Difference between revisions of "Apatite" - New World Encyclopedia
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| − | '''Apatite''' is the name given to a group of [[phosphate minerals]], usually referring to | + | '''Apatite''' is the name given to a group of [[phosphate minerals]], usually referring to '''hydroxylapatite''' (or '''hydroxyapatite'''), '''fluorapatite''' (or '''fluoroapatite'''), and '''chlorapatite''' (or '''chloroapatite'''). They are named for the presence of hydroxide ([[Hydroxyl|OH]]<sup>-</sup>), fluoride ([[Fluorine|F]]<sup>-</sup>), and chloride ([[Chlorine|Cl]]<sup>-</sup>) [[ion]]s, respectively, in the [[crystal]] lattice. These three forms of apatite are not readily distinguishable, as each specimen usually contains all three types of ions. Impure, massive apatite is called '''phosphorite'''. |
| + | |||
| + | Apatite is distributed widely in [[igneous]], [[metamorphic]], and [[sedimentary]] rocks, often in the form of small, cryptocrystalline fragments. It is usually green, but blue, yellow, purple, and brown varieties have also been found. The crystals range from transparent to translucent, with a vitreous to greasy luster. | ||
| + | |||
| + | * "Very gemmy crystals of apatite can be cut as gems but the softness of apatite prevents wide distribution or acceptance of apatite as a gemstone." | ||
In addition, phosphate, arsenate, and vanadate minerals with similar crystalline structures (hexagonal or pseudohexagonal monoclinic crystals) are known as the Apatite Group. This group includes minerals such as apatite, [[mimetite]], [[pyromorphite]], and [[vanadinite]]. | In addition, phosphate, arsenate, and vanadate minerals with similar crystalline structures (hexagonal or pseudohexagonal monoclinic crystals) are known as the Apatite Group. This group includes minerals such as apatite, [[mimetite]], [[pyromorphite]], and [[vanadinite]]. | ||
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== Characteristics == | == Characteristics == | ||
| − | The chemical formulas | + | The overall chemical formula for apatite is generally given as [[Calcium|Ca]]<sub>5</sub>([[Phosphate|PO<sub>4</sub>]])<sub>3</sub>(OH, F, Cl). The formulas for the three common species may be written as: |
* Hydroxylapatite: Ca<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>(OH) | * Hydroxylapatite: Ca<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>(OH) | ||
* Fluoroapatite: Ca<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>F | * Fluoroapatite: Ca<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>F | ||
* Chlorapatite: Ca<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>Cl | * Chlorapatite: Ca<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>Cl | ||
| − | |||
| − | + | Apatite has a hardness of 5 on the [[Mohs scale]], and its [[specific gravity]] is between 3.1 and 3.2. Its crystals belong to the hexagonal crystal system, and the crystal habit is typically hexagonal prism, terminating with a hexagonal pyramid or pinacoid shape. In addition, apatite may occur in acicular (needle-like), granular, reniform, and massive forms. | |
| − | + | == Occurrence == | |
| − | + | '''Biological:''' | |
| + | Apatite is one of few minerals that are produced and used by biological micro-environmental systems. Hydroxylapatite is the main component of [[tooth enamel]]. A relatively unique form of apatite—in which most OH groups are absent and containing many carbonate and acid phosphate substitutions—is a large component of [[bone]] material. | ||
| + | |||
| + | '''Mineralogical:''' | ||
| + | In mineral form, noteworthy areas of occurrence include Bancroft, Ontario; Durango, Mexico; Germany; and Russia. | ||
| − | [[ | + | == Hydroxylapatite == |
| + | [[Image:Mineraly.sk - hydroxylapatit.jpg|right|thumb|250px|A sample of hydroxylapatite]] | ||
| − | + | Hydroxylapatite is the [[hydroxyl]] endmember of the apatite group. The OH<sup>-</sup> [[ion]] can be replaced by [[fluorine|fluoride]], [[chlorine|chloride]] or [[carbonate]]. As noted above, its formula may be written as Ca<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>(OH). The formula may also be written as Ca<sub>10</sub>(PO<sub>4</sub>)<sub>6</sub>(OH)<sub>2</sub>, to indicate that each crystal unit cell combines two molecules. | |
| − | + | Purified hydroxylapatite powder is white. Naturally occurring forms can also be brown, yellow, or green. | |
| − | + | Hydroxylapatite is the main mineral component of [[osseous tissue|bone]]. Hydroxylapatite that is deficient in carbonated calcium is the main constituent of dental enamel and dentin. | |
== Fluoroapatite == | == Fluoroapatite == | ||
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|} | |} | ||
