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Hydroxyapatite
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<p>Hydroxyapatite (<a href="/facts/International_Mineralogical_Association/BzSmsAYQ">IMA</a> name: hydroxylapatite) (Hap, HAp, or HA) is a naturally occurring <a href="/facts/Mineral/LkUFbNmv">mineral</a> form of <a href="/facts/Calcium/hf252CC0">calcium</a> <a href="/facts/Apatite/f5tYvl1n">apatite</a> with the <a href="/facts/Chemical_formula/hghUtthl">formula</a> Ca5(PO4)3(OH), often written Ca10(PO4)6(OH)2 to denote that the <a href="/facts/Crystal_structure/bJ4bCeHd">crystal unit</a> cell comprises two entities. It is the <a href="/facts/Hydroxy_group/1Hs2QAEv">hydroxyl</a> <a href="/facts/Endmember/ftb6zfhy">endmember</a> of the complex <a href="/facts/Apatite/f5tYvl1n">apatite group</a>. The OH− <a href="/facts/Ion/3bePpDna">ion</a> can be replaced by <a href="/facts/Fluorine/YUV0Fb8m">fluoride</a> or <a href="/facts/Chlorine/y27UwYBM">chloride</a>, producing <a href="/facts/Fluorapatite/T41AaVmK">fluorapatite</a> or <a href="/facts/Chlorapatite/f5tYvl1n">chlorapatite</a>. It crystallizes in the <a href="/facts/Hexagonal_(crystal_system)/sSSok8wo">hexagonal</a> <a href="/facts/Crystal_system/dMQLk1q4">crystal system</a>. Pure hydroxyapatite powder is white. Naturally occurring apatites can, however, also have brown, yellow, or green colorations, comparable to the discolorations of <a href="/facts/Dental_fluorosis/EuPM2ISy">dental fluorosis</a>. </p><p>Up to 50% by volume and 70% by weight of <a href="/facts/Human_bone/LKMMXUwD">human bone</a> is a modified form of hydroxyapatite, known as <a href="/facts/Bone_mineral/L6LpbEYE">bone mineral</a>. Carbonated calcium-deficient hydroxyapatite is the main mineral of which <a href="/facts/Dental_enamel/ffUMmssl">dental enamel</a> and <a href="/facts/Dentin/UMpkJ3UV">dentin</a> are composed. Hydroxyapatite crystals are also found in pathological calcifications such as those found in <a href="/facts/Breast_cancer/6hME580v">breast tumors</a>, as well as calcifications within the <a href="/facts/Pineal_gland/eN9EBq5Q">pineal gland</a> (and other structures of the brain) known as <a href="/facts/Corpora_arenacea/LD763mmK">corpora arenacea</a> or "brain sand". </p>