Apatite

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Categories: Calcium minerals | Phosphate minerals | Halide minerals | Piezoelectric materials

Not to be confused with appetite, the desire to eat.

Apatite
Image:Apatite crystals.jpg
General
Category	Phosphate mineral group
Chemical formula	Ca₅(PO₄)₃(F,Cl,OH)
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]
Luster	Vitreous^[1] to subresinous
Polish luster	Vitreous^[1]
Refractive index	1.634 - 1.638 (+.012, -.006)^[1]
Optical Properties	Double refractive, uniaxial negative^[1]
Birefringence	.002-.008^[1]
Dispersion	.013^[1]
Pleochroism	Blue stones - strong, blue and yellow to colorless. Other colors are weak to very weak.^[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]
Streak	White
Specific gravity	3.16 - 3.22^[2]
Diaphaneity	Transparent to translucent^[2]

Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite, and chlorapatite, named for high concentrations of OH⁻, F⁻, or Cl⁻ ions, respectively, in the crystal. The formula of the admixture of the three most common endmembers is written as Ca₅(PO₄)₃(OH, F, Cl), and the formulae of the individual minerals are written as Ca₅(PO₄)₃(OH), Ca₅(PO₄)₃F and Ca₅(PO₄)₃Cl, respectively.

Apatite is one of few minerals that are produced and used by biological micro-environmental systems. Hydroxylapatite is the major component of tooth enamel. A relatively rare form of apatite in which most of the OH groups are absent and containing many carbonate and acid phosphate substitutions is a large component of bone material.

Fluorapatite (or fluoroapatite) is more resistant to acid attack than is hydroxyapatite. For this reason, toothpaste typically contain a source of fluoride anions (e.g. sodium fluoride, sodium monofluorophosphate). Similarly, fluoridated water allows exchange in the teeth of fluoride ions for hydroxyl groups in apatite. Too much fluoride results in dental fluorosis and/or skeletal fluorosis.

In the United States, apatite is often used to fertilize tobacco. It partially starves the plant of nitrogen, which gives American cigarettes a different taste from those of other countries.

Fission tracks in apatite are commonly used to determine the thermal history of orogenic (mountain) belts and of sediments in sedimentary basins.

Phosphorite is the name given to impure, massive apatite.

1 Gemology
2 See also
3 References
4 Further reading

Gemology

Apatite is infrequently used as a gemstone. Transparent stones of clean color have been faceted, and chatoyant specimens have been cabochon cut.^[1] Chatoyant stones are known as cat's-eye apatite,^[1] transparent green stones are known as asparagus stone,^[1] and blue stones have been called moroxite.^[3] Crystals of rutile may have grown in the crystal of apatite so when in the right light, the cut stone displays a cat's eye effect. Major sources for gem apatite are^[1] Brazil, Burma, and Mexico. Other sources include^[1] Canada, Czechoslovakia, Germany, India, Madagascar, Mozambique, Norway, South Africa, Spain, Sri Lanka, and the US.