| − | Fluoroapatite is a | + | Fluoroapatite is a hard crystalline solid that may be classified as a calcium halophosphate. The pure mineral is colorless, but naturally occurring samples can have various colors, such as green, brown, blue, or violet. It is an important constituent of [[tooth enamel]]. It is often combined as a solid solution with [[hydroxylapatite]] (Ca<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>OH) in biological matrices. |
| − | |||
| − | |||
Fluoroapatite can be synthesized in a two step process. First, [[calcium phosphate]] is generated by combining calcium and phosphate [[salt]]s at neutral [[pH]].This material then reacts further with fluoride sources (often [[sodium monofluorophosphate]] or [[calcium fluoride]] (CaF<sub>2</sub>)) to give the mineral. This reaction is integral in the global [[phosphorous cycle]].<ref>Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.</ref> | Fluoroapatite can be synthesized in a two step process. First, [[calcium phosphate]] is generated by combining calcium and phosphate [[salt]]s at neutral [[pH]].This material then reacts further with fluoride sources (often [[sodium monofluorophosphate]] or [[calcium fluoride]] (CaF<sub>2</sub>)) to give the mineral. This reaction is integral in the global [[phosphorous cycle]].<ref>Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.</ref> | ||
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:3 Ca<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub> + CaF<sub>2</sub> → 2 Ca<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>F | :3 Ca<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub> + CaF<sub>2</sub> → 2 Ca<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>F | ||
| − | Fluoroapatite can also be used as a precursor for the production of [[phosphorus]]. | + | Fluoroapatite can also be used as a precursor for the production of [[phosphorus]]. The mineral can be reduced by [[carbon]] in the presence of [[quartz]], ultimately generating [[white phosphorus]], P<sub>4</sub>: |
:Ca<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>F + 3SiO<sub>2</sub> + 5C → 3CaSiO<sub>3</sub> + 5CO + P<sub>2</sub> | :Ca<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>F + 3SiO<sub>2</sub> + 5C → 3CaSiO<sub>3</sub> + 5CO + P<sub>2</sub> | ||
2P<sub>2</sub> → P<sub>4</sub> after cooling | 2P<sub>2</sub> → P<sub>4</sub> after cooling | ||
| − | == | + | == Applications == |
| − | [[ | + | |
| + | [[Fluorapatite]] (or fluoroapatite) is more resistant to acid attack than is hydroxyapatite. For this reason, toothpastes typically contain a source of fluoride anions (such as sodium fluoride or [[sodium monofluorophosphate]]), allowing for the exchange of fluoride ions for hydroxy groups in the apatite in [[tooth|teeth]]. [[Fluoridated water]] has a similar effect. Too much fluoride, however, results in [[dental fluorosis]] or [[skeletal fluorosis]]. | ||
| + | |||
| + | In the United States, apatite is often used to fertilize [[tobacco]]. It partially starves the plant of nitrogen, which gives American [[cigarette]]s a different taste from those of other countries. | ||
| − | + | [[Fission track dating|Fission tracks]] in apatite are commonly used to determine the thermal history of orogenic (mountain-forming) belts and sediments in [[sedimentary basin]]s. | |
| − | + | ===Medical uses of hydroxylapatite=== | |
| − | |||
Hydroxylapatite can be used as a filler to replace amputated bone or as a coating to promote bone ingrowth into [[prosthesis|prosthetic]] implants. Although many other [[phase (matter)|phase]]s exist with similar or even identical chemical makeup, the body responds much differently to them. [[Coral]] skeletons can be transformed into hydroxylapatite by high temperatures; their porous structure allows relatively rapid ingrowth at the expense of initial mechanical strength. The high temperature also burns away any organic molecules such as [[protein]]s, preventing [[Transplant_rejection|host vs. graft]] disease. | Hydroxylapatite can be used as a filler to replace amputated bone or as a coating to promote bone ingrowth into [[prosthesis|prosthetic]] implants. Although many other [[phase (matter)|phase]]s exist with similar or even identical chemical makeup, the body responds much differently to them. [[Coral]] skeletons can be transformed into hydroxylapatite by high temperatures; their porous structure allows relatively rapid ingrowth at the expense of initial mechanical strength. The high temperature also burns away any organic molecules such as [[protein]]s, preventing [[Transplant_rejection|host vs. graft]] disease. | ||
Some modern [[dental implants]] are coated with hydroxylapatite. It has been suggested that this may promote [[osseointegration]], but there is not yet conclusive clinical proof of this. | Some modern [[dental implants]] are coated with hydroxylapatite. It has been suggested that this may promote [[osseointegration]], but there is not yet conclusive clinical proof of this. | ||
