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.ydroxylapatite 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.