apatite

apatite

[ap-uh-tahyt]
apatite, mineral, a phosphate of calcium containing chlorine or fluorine, or both, that is transparent to opaque in shades of green, brown, yellow, white, red, and purple. Apatite is a minor constituent in igneous and metamorphic rocks. Yellow-green asparagus stone and blue-green manganapatite are used in jewelry. Apatite is mined to make phosphatic fertilizers and is used in fission track dating of rocks (see dating). Commercial deposits are mined in Idaho, Tennessee, and Wyoming, and in N Africa and Russia.

A member of the phosphate group of minerals, the world's major source of phosphorus, found as variously coloured, glassy crystals, masses, or nodules. Much of it has a chemical composition approximating Ca5(PO4)3(F,Cl,OH). If not for its softness, apatite would be a popular gemstone; some of the material found is clear, but it is fragile and difficult to cut and polish.

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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. Apatite has a Moh's Scale hardness of 5. 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. (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

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. The primary use of apatite is in the manufacture of fertilizer - it is a source of phosphorus. It is occasionally used as a gemstone.

Gemology

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 US.

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

See also

References

Further reading

  • Schmittner Karl-Erich and Giresse Pierre, 1999. Micro-environmental controls on biomineralization: superficial processes of apatite and calcite precipitation in Quaternary soils, Roussillon, France. Sedimentology 46/3: 463-476.

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