Orientation-Dependent Etchpit Penetration and Dissolution of Fluoroapatite

In this study, fluoroapatite crystals were etched with citric acid (pH 2) at 60 °C parallel and perpendicular to Apatite the c-axis. Quantitative measurements on a number of etchpits, visualized with the scanning electron microscope were combined with accurate chemical solubility studies with an atomic absorption spectrophotometer, pertaining to the same crystal surface. Whereas the difference in chemical solubility with respect to the prism or basal plane is small, the velocity of etchpit penetration is 5 times quicker parallel to the c-axis than perpendicular to it. Etchpits and dislocations existing in a one-to-one relation, these experiments strongly indicate that the dislocations present in apatite crystallites in enamel, are the sites where the very first dissolution during the caries process occurs.