Fluorapatite

Fluorapatite is the most common member of the apatite series, a group of phosphates that also includes chlorapatite and hydroxyapatite. Its formula Ca₅(PO₄)₃F reflects a hexagonal crystal structure where calcium ions occupy two distinct crystallographic sites, while phosphate groups form the supporting structure and fluorine ions are positioned in the axial channels. This structural arrangement confers on fluorapatite remarkable thermal and chemical stability, making it particularly resistant to alteration.

Fluorapatite forms in an extraordinary variety of geological environments: in granitic pegmatites as a primary accessory phase, in alkaline igneous rocks, in biochemical sedimentary phosphate deposits, and even in some hydrothermal environments. Biologically, it is the principal mineral constituent of dental enamel (96% by weight) and the inorganic phase of bones, where its chemical stability is fundamental for protecting teeth from acidity. Fluorapatite crystals are often colorless or slightly colored (yellow, green, blue) due to trace elements such as manganese or iron, and can reach significant sizes in high-quality pegmatites.

Fluorapatite — Ca₅(PO₄)₃F; apatite group, primary phosphates. Crystal system: hexagonal, space group P63/m, lattice parameters a ≈ 9.37 Å, c ≈ 6.88 Å. Mohs hardness: 5. Density: 3.15–3.20 g/cm³. Cleavage: absent; fracture: conchoidal. Luster: vitreous. Color: colorless, white, yellow, green, blue (due to trace element inclusions). Transparency: transparent to translucent. Refractive index: nω ≈ 1.602–1.607, nε ≈ 1.633–1.638 (birefringent, uniaxial positive). Dispersion: moderate. Pleochroism: weak or absent. Raman spectroscopy: characteristic phosphate bands around 960 cm⁻¹ (PO₄ stretching), 1050 cm⁻¹ (asymmetric stretching). IR spectroscopy: intense absorptions at 1090–1020 cm⁻¹ (PO₄³⁻), 600–500 cm⁻¹ (PO₄ bending). UV fluorescence: generally inert, occasionally weakly fluorescent in blue-green. Thermal analysis: stable up to 1200 °C; decomposition begins above this temperature. Solubility: insoluble in water; slightly soluble in dilute acids. Common mineral associations: alkali feldspars, quartz, mica, beryl, tourmaline (in pegmatites); with green apatite, monazite, xenotime (in alkaline igneous rocks); with calcite, dolomite, silica (in phosphatic sediments).