Ca5PO43ClOH

Ca5PO43ClOH is a composition associated with the apatite family of calcium phosphate minerals, representing a mixed anion variant in which the channel site in the apatite structure is partially occupied by chloride (Cl−) and hydroxide (OH−). The endmembers Ca5(PO4)3Cl (chlorapatite) and Ca5(PO4)3OH (hydroxyapatite) are well established, so Ca5(PO4)3ClOH denotes a solid solution between these ends. In the apatite structure, Ca2+ ions occupy two distinct sites, phosphate groups form PO4 tetrahedra, and a one-dimensional channel accommodates the X-site anions. Substitution of OH− by Cl− or a mixture of both leads to gradual changes in lattice parameters, solubility, and bioactivity.

Chlorapatite and hydroxyapatite occur naturally or are synthesized; mixed Cl/OH apatites can form in laboratory systems or as natural solid solutions in apatite-bearing rocks. In biomaterials research, halide- or hydroxyl-substituted apatites are studied for bone grafts, coatings, and dental applications due to their biocompatibility and ability to modulate solubility.

See also: Apatite, chlorapatite, hydroxyapatite, fluorapatite.