| − | Bioactive | + | Bioactive glasses are the only manmade materials known to bond to both bone and soft tissue and have been clinically used as a bone grafting material for over 20 years in dental, maxillofacial, and orthopedic procedures. The material has been used in both solid form, as a middle ear prosthetic for conducted hearing loss, as well as in particulate form for filling boney defects throughout the body. Unlike hydroxyapatite, which is said to be “osteoconductive” by conducting new bone growth along the materials surface, bioactive glasses are “osteostimulative” in that the material stimulates the recruitment and differentiation of osteoblasts—cells that produce new bone. As a result, bioactive glass rapidly enhances the production of new bone and is completely resorbed by the body and replaced by new bone. Bioactive glasses have also been used in oral care applications as a tooth remineralizer [[(calcium sodium phosphosilicate)]] in both professional dental and consumer oral care products. |
| − | Bioactive | ||
===Hydroxyapatite uses in chromatography=== | ===Hydroxyapatite uses in chromatography=== | ||
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The mechanism of Hydroxyapatite (HAP) [[chromatography]] is complicated and has been described as "mixed-mode" ion exchange. It involves nonspecific interactions between positively charged calcium ions and negatively charged phosphate ions on the stationary phase HAP resin with protein negatively charged carboxyl groups and positively charged amino groups. It may be difficult to predict the effectiveness of HAP chromatography based on physical and chemical properties of the desired protein to be purified. For elution, a buffer with increasing phosphate concentration is typically used. | The mechanism of Hydroxyapatite (HAP) [[chromatography]] is complicated and has been described as "mixed-mode" ion exchange. It involves nonspecific interactions between positively charged calcium ions and negatively charged phosphate ions on the stationary phase HAP resin with protein negatively charged carboxyl groups and positively charged amino groups. It may be difficult to predict the effectiveness of HAP chromatography based on physical and chemical properties of the desired protein to be purified. For elution, a buffer with increasing phosphate concentration is typically used. | ||
| − | ==Gemology== | + | ===Gemology=== |
Apatite is infrequently used as a [[gemstone]]. Transparent stones of clean color have been faceted, and [[chatoyant]] specimens have been [[cabochon]] cut.<ref name="GRG"/> Chatoyant stones are known as ''cat's-eye apatite'',<ref name="GRG"/> transparent green stones are known as ''asparagus stone'',<ref name="GRG"/> and blue stones may be called ''[[moroxite]]''.<ref>Edwin W. Streeter, [http://www.farlang.com/gemstones/streeter-precious-stones/page_306/view?searchterm=moroxite Moroxite] ''Precious Stones and Gems'' 6th ed. (London: George Bell and Sons, 1898, p. 306). Retrieved May 8, 2007.</ref> If crystals of rutile have grown in the apatite crystal, the cut stone displays a cat's eye effect when viewed in the right lighting. | Apatite is infrequently used as a [[gemstone]]. Transparent stones of clean color have been faceted, and [[chatoyant]] specimens have been [[cabochon]] cut.<ref name="GRG"/> Chatoyant stones are known as ''cat's-eye apatite'',<ref name="GRG"/> transparent green stones are known as ''asparagus stone'',<ref name="GRG"/> and blue stones may be called ''[[moroxite]]''.<ref>Edwin W. Streeter, [http://www.farlang.com/gemstones/streeter-precious-stones/page_306/view?searchterm=moroxite Moroxite] ''Precious Stones and Gems'' 6th ed. (London: George Bell and Sons, 1898, p. 306). Retrieved May 8, 2007.</ref> If crystals of rutile have grown in the apatite crystal, the cut stone displays a cat's eye effect when viewed in the right lighting. | ||
Revision as of 22:03, 23 May 2007
| Apatite | |
|---|---|
 |
|
| 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 |
| Refractive index | 1.634 - 1.638 (+.012, -.006)[1] |
| Optical Properties | Double refractive, uniaxial negative[1] |
| Birefringence | .002-.008[1] |
| Pleochroism | Blue stones - strong, blue and yellow to colorless. Other colors are weak to very weak.[1] |
| Streak | White |
| Specific gravity | 3.16 - 3.22[2] |
| Diaphaneity | Transparent to translucent[2] |
Apatite is the name given to a group of phosphate minerals, usually referring to hydroxylapatite (or hydroxyapatite), fluorapatite (or fluoroapatite), and chlorapatite (or chloroapatite). They are named for the presence of hydroxide (OH-), fluoride (F-), and chloride (Cl-) ions, respectively, in the crystal lattice. These three forms of apatite are not readily distinguishable, as each specimen usually contains all three types of ions. Impure, massive apatite is called phosphorite.
