Fluorapatite

Fluorapatite is a fluorine-substituted phosphate mineral in the apatite group, with the ideal chemical formula Ca5(PO4)3F. It is part of the hexagonal apatite structure and forms a continuous solid solution with chlorapatite Ca5(PO4)3Cl and hydroxyapatite Ca5(PO4)3(OH). The fluoride ion substitutes for hydroxyl or chloride in the lattice, producing fluorapatite.

Occurrence and formation: Fluorapatite occurs as a primary mineral in phosphate-rich igneous rocks such as carbonatites and granitic pegmatites, as well as in metamorphic and sedimentary phosphate deposits. It crystallizes from phosphate-bearing magmas and fluids during late-stage alteration and is a major ore mineral for phosphorus.

Physical properties: Fluorapatite typically appears colorless to pale yellow, green, brown, or gray, with a vitreous luster. It has a Mohs hardness of about 5 and a specific gravity near 3.1–3.2. It crystallizes in the hexagonal system and commonly occurs as prismatic crystals or in massive aggregates. Cleavage is imperfect and generally limited to one direction.

Economic and biological relevance: Fluorapatite is the principal source of phosphorus for fertilizer production, obtained from phosphate-bearing rocks after processing. In biological systems, fluoride can substitute for hydroxyl in hydroxyapatite to form fluorapatite, which is more resistant to acid dissolution and is associated with dental enamel and bone mineral.

Notes: The fluorapatite–chlorapatite–hydroxyapatite system displays extensive solid solution, allowing variation in fluoride, chloride, and hydroxyl contents across natural samples.