apatite

[From Greek apatē, deceit (from its often being mistaken for other minerals).]

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The most abundant and widespread of the phosphate minerals, crystallizing in the hexagonal system. The apatite structure type includes no less than 10 mineral species and has the general formula X₅(YO₄)₃Z, where X is usually Ca²⁺ or Pb²⁺, Y is P⁵⁺ or As⁵⁺, and Z is F⁻, Cl⁻, or (OH)⁻. The apatite series takes X = Ca, whereas the pyromorphite series includes those members with X = Pb. Three end members form a complete solid-solution series involving the halide and hydroxyl anions. These are fluorapatite, Ca₅(PO₄)F₃; chlorapatite, Ca₅(PO₄)₃Cl; and hydroxyapatite, Ca₅(PO₄)₃(OH). Thus, the general series can be written Ca₅(PO₄)₃(F,Cl,OH), the fluoride member being the most frequent and often simply called apatite.

The apatite isomorphous series of minerals occurs as grains, blebs, or short to long hexagonal prisms terminated by pyramids, dipyramids, and the basal pinacoid. The minerals are transparent to opaque, and can be asparagus-green (asparagus stone), grayish-green, greenish-yellow, gray, brown, brownish-red, and more rarely violet, pink, or colorless. Apatites are brittle, with hardness 5 on Mohs scale, and specific gravity 3.1–3.2; they are also botryoidal, fibrous, and earthy.

Apatite occurs in nearly every rock type as an accessory mineral. It often crystallizes in regional and contact metamorphic rocks, especially in limestone and associated with chondrodite and phlogopite. It is very common in basic to ultrabasic rocks; enormous masses occur associated with nephelinesyenites in the Kola Peninsula, Russia, and constitute valuable ores which also contain rare-earth elements. Large beds of oolitic, pulverulent, and compact fine-grained carbonate-apatites occur as phosphate rock, phosphorites, or collophanes. Extensive deposits of this kind occur in the United States in Montana and Florida and in North Africa. The material is mined for fertilizer and for the manufacture of elemental phosphorus. See also Fertilizer; Phosphorus; Pyromorphite.

The inorganic mineral substance of teeth and bone. See also carbonate hydroxyapatite.

Environment

Plutonic rocks, pegmatite dikes, ore veins, bedded sedimentary deposits.

Often crystallized, with considerable variation in crystal habit: long-prismatic, short-prismatic, to tabular. Also in botryoidal crusts and in great massive beds.

Colorless, white, brown, green, violet, blue, or yellow. Luster glassy; hardness 5; specific gravity 3.1-3.2; fracture conchoidal; cleavage inconspicuous basal and prism. Brittle; transparent to translucent; sometimes fluorescent yellow-orange (manganapatite--to 10.5% Mn replacing Ca), and thermoluminescent blue-white; usually becomes fluorescent orange (longwave ultraviolet light) after strong heating.

Calcium fluophosphate or calcium chlorophosphate, or an intermediate (about 54.5% Ca, 41.5% P ₂ O ₅ , and about 4% F and Cl). There is so large a range of composition in the various apatites that they are labeled a group by some authorities. However, the examples usually called apatite are relatively constant in appearance and associations, and the mere fact that some vanadates, arsenates, or metallic phosphates have a like structure does not make them apatites. Ordinary apatite is not isomorphous with the far-afield species such as pyromorphite and mimetite.

Does not fuse, but chip held in the Bunsen burner flame melts on the edges, coloring the flame reddish yellow (calcium). Crushed and moistened with sulfuric acid gives green-white flame (phosphorus). Soluble in acids; fluorescent after heating (if not already so).

Crystals resemble beryl but can be distinguished by the hardness. Manganapatite resembles green tourmaline, but also is softer than that mineral, and is usually fluorescent. Herderite and beryllonite fuse.

