Hydroxyapatite, also called hydroxylapatite (HA), is a naturally occurring mineral with the formula Ca5(PO4)3(OH), but it is usually written Ca10(PO4)6(OH)2 to denote that the crystal unit cell comprises two entities. Hydroxyapatite is the hydroxyl of the complex apatite group can be replaced by fluoride, producing fluorapatite. It crystallizes in the hexagonal crystal system. Pure hydroxyapatite powder is white. Naturally occurring apatites can, however, also have brown, yellow, or green colorations, comparable to the discolorations of dental fluorosis.
Up to 50% by volume and 70% by weight of human bone is a modified form of hydroxyapatite, known as bone mineral Carbonated calcium-deficient hydroxyapatite is the main mineral of which dental enamel are composed. Hydroxyapatite crystals are also found in the small calcifications, within the pineal gland and other structures, known as corpora arenacea