Apatite is distributed widely in igneous, metamorphic, and sedimentary rocks, often in the form of small, cryptocrystalline fragments. It is usually green, but blue, yellow, purple, and brown varieties have also been found. The crystals range from transparent to translucent, with a vitreous to greasy luster.
- "Very gemmy crystals of apatite can be cut as gems but the softness of apatite prevents wide distribution or acceptance of apatite as a gemstone."
In addition, phosphate, arsenate, and vanadate minerals with similar crystalline structures (hexagonal or pseudohexagonal monoclinic crystals) are known as the Apatite Group. This group includes minerals such as apatite, mimetite, pyromorphite, and vanadinite.
Etymology
The name apatite is derived from a Greek word that means "to deceive," because it appears similar to other minerals, particularly olivine, beryl, and peridot.
Characteristics
The overall chemical formula for apatite is generally given as Ca5(PO4)3(OH, F, Cl). The formulas for the three common species may be written as:
- Hydroxylapatite: Ca5(PO4)3(OH)
- Fluoroapatite: Ca5(PO4)3F
- Chlorapatite: Ca5(PO4)3Cl
Apatite has a hardness of 5 on the Mohs scale, and its specific gravity is between 3.1 and 3.2. Its crystals belong to the hexagonal crystal system, and the crystal habit is typically hexagonal prism, terminating with a hexagonal pyramid or pinacoid shape. In addition, apatite may occur in acicular (needle-like), granular, reniform, and massive forms.
Occurrence
Biological: Apatite is one of few minerals that are produced and used by biological micro-environmental systems. Hydroxylapatite is the main component of tooth enamel. A relatively unique form of apatite—in which most OH groups are absent and containing many carbonate and acid phosphate substitutions—is a large component of bone material.
Mineralogical: In mineral form, noteworthy areas of occurrence include Bancroft, Ontario; Durango, Mexico; Germany; and Russia.
Hydroxylapatite
Hydroxylapatite is the hydroxyl endmember of the apatite group. The OH- ion can be replaced by fluoride, chloride or carbonate. As noted above, its formula may be written as Ca5(PO4)3(OH). The formula may also be written as Ca10(PO4)6(OH)2, to indicate that each crystal unit cell combines two molecules.
Purified hydroxylapatite powder is white. Naturally occurring forms can also be brown, yellow, or green.
Hydroxylapatite is the main mineral component of bone. Hydroxylapatite that is deficient in carbonated calcium is the main constituent of dental enamel and dentin.
Fluoroapatite
| Fluoroapatite | |
|---|---|
| |
| General | |
| Systematic name | Fluoroapatite |
| Other names | Fluorapatite |
| Molecular formula | Ca5(PO4)3F |
| Molar mass | 504.3 g/mol |
| Appearance | hard solid, various colors |
| CAS number | 68877-08-7 |
| Properties | |
| Solubility in water | almost insoluble |
| Structure | |
| Crystal structure | hexagonal |
| Related compounds | |
| Related compounds | Ca5(PO4)3OH Ca5(PO4)3Cl |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references | |
Fluoroapatite is a hard crystalline solid that may be classified as a calcium halophosphate. The pure mineral is colorless, but naturally occurring samples can have various colors, such as green, brown, blue, or violet. It is an important constituent of tooth enamel. It is often combined as a solid solution with hydroxylapatite (Ca5(PO4)3OH) in biological matrices.
Fluoroapatite can be synthesized in a two step process. First, calcium phosphate is generated by combining calcium and phosphate salts at neutral pH.This material then reacts further with fluoride sources (often sodium monofluorophosphate or calcium fluoride (CaF2)) to give the mineral. This reaction is integral in the global phosphorous cycle.[3]
- 3Ca2+ + 2PO43- → Ca3(PO4)2
- 3 Ca3(PO4)2 + CaF2 → 2 Ca5(PO4)3F
Fluoroapatite can also be used as a precursor for the production of phosphorus. The mineral can be reduced by carbon in the presence of quartz, ultimately generating white phosphorus, P4:
- Ca5(PO4)3F + 3SiO2 + 5C → 3CaSiO3 + 5CO + P2
2P2 → P4 after cooling
Applications
Fluorapatite (or fluoroapatite) is more resistant to acid attack than is hydroxyapatite. For this reason, toothpastes typically contain a source of fluoride anions (such as sodium fluoride or sodium monofluorophosphate), allowing for the exchange of fluoride ions for hydroxy groups in the apatite in teeth. Fluoridated water has a similar effect. Too much fluoride, however, results in dental fluorosis 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-forming) belts and sediments in sedimentary basins.
