Fluorapatite

Fluorapatite
General
Category	Phosphate mineral Apatite group
Formula (repeating unit)	Ca5(PO4)3F
Strunz classification	8.BN.05
Crystal system	Hexagonal
Crystal class	Dipyramidal (6/m) H-M symbol: (6/m)
Space group	P63/m
Identification
Color	Sea-green, violet, purple, blue, pink, yellow, brown, white, colorless, may be zoned
Crystal habit	Massive to prismatic cyrstaliine
Twinning	Contact twins rare
Cleavage	Indistinct
Fracture	Brittle to conchoidal
Mohs scale hardness	5
Luster	Vitreous, resinous to dull
Streak	White
Diaphaneity	Transparent to opaque
Specific gravity	3.1 to 3.2
Optical properties	Uniaxial (-)
Refractive index	nω = 1.631 - 1.650 nε = 1.633 - 1.646
Birefringence	δ = 0.002
Ultraviolet fluorescence	Fluorescent and phosphorescent.
References	[1][2][3]

Fluorapatite, often with the alternate spelling of fluoroapatite, is a phosphate mineral with the formula Ca₅(PO₄)₃F (calcium fluorophosphate). 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. Along with hydroxylapatite, it is an important component of tooth enamel.^[4]

Fluorapatite crystallizes in a hexagonal crystal system. It is often combined as a solid solution with hydroxylapatite (Ca₅(PO₄)₃OH or Ca₁₀(PO₄)₆(OH)₂) in biological matrices. Chlorapatite (Ca₅(PO₄)₃Cl) is another related structure.^[4] Industrially, the mineral is an important source of both phosphoric and hydrofluoric acids.

Fluorapatite as a mineral 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]

Fluorapatite is found in the teeth of sharks and other fishes in varying concentrations. It is also present in human teeth that have been exposed to fluoride ions, for example, through water fluoridation or by using fluoride-containing toothpaste. The presence of fluorapatite helps prevent tooth decay or dental caries.^[5] Fluoroapatite has a critical pH of 4.5, thus, it makes tooth structure more resistant to additional caries attack. It has a mild bacteriostatic property as well which helps in decreasing the proliferation of Streptococcus mutans, the predominant bacteria related to dental caries.^[6]

Synthesis

Fluoroapatite with albite

Fluorapatite. São Geraldo do Baixio, Doce valley, Minas Gerais, Brazil.

Fluorapatite can be synthesized in a three 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.^[7]

3 Ca2+
+ 2 PO3−
4 → Ca
3(PO
4)
2

3 Ca
3(PO
4)
2 + CaF
2 → 2 Ca
5(PO
4)
3F

Applications

Fluorapatite as a naturally occurring impurity in apatite generates hydrogen fluoride as a byproduct during the production of phosphoric acid, as apatite is digested by sulfuric acid. The hydrogen fluoride byproduct is now one of the industrial sources of hydrofluoric acid, which in turn is used as a starting reagent for synthesis of a range of important industrial and pharmaceutical fluorine compounds.

Synthetic fluorapatite doped with manganese-II and antimony-V formed the basis for the second generation of fluorescent tube phosphors referred to as halophosphors. When irradiated with 253.7 nm mercury resonance radiation they fluoresced with broad emission which appeared within the range of acceptable whites. The antimony-V acted as the primary activator and produced a broad blue emission. Addition of manganese-II produced a second broad peak to appear at the red end of the emission spectrum at the expense of the antimony peak, excitation energy being transferred from the antimony to the manganese by a non radiative process and making the emitted light appear less blue and more pink. Replacement of some of the fluoride ions with chloride ions in the lattice caused a general shift of the emission bands to the longer wavelength red end of the spectrum. These alterations allowed phosphors for Warm White, White and Daylight tubes, (with corrected color temperatures of 2900, 4100 and 6500 K respectively), to be made. The amounts of the manganese and antimony activators vary between 0.05 and 0.5 mole percent. The reaction used to create halophosphor is shown below. The antimony and manganese must be incorporated in the correct trace amounts if the product is to be fluorescent.

6 CaHPO
4 + (3+x) CaCO
3 + (1-x) CaF
2 + (2x) NH
4Cl → 2 Ca
5(PO
4)
3(F
1-xCl
x) + (3+x)CO
2 + (3+x)H
2O+ (2x)NH
3

Sometimes some of the calcium was substituted with strontium giving narrower emission peaks. For special purpose or colored tubes the halophosphor was mixed with small quantities of other phosphors, particularly in De-Luxe tubes with higher color rendering index for use in food market or art studio lighting.

Prior to the development of halophosphor in 1942, the first generation willemite latticed, manganese-II activated zinc orthosilicate and zinc beryllium orthosilicate phosphors were used in fluorescent tubes. Due to the respiratory toxicity of beryllium compounds the obselescence of these early phosphor types were advantageous to health.

Since about 1990 the third generation TriPhosphors, three separate red, blue and green phosphors activated with rare earth ions and mixed in proportions to produce acceptable whites, have largely replaced halophosphors.^[8]

Fluorapatite can be used as a precursor for the production of phosphorus. It can be reduced by carbon in the presence of quartz:

4 Ca
5(PO
4)
3F + 21 SiO
2 + 30 C → 20 CaSiO
3 + 30 CO + SiF
4 + 6 P
2

Upon cooling, white phosphorus (P₄) is generated:

2 P
2 → P
4

Fluorapatite is also used as a gemstone.^[9]

References

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