Apatite General Category Phosphate mineral group Chemical formula 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  indistinct,  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, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br− 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,Br)2, and the crystal unit cell formulae of the individual minerals are written as Ca10(PO4)6(OH)2, Ca10(PO4)6(F)2, Ca10(PO4)6(Cl)2 and Ca10(PO4)6(Br)2.
Apatite is one of a few minerals that are 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. For this reason, toothpaste typically contains 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.
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.
The primary use of apatite is in the manufacture of fertilizer – it is a source of phosphorus. It is occasionally used as a gemstone.
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. Now almost entirely replaced by the Tri-Phosphor system.
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. 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 Brazil, Burma, and Mexico. Other sources include Canada, Czechoslovakia, Germany, India, Madagascar, Mozambique, Norway, South Africa, Spain, Sri Lanka, and the United States.
Use as an ore mineral
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. Except some apatite in Florida used to produce phosphate for U.S. tobacco crops contains uranium, radium, lead 210 and polonium 210 and radon.
The standard (p = 0.1 MPa) molar enthalpies of formation in the crystalline state of hydroxyapatite, chlorapatite and a preliminary value for bromapatite, at T = 298.15 K, have already 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.
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.
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