High-resolution electron microscopy of hydroxyapatite grown in dilute solutions.

High-resolution electron microscopy (HREM) was applied to the study of seeded crystal growth of hydroxyapatite (HA) in supersaturated solutions. The HA seed crystals were rod-shaped, elongated along the c-axis, and showed smooth contours. The seed crystals exhibited hexagonal prism facets, and one end was rhombohedrally terminated, whereas the opposite end was blunt. Growth experiments were carried out at 37 degrees C, and solution compositions were carefully selected to avoid the involvement of precursor phases during the HA growth. After five-hour growth, the total amount of precipitation was from 3 to 7% of the initial crystal mass added to the solutions; examination of the crystals by HREM disclosed the formation of projections on the end surfaces of the HA crystals. High-resolution TEM clearly showed lattice fringes with predictable spacings over the entire crystal specimen, including the formed projections. Analysis of selected regions by optical diffraction of the HRTEM fringe negatives showed that the nature and orientation of the projections were similar to those of the underlying seed crystal. High-resolution SEM of the HA crystal after five-hour growth disclosed step-like structures on the prism faces, as well as the projections localized on the ends. These results strongly suggest that more than one growth process may be involved, perhaps related to the distinct faces of the HA seed crystals.