Ca5PO43OH

Ca5(PO4)3OH is the empirical formula associated with the hydroxyapatite family, a major calcium phosphate mineral. In the more common full formula, hydroxyapatite is written as Ca10(PO4)6(OH)2, so Ca10(PO4)6(OH)2 equals two units of Ca5(PO4)3OH. Thus Ca5(PO4)3OH represents a structural unit of the apatite lattice.

The compound belongs to the apatite group and crystallizes in a hexagonal (pseudo-crystal) system. Its structure consists of PO4 tetrahedra linked by calcium ions, forming a three-dimensional framework with channels along the c-axis that house hydroxide ions. Calcium occupies distinct crystallographic sites, contributing to the characteristic stability and ion-exchange properties of apatites.

Biological and geological relevance is a defining feature. Hydroxyapatite is the principal inorganic component of bone and dentin, and a major mineral phase in teeth. In bones and teeth, the mineral provides rigidity and a reservoir for calcium and phosphate. The Ca/P ratio of Ca5(PO4)3OH is 5/3 (approximately 1.67), matching the ratio in Ca10(PO4)6(OH)2.

Synthesis and variability are common themes. Ca5(PO4)3OH can form by precipitation from aqueous calcium and phosphate solutions and can serve as a basis for synthetic calcium phosphate materials. Substitutions are widespread: carbonate, fluoride, or chloride can replace phosphate or hydroxide to yield carbonated, fluorinated, or chlorinated apatites, which are relevant to biology and biomedical engineering.

Applications and properties. Hydroxyapatite and related apatites are biocompatible and osteoconductive, with low solubility at physiological pH. They are used as bone graft substitutes, coatings for implants, and dental materials, as well as in research on biomineralization and material science.