Apatite

Not to be confused with appetite.

Apatite is a group of phosphate minerals with high concentrations of OH⁻, F⁻, Cl⁻, respectively, in the crystal.

Apatite group
General
Category	Phosphate mineral
Formula (repeating unit)	Ca5(PO4)3(F,Cl,OH)
Strunz classification	08.BN.05
Identification
Color	Transparent to translucent, usually green, less often colorless, yellow, blue to violet, pink, brown.[1]
Crystal habit	Tabular, prismatic crystals, massive, compact or granular
Crystal system	Hexagonal dipyramidal (6/m)[2]
Cleavage	[0001] indistinct, [1010] indistinct[2]
Fracture	Conchoidal to uneven[1]
Mohs scale hardness	5[1] (defining mineral)
Luster	Vitreous[1] to subresinous
Streak	White
Diaphaneity	Transparent to translucent[2]
Specific gravity	3.16–3.22[2]
Polish luster	Vitreous[1]
Optical properties	Double refractive, uniaxial negative[1]
Refractive index	1.634–1.638 (+0.012, −0.006)[1]
Birefringence	0.002–0.008[1]
Pleochroism	Blue stones – strong, blue and yellow to colorless. Other colors are weak to very weak.[1]
Dispersion	0.013[1]
Ultraviolet fluorescence	Yellow stones – purplish-pink, which is stronger in long wave; blue stones – blue to light-blue in both long and short wave; green stones – greenish-yellow, which is stronger in long wave; violet stones – greenish-yellow in long wave, light-purple in short wave.[1]

Apatite is characteristic of biological systems. It is one of a few minerals produced and used by biological micro-environmental systems. It hardness defines 5 on the Mohs scale. Hydroxyapatite is the main component of tooth enamel and bone mineral.

Much bone material is in a relatively rare form of apatite. In this form most of the OH groups are absent, and there are many carbonate and acid phosphate substitutions.

Fluorapatite (or fluoroapatite) is more resistant to acid attack than is hydroxyapatite. In the mid-20th century it was discovered that communities whose water supply naturally contained fluorine anions had lower rates of dental caries.^[3] Fluoridated water allows exchange in the teeth of fluorine ions for hydroxyl groups in apatite. Similarly, toothpaste often includes a source of fluoride (e.g. sodium fluoride, sodium monofluorophosphate).