Chloroapatite

Chloroapatite, also known as chlorapatite, is a naturally occurring calcium phosphate mineral with the chemical formula Ca5(PO4)3Cl. It belongs to the apatite group and is isostructural with hydroxyapatite, differing primarily by the substitution of the hydroxyl anion (OH−) with chloride (Cl−). The crystal system is hexagonal, with common symmetry P63/m and unit cell parameters a ≈ 9.4 Å and c ≈ 6.9 Å.

The mineral frequently appears in vertebrate hard tissues. It is a principal component of human and animal teeth, where it makes up roughly 85–90 % of the enamel mineral content. Small amounts of chloride are incorporated during enamel maturation, forming small amounts of chloroapatite crystals. In bone, hydroxyapatite is predominant, but chloroapatite can be found in mineralized tissues that have been exposed to chloride-rich environments, such as in certain pathological conditions or after long-term exposure to saline solutions.

Chloroapatite occurs naturally in various geological settings as a secondary mineral. It is typically found in sedimentary rocks, especially in evaporite deposits where high concentrations of chloride are present. It can also form in the weathering ash of volcanic rocks and in some phosphatic nodules. In these contexts, it often occurs as fine, submicron particles that can be distinguished from hydroxyapatite by electron microscopy and X‑ray diffraction.

Because of its biochemical similarity to hydroxyapatite, chloroapatite has been studied as a model for ion‑exchanged apatites in biomedical research. It is used in the fabrication of bioactive glass coatings and calcium phosphate cements to promote bone growth and repair. In addition, chloroapatite can be employed in water treatment as a source of calcium and phosphate ions.

Although chloroapatite is less stable than hydroxyapatite under normal conditions, it remains an important component of biological calcification and is a subject of ongoing research in both materials science and geology.