Hydroxylapatite


Hydroxylapatite, also frequently called hydroxyapatite, is a mineral. It is a naturally occurring form of calcium apatite with the formula Ca<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>(OH), but is usually written Ca<sub>10</sub>(PO<sub>4</sub>)<sub>6</sub>(OH)<sub>2</sub> to denote that the crystal unit cell comprises two molecules. Hydroxylapatite is the hydroxyl endmember of the complex apatite group. The OH<sup>-</sup> ion can be replaced by fluoride, chloride or carbonate. It crystallizes in the hexagonal crystal system. It has a specific gravity of 3.08 and is 5 on the Mohs hardness scale. Pure hydroxylapatite powder is white. Naturally occurring apatites can however also have brown, yellow or green colorations. Compare to the discolorations of dental fluorosis.