Hydroxylapatite, also called
hydroxyapatite (HA), is a naturally occurring
mineral form of calcium
apatite with the formula Ca
5(PO
4)
3(OH), but is usually written Ca
10(PO
4)
6(OH)
2 to denote that the crystal unit cell comprises two entities. Hydroxylapatite is the
hydroxyl endmember of the complex
apatite group. The OH
- ion can be replaced by
fluoride,
chloride or
carbonate, producing
fluorapatite or
chlorapatite. It crystallizes in the
hexagonal crystal system. Pure hydroxylapatite powder is white. Naturally occurring apatites can, however, also have brown, yellow, or green colorations, comparable to the discolorations of
dental fluorosis.