GROWTH AND STRUCTURE OF LAMELLAR MIXED-CRYSTALS OF OCTACALCIUM PHOSPHATE AND APATITE IN A MODEL SYSTEM OF ENAMEL FORMATION

Lamellar mixed crystals of octacalcium phosphate (OCP) and apatite were synthesized in a model system of enamel formation in the presence of 1 ppm F- at 37-degrees-C and at pH 6.5. The crystal has long and thin plate-like morphology and contained a distinct OCP lamella in the center of the apatite matrix. The thickness of the OCP lamella in the a-axis direction is one to several unit cells. Some apatite crystals embed a central layer instead of the distinct OCP lamella. The OCP lamella and the central layer are parallel to the (100) plane of the apatite, while the c-axis of the OCP is parallel to the c-axis of the apatite. Analysis suggests that (1) F- causes the growth of apatite on OCP and regulates the formation of the lamellar mixed crystals of OCP and apatite, (2) the OCP lamella acts as a template for the subsequent epitaxial growth of apatite, and (3) the lamellar mixed crystals grow mainly in the c-axis direction of both the OCP and apatite. These results strongly support the idea that enamel crystals take a thin and long ribbon-like morphology when the initially formed OCP acts as a template for the subsequent growth of apatite in the enamel formation.