Medical uses of hydroxylapatite
Hydroxylapatite can be used as a filler to replace amputated bone or as a coating to promote bone ingrowth into prosthetic implants. Although many other phases exist with similar or even identical chemical makeup, the body responds much differently to them. Coral skeletons can be transformed into hydroxylapatite by high temperatures; their porous structure allows relatively rapid ingrowth at the expense of initial mechanical strength. The high temperature also burns away any organic molecules such as proteins, preventing host vs. graft disease.
Some modern dental implants are coated with hydroxylapatite. It has been suggested that this may promote osseointegration, but there is not yet conclusive clinical proof of this.
Bioactive glasses are the only manmade materials known to bond to both bone and soft tissue and have been clinically used as a bone grafting material for over 20 years in dental, maxillofacial, and orthopedic procedures. The material has been used in both solid form, as a middle ear prosthetic for conducted hearing loss, as well as in particulate form for filling boney defects throughout the body. Unlike hydroxyapatite, which is said to be “osteoconductive” by conducting new bone growth along the materials surface, bioactive glasses are “osteostimulative” in that the material stimulates the recruitment and differentiation of osteoblasts—cells that produce new bone. As a result, bioactive glass rapidly enhances the production of new bone and is completely resorbed by the body and replaced by new bone. Bioactive glasses have also been used in oral care applications as a tooth remineralizer (calcium sodium phosphosilicate) in both professional dental and consumer oral care products.
Hydroxyapatite uses in chromatography
The mechanism of Hydroxyapatite (HAP) chromatography is complicated and has been described as "mixed-mode" ion exchange. It involves nonspecific interactions between positively charged calcium ions and negatively charged phosphate ions on the stationary phase HAP resin with protein negatively charged carboxyl groups and positively charged amino groups. It may be difficult to predict the effectiveness of HAP chromatography based on physical and chemical properties of the desired protein to be purified. For elution, a buffer with increasing phosphate concentration is typically used.
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 may be called moroxite.[4] If crystals of rutile have grown in the apatite crystal, the cut stone displays a cat's eye effect when viewed in the right lighting.
Major sources[1] for gem-quality apatite are: Brazil, Burma, and Mexico. Additional sources include: Canada, Czechoslovakia, Germany, India, Madagascar, Mozambique, Norway, South Africa, Spain, Sri Lanka, and the United States.
See also
Notes
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 GIA Gem Reference Guide (Carlsbad, CA: Gemological Institute of America, 1995), ISBN 0-87311-019-6.
- ↑ 2.0 2.1 2.2 2.3 Apatite Mineral Data. Webmineral.com. Retrieved May 8, 2007.
- ↑ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
- ↑ Edwin W. Streeter, Moroxite Precious Stones and Gems 6th ed. (London: George Bell and Sons, 1898, p. 306). Retrieved May 8, 2007.
ReferencesISBN links support NWE through referral fees
- Farndon, John. 2006. The Practical Encyclopedia of Rocks & Minerals: How to Find, Identify, Collect and Maintain the World's best Specimens, with over 1000 Photographs and Artworks. London: Lorenz Books. ISBN 0754815412.
- Klein, Cornelis, and Barbara Dutrow. 2007. Manual of Mineral Science. 23rd ed. New York: John Wiley. ISBN 978-0471721574.
- Pellant, Chris. 2002. Rocks and Minerals. Smithsonian Handbooks. New York: Dorling Kindersley. ISBN 0789491060.
- Shaffer, Paul R., Herbert S. Zim, and Raymond Perlman. 2001. Rocks, Gems and Minerals. Rev. ed. New York: St. Martin's Press. ISBN 1582381321.
- Mineral Gallery. 2006. The Mineral Apatite. Amethyst Galleries. Retrieved May 8, 2007.
- Mineral Gallery. 2006. The Apatite Group of Minerals. Amethyst Galleries. Retrieved May 8, 2007.
External links
- Apatite. Mindat.org. Retrieved May 8, 2007.
- Apatite Group. Mindat.org. Retrieved May 8, 2007.
- Hydroxylapatite. Mindat.org. Retrieved May 8, 2007.
- Apatite Mineral Data. Webmineral.com. Retrieved May 8, 2007.
- Hydroxylapatite Mineral Data. Webmineral.com. Retrieved May 8, 2007.
- Fluoroapatite Mineral Data. Webmineral.com. Retrieved May 8, 2007.
- Hydroxyapatite. Azom.com. Retrieved May 8, 2007.
- Hydroxylapatite. Center for Advanced Microstructures and Devices, Louisiana State University. Retrieved May 8, 2007.
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