of the crystal growth sites. An increase in the ionic strength did not influence the crystal morphogenesis.