Figure 1.
Crystal structure of hydroxyapatite showing a single phosphate-to-carbonate substitution. Substitution is shown in the center, along with two typical vacancies forming to balance the resulting cationic charge excess: Ca2+ and OH−. Different ionic elements are represented with different colors: columnar, Ca1 calcium in green, hexagonal, Ca2 calcium in yellow, hydroxyl in magenta, phosphate in blue, and carbonate in gray. Red arrows indicate that these ions will be removed, and vacancies formed with carbonate substitution.