Apatite is a common minor constituent of rocks, and is the source of the phosphorus required by plants. Specimens come from crystallized concentrations in pegmatites, in some ore veins, and in the form of the occasional rich masses of igneous segregations. Green manganapatite is a common mineral of the early stages of mineral formation in pegmatites; it occurs embedded in feldspar and quartz. Colorful short-prismatic and tabular apatite crystals form in cavities in cleavelandite in the late replacement phases of complex pegmatite formation. Apatite also forms good crystals in some ore veins, such as the violet crystals in the Ehrenfriedersdorf tin veins in Germany, and the gemmy yellow crystals associated with the Durango iron deposits, Mexico. Among the most abundant crystals are the yellow ones from Durango, Mexico, which can be 3 in. long (8 cm) and are often gemmy. Giant crystals of this type have been found near Copiapó, Chile, and in Brazil. The colorless, brilliant plates in the Austrian Tyrol reflect an alpine assemblage.

Entirely different in occurrence are the indigo blue apatites of Campo Formosa, Bahia, Brazil, and the large brown and green corroded crystals found in Ontario, embedded in flesh-colored calcite. These crystals are to 18 in. (40 cm) or more in length and may be several inches deep. Clear, gemmy, violet crystals to 1 in. (2.5 cm) across have been collected in some New England pegmatites, especially at Mt. Apatite, Maine. Granular beds of apatite that can be mined for fertilizer use are found in the Russian Kola Peninsula, and in Brazil. The apatites of Panasqueira, Portugal, are among the most attractive ones of open pockets. Bolivia yields fine colorless to violet crystals.

Bone has essentially an apatite composition and structure. Apatite has an interesting crystal symmetry often revealed in the smaller, shiny crystals by faces to the right or left of the horizontal axis unpaired with a corresponding face on the other side. These are known as third-order faces.

1. calcium phosphate; one of the two mineral constituents of bones and teeth.
2. fluorapatite; a naturally occurring rock mineral containing fluorine.

• a. calculi — see apatite urolith.

Tutor's tip: No matter how big your "appetite" (hunger) is, you still can't eat a mineral like "apatite."

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
Streak	White
Diaphaneity	Transparent to translucent[2]
Specific gravity	3.16 - 3.22[2]
Polish luster	Vitreous[1]
Optical properties	Double refractive, uniaxial negative[1]
Refractive index	1.634 - 1.638 (+.012, -.006)[1]
Birefringence	.002-.008[1]
Pleochroism	Blue stones - strong, blue and yellow to colorless. Other colors are weak to very weak.[1]
Dispersion	.013[1]
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.[1]

Apatite is a group of phosphate minerals, usually referring to hydroxyapatite, 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 few minerals that are produced and used by biological micro-environmental systems. Apatite has a Mohs 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 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.

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.

Contents 1 Uses 2 Gemology 3 Use as an ore mineral 4 Thermodynamics 5 See also 6 References 7 Further reading

Uses

Apatity City, Russia, 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.

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.^[3]

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. This is also the source of radioactive polonium found in cigarettes.^[4]

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.^[5] 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. It is also used in other countries.

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 ^[6]. This is preferable to traditional rare earth ores, as Apatite is non-radioactive ^[7] and does not pose an environmental hazard in mine tailings. Apatite is an ore mineral at the Hoidas Lake rare earth project^[8].

Thermodynamics

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 ^[9].

See also

Apatite Crystal, Mexico

• List of minerals
• Thermal history modelling

References

1. ^ ^{a b c d e f g h i j k l m n o p} Gemological Institute of America, GIA Gem Reference Guide 1995, ISBN 0-87311-019-6
2. ^ ^{a b c d} http://webmineral.com/data/Apatite.shtml Webmineral data
3. ^ Henderson and Marsden, "Lamps and Lighting", Edward Arnold Ltd, 1972, ISBN 0 7131 3267 1
4. ^ http://www.webspawner.com/users/radioactivethreat/
5. ^ Streeter, Edwin W., Precious Stones and Gems 6th edition, George Bell and Sons, London, 1898, p306
6. ^ 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
7. ^ 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
8. ^ Great Western Minerals Group Ltd. | Projects - Hoidas Lake, Saskatchewan
9. ^ Cruz, F.J.A.L.; Minas da Piedade, M.E.; Calado, J.C.G. "Standard molar enthalpies of formation of hydroxy-, chlor-, and bromapatite" J. Chem. Thermodyn. 37 (2005) 1061-1070

• Apatite on Mineral galleries

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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