hydroxylapatite

{{Infobox mineral | name = Hydroxyapatite | category = Phosphate mineral Apatite group | image = Mineraly.sk - hydroxylapatit.jpg | caption = Hydroxyapatite crystals on matrix | formula = | IMAsymbol = Hap | molweight = 502.31 g/mol | strunz = 8.BN.05 | system = Hexagonal | class = Dipyramidal (6/m) H-M symbol (6/m) | symmetry = P63/m | unit cell = a = 9.41 Å, c = 6.88 Å; Z = 2 | color = Colorless, white, gray, yellow, yellowish green | habit = As tabular crystals and as stalagmites, nodules, in crystalline to massive crusts | twinning = | cleavage = Poor on {0001} and {100} | fracture = Conchoidal | tenacity = Brittle | mohs = 5 | luster = Vitreous to subresinous, earthy | polish = | refractive = nω = 1.651 nε = 1.644 | opticalprop = Uniaxial (−) | birefringence = δ = 0.007 | dispersion = | pleochroism = | fluorescence= | absorption = | streak = White | gravity = 3.14–3.21 (measured), 3.16 (calculated) | density = | melt = | fusibility = | diagnostic = | solubility = | diaphaneity = Transparent to translucent | other = | references = }} thumb|Hydroxyapatite thumb|Needle-like hydroxyapatite crystals on stainless steel. Scanning electron microscope picture from [[University of Tartu.]] thumb|Nanoscale coating of Ca-HAp, image taken with Scanning probe microscopy|scanning probe microscope thumbnail|right|A 3D visualization of half of a hydroxyapatite unit cell, from x-ray crystallography Hydroxyapatite (IMA name: hydroxylapatite) (Hap, HAp, or HA) is a naturally occurring mineral form of apatite with the formula , often written to denote that the crystal unit cell comprises two entities. It is the hydroxyl endmember of the complex apatite group. The ion can be replaced by fluoride or chloride, producing fluorapatite or chlorapatite. 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 and dentin are composed. Hydroxyapatite crystals are also found in pathological calcifications such as those found in breast tumors, as well as calcifications within the pineal gland (and other structures of the brain) known as corpora arenacea or "brain sand".