Hydroxylapatite, also called hydroxyapatite,'s a naturally occurring mineral form of calcium apatite with the formula Ca5(PO4)3(OH), but's usually written Ca10(PO4)6(OH)2 to denote that the crystal unit cell comprises two entities. Hydroxylapatite's the hydroxyl endmember of the complex apatite group. The OH- ion can be replaced by fluoride, chloride or carbonate. It crystallizes in the hexagonal crystal system. It's a specific gravity of 3.08 and's 5 on the Mohs hardness scale. Pure hydroxylap… (More on Hydroxylapatite)