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Fluorapatite

Fluorapatite

General
Category	Phosphate mineral
Chemical formula	Ca₅(PO₄)₃F
Identification
Color	Sea-green, violet, purple, blue, pink, yellow, brown, white, colorless, may be zoned
Crystal habit	Massive to prismatic cyrstaliine
Crystal system	Hexagonal - dipyramidal
Twinning	Contact twins rare
Cleavage	Indistinct
Fracture	Brittle to conchoidal
Mohs Scale hardness	5
Luster	Vitreous, resinous to dull
Refractive index	nω = 1.631 - 1.650 nε = 1.633 - 1.646
Optical Properties	Uniaxial (-)
Birefringence	δ = 0.002
Ultraviolet fluorescence	Fluorescent and phosphorescent.
Streak	White
Specific gravity	3.1 to 3.2
Diaphaneity	Transparent to Opaque
References	^[1]^[2]^[3]

Fluorapatite, often with the alternate spelling of fluoroapatite, is a mineral with the formula Ca₅(PO₄)₃F (calcium halophosphate). Fluorapatite is a hard crystalline solid. Although samples can have various color (green, brown, blue, violet, or colorless), the pure mineral is colorless as expected for a material lacking transition metals. It is an important constituent of tooth enamel.^[4]

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Fluorapatite crystallizes in a hexagonal crystal system. It is often combined as a solid solution with hydroxylapatite (Ca₅(PO₄)₃OH) in biological matrices. Chloroapatite (Ca₅(PO₄)₃Cl) is another related structure.^[4]

Fluorapatite is the most common phosphate mineral. It occurs widely as an accessory mineral in igneous rocks and in calcium rich metamorphic rocks. It commonly occurs as a detrital or diagenic mineral in sedimentary rocks and is an essential component of phosphorite ore deposits. It occurs as a residual mineral in lateritic soils.^[1]

Synthesis

Fluorapatite 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 phosphorus cycle.^[5]

3Ca²⁺ + 2PO₄^3- → Ca₃(PO₄)₂

3 Ca₃(PO₄)₂ + CaF₂ → 2 Ca₅(PO₄)₃F

Fluorapatite 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.

References

^ ^a ^b http://rruff.geo.arizona.edu/doclib/hom/fluorapatite.pdf Mineral Handbook
^ http://webmineral.com/data/Fluorapatite.shtml Webmineral
^ http://www.mindat.org/min-1572.html Mindat
^ ^a ^b Hurlbert and Klein. "Manual of Mineralogy, 19th Edition". 1977. ISBN 0471251771
^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.

Category: Phosphate minerals

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Fluorapatite". A list of authors is available in Wikipedia.