Hydroxylapatite, also called hydroxyapatite, phosphate mineral, calcium hydroxide phosphate [Ca5(PO4)3OH], that forms glassy, often green crystals and masses. It is seldom pure in nature but often occurs mixed with fluorapatite, in which fluorine substitutes for the hydroxyl (OH) group in the molecule. This mixture, called a solid-solution series, is a continuous chemical variation between the two pure substances. Collophane, the apatite in fossil bone and phosphate rock belongs to the series. For physical properties, see phosphate mineral (table).