Alkali-bearing apatite was synthesized at 25°C and 100°C in solutions bearing a wide range of alkali and fluoride concentration. For apatite synthesized at 100°C in the pH range of 7.4-8.1, CaO, P2O5, and CO2 content failed to show a relationship to fluoride concentration; however, the alkali concentration increased with increasing fluoride concentrations. These crystals have a low cation to anion ratio and a low mean index of refraction relative to stoichiometric apatite. Apatite synthesized in very basic solutions contains more alkali, lime, and carbon dioxide, and less P2O5, and thus has a high cation to anion ratio relative to crystals formed under less basic conditions.
Increasing fluoride concentration in the crystals results in a reduced a lattice parameter; whereas an increased cation to anion ratio results in an increased c.
Apatite formed at 25°C by reaction of calcite with a sodium phosphate solution (pH 7.06-8.27), bearing 10-1 M. fluoride, contains an average 3.7 percent fluoride and 4.6 percent water. The cation/anion ratio of these crystals averages 1.69; and a averages 9.35 Å, a value significantly less than found for apatite synthesized at 100°C.
Balanced structural formula require the incorporation of water in various combinations of H3O+, H4O44-, and H2O° in the calcium, phosphate, and hydroxyl site respectively. Some crystals also require a reduction of charge of the phosphate site, a requirement that can be satisfied by substitution of PO3F2- for PO43-.