Apatite

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Apatite
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General
CategoryPhosphate mineral group
Chemical formulaCa5(PO4)3(F,Cl,OH)
Identification
ColorTransparent to translucent, usually green, less often colorless, yellow, blue to violet, pink, brown.[1]
Crystal habitTabular, prismatic crystals, massive, compact or granular
Crystal systemHexagonal Dipyramidal (6/m)[2]
Cleavage[0001] Indistinct, [1010] Indistinct[2]
FractureConchoidal to uneven[1]
Mohs Scale hardness5[1]
LusterVitreous[1] to subresinous
Polish lusterVitreous[1]
Refractive index1.634 - 1.638 (+.012, -.006)[1]
Optical PropertiesDouble refractive, uniaxial negative[1]
Birefringence.002-.008[1]
Dispersion.013[1]
PleochroismBlue stones - strong, blue and yellow to colorless. Other colors are weak to very weak.[1]
Ultraviolet fluorescenceYellow 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]
StreakWhite
Specific gravity3.16 - 3.22[2]
DiaphaneityTransparent 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. (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.

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.

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 [4]. This is preferable to traditional rare earth ores, as Apatite is non-radioactive [5] and does not pose an environmental hazard in mine tailings. Apatite is an ore mineral at the Hoidas Lake rare earth project[6].

See also

File:Apatite09.jpg
Apatite Crystal, Mexico

References

  1. 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 Gemological Institute of America, GIA Gem Reference Guide 1995, ISBN 0-87311-019-6
  2. 2.0 2.1 2.2 2.3 http://webmineral.com/data/Apatite.shtml Webmineral data
  3. Streeter, Edwin W., Precious Stones and Gems 6th edition, George Bell and Sons, London, 1898, p306
  4. Salvi S, Williams‐Jones A. 2004. Alkaline granite‐syenite deposits. In Linnen RL, Samson IM, editors. Rare element geochemistry and mineral deposits. St. Catharines (ON): Geological Association of Canada. pp. 315‐341
  5. Haxel G, Hedrick J, Orris J. 2006. Rare earth elements critical resources for high technology. Reston (VA): United States Geological Survey. USGS Fact Sheet: 087‐02. Available online from the USGS at http://pubs.usgs.gov/fs/2002/fs087-02/fs087-02.pdf
  6. http://www.gwmg.ca/projects/hoidas-lake

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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