Apatite

Not to be confused with appetite.

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.
Crystal habit	Tabular, prismatic crystals, massive, compact or granular
Crystal system	Hexagonal dipyramidal (6/m)
Cleavage	[0001] indistinct, [1010] indistinct
Fracture	Conchoidal to uneven
Mohs scale hardness	5 (defining mineral)
Luster	Vitreous to subresinous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	3.16–3.22
Polish luster	Vitreous
Optical properties	Double refractive, uniaxial negative
Refractive index	1.634–1.638 (+0.012, −0.006)
Birefringence	0.002–0.008
Pleochroism	Blue stones – strong, blue and yellow to colorless. Other colors are weak to very weak.
Dispersion	0.013
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.

Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite and chlorapatite, with high concentrations of OH⁻, F⁻ and Cl⁻ ions, respectively, in the crystal. The formula of the admixture of the four most common endmembers is written as Ca₁₀(PO₄)₆(OH,F,Cl)₂, and the crystal unit cell formulae of the individual minerals are written as Ca₁₀(PO₄)₆(OH)₂, Ca₁₀(PO₄)₆(F)₂ and Ca₁₀(PO₄)₆(Cl)₂.

The mineral was named apatite by the German geologist Abraham Gottlob Werner in 1786, although the specific mineral he had described was reclassified as fluorapatite in 1860 by the German mineralogist Karl Friedrich August Rammelsberg. Apatite is a mineral that is often mistaken for other minerals. This tendency is reflected in the mineral's name, which is derived from the Greek word απατείν (apatein), which means to deceive or to be misleading.

Apatite is one of a few minerals produced and used by biological micro-environmental systems. Apatite is the defining mineral for 5 on the Mohs scale. Hydroxyapatite, also known as hydroxylapatite, is the major component of tooth enamel and bone mineral. 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; in the mid-20th century, it was discovered that communities whose water supply naturally contained fluorine had lower rates of dental caries. Fluoridated water allows exchange in the teeth of fluoride ions for hydroxyl groups in apatite. Similarly, toothpaste typically contains a source of fluoride anions (e.g. sodium fluoride, sodium monofluorophosphate). Too much fluoride results in dental fluorosis and/or skeletal fluorosis.

Fission tracks in apatite are commonly used to determine the thermal history of orogenic (mountain) belts and of sediments in sedimentary basins. (U-Th)/He dating of apatite is also well established for use in determining thermal histories and other, less typical applications such as paleo-wildfire dating.

Phosphorite is a phosphate-rich sedimentary rock, that contains between 18% and 40% P₂O₅. The apatite in phosphorite is present as cryptocrystalline masses referred to as collophane.

Uses

Apatity, Russia, a site of apatite mines and processing facilities

The primary use of apatite is in the manufacture of fertilizer – it is a source of phosphorus. It is occasionally used as a gemstone. Green and blue varieties, in finely divided form, are pigments with excellent covering power.

During digestion of apatite with sulfuric acid to make phosphoric acid, hydrogen fluoride is produced as a byproduct from any fluorapatite content. This byproduct is a minor industrial source of hydrofluoric acid.

Fluoro-chloro apatite forms the basis of the now obsolete Halophosphor fluorescent tube phosphor system. Dopant elements of manganese and antimony, at less than one mole-percent, in place of the calcium and phosphorus impart the fluorescence, and adjustment of the fluorine-to-chlorine ratio adjusts the shade of white produced. This system has been almost entirely replaced by the Tri-Phosphor system.

In the United States, apatite-derived fertilizers are used to supplement the nutrition of many agricultural crops by providing a valuable source of phosphate.

Apatites are also a proposed host material for storage of nuclear waste, along with other phosphates.

Gemology

Faceted blue apatite, Brasil

Apatite is infrequently used as a gemstone. Transparent stones of clean color have been faceted, and chatoyant specimens have been cabochon-cut. Chatoyant stones are known as cat's-eye apatite, transparent green stones are known as asparagus stone, and blue stones have been called moroxite. If crystals of rutile have grown in the crystal of apatite, in the right light the cut stone displays a cat's-eye effect. Major sources for gem apatite are Brazil, Burma, and Mexico. Other sources include Canada, Czech Republic, Germany, India, Madagascar, Mozambique, Norway, South Africa, Spain, Sri Lanka, and the United States.

Use as an ore mineral

Apatite crystal, Mexico

Apatite is occasionally found to contain significant amounts of rare-earth elements and can be used as an ore for those metals. This is preferable to traditional rare-earth ores, as apatite is non-radioactive and does not pose an environmental hazard in mine tailings. However, some apatite in Florida used to produce phosphate for agriculture does contain uranium, radium, lead-210, polonium-210, and radon.

Apatite is an ore mineral at the Hoidas Lake rare-earth project.

Thermodynamics

The standard enthalpies of formation in the crystalline state of hydroxyapatite, chlorapatite and a preliminary value for bromapatite, have been determined by reaction-solution calorimetry. Speculations on the existence of a possible fifth member of the calcium apatites family, iodoapatite, have been drawn from energetic considerations.

Structural and thermodynamic properties of crystal hexagonal calcium apatites, Ca10(PO4)6(X)2 (X= OH, F, Cl, Br), have been investigated using an all-atom Born-Huggins-Mayer potential by a molecular dynamics technique. The accuracy of the model at room temperature and atmospheric pressure was checked against crystal structural data, with maximum deviations of ca. 4% for the haloapatites and 8% for hydroxyapatite. High-pressure simulation runs, in the range 0.5-75 kbar, were performed in order to estimate the isothermal compressibility coefficient of those compounds. The deformation of the compressed solids is always elastically anisotropic, with BrAp exhibiting a markedly different behavior from those displayed by HOAp and ClAp. High-pressure p-V data were fitted to the Parsafar-Mason equation of state with an accuracy better than 1%.

The monoclinic solid phases Ca10(PO4)6(X)2 (X= OH, Cl) and the molten hydroxyapatite compound have also been studied by molecular dynamics.

Lunar science

Moon rocks collected by astronauts during the Apollo program contain traces of apatite. Re-analysis of these samples in 2010 revealed water trapped in the mineral as hydroxyl, leading to estimates of water on the lunar surface at a rate of at least 64 parts per billion – 100 times greater than previous estimates – and as high as 5 parts per million. If the minimum amount of mineral-locked water was hypothetically converted to liquid, it would cover the Moon's surface in roughly one meter of water.

Bio-leaching

The ectomycorrhizal fungi Suillus granulatus and Paxillus involutus can release elements from apatite.

See also

• List of minerals
• Thermal history modelling
• Hexafluorosilicic acid
• Hydroxylapatite in bone