Fluorapatite, often with the alternate spelling of fluoroapatite, is a
phosphate mineral with the formula Ca
5(PO
4)
3F (calcium fluorophosphate). Fluorapatite is a hard crystalline solid. Although samples can have various color (green, brown, blue, yellow, violet, or colorless), the pure mineral is colorless, as expected for a material lacking
transition metals. Along with
hydroxylapatite, it can be a component of
tooth enamel, especially in individuals who use
fluoridated toothpaste, but for industrial use both minerals are mined in the form of
phosphate rock, whose usual mineral composition is primarily fluorapatite but often with significant amounts of the other.
[
]
Fluorapatite crystallizes in a hexagonal crystal system. It is often combined as a
solid solution with hydroxylapatite (Ca
5(PO
4)
3OH or Ca
10(PO
4)
6(OH)
2) in biological matrices.
Chlorapatite (Ca
5(PO
4)
3Cl) is another related structure.
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
Igneous rock ( ), or magmatic rock, is one of the three main Rock (geology)#Classification, rock types, the others being sedimentary rock, sedimentary and metamorphic rock, metamorphic. Igneous rocks are formed through the cooling and solidifi ...
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.
[
Fluorapatite is found in the teeth of ]shark
Sharks are a group of elasmobranch cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the ...
s and other fishes in varying concentrations. It is also present in human teeth that have been exposed to fluoride
Fluoride (). According to this source, is a possible pronunciation in British English. is an Inorganic chemistry, inorganic, Monatomic ion, monatomic Ion#Anions and cations, anion of fluorine, with the chemical formula (also written ), whose ...
ions, for example, through water fluoridation or by using fluoride-containing toothpaste
Toothpaste is a paste or gel dentifrice that is used with a toothbrush to clean and maintain the aesthetics of Human tooth, teeth. Toothpaste is used to promote oral hygiene: it is an abrasive that aids in removing dental plaque and food from th ...
. The presence of fluorapatite helps prevent tooth decay or dental caries. Fluoroapatite has a mild bacteriostatic property as well, which helps decrease the proliferation of ''Streptococcus mutans
''Streptococcus mutans'' is a Facultative anaerobic organism, facultatively anaerobic, gram-positive coccus (round bacteria, bacterium) commonly found in the human oral cavity and is a significant contributor to dental caries, tooth decay.
The m ...
'', the predominant bacterium related to dental caries.
Synthesis
Fluorapatite can be synthesized in a three step process. First, calcium phosphate is generated by combining calcium and phosphate salt
In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...
s 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.
:3 + 2 →
:3 + → 2
Applications
Fluorapatite as a naturally occurring impurity in apatite
Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of Hydroxide, OH−, Fluoride, F− and Chloride, Cl− ion, respectively, in the crystal. The formula of the admixture of ...
generates hydrogen fluoride as a byproduct during the production of phosphoric acid
Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula . It is commonly encountered as an 85% aqueous solution, ...
, as apatite is digested by sulfuric acid
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...
. 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
Fluorine is a chemical element; it has Chemical symbol, symbol F and atomic number 9. It is the lightest halogen and exists at Standard temperature and pressure, standard conditions as pale yellow Diatomic molecule, diatomic gas. Fluorine is extre ...
compounds.
Synthetic fluorapatite doped with manganese
Manganese is a chemical element; it has Symbol (chemistry), symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese was first isolated in the 1770s. It is a transition m ...
-II and antimony
Antimony is a chemical element; it has chemical symbol, symbol Sb () and atomic number 51. A lustrous grey metal or metalloid, it is found in nature mainly as the sulfide mineral stibnite (). Antimony compounds have been known since ancient t ...
-V formed the basis for the second generation of fluorescent tube phosphor
A phosphor is a substance that exhibits the phenomenon of luminescence; it emits light when exposed to some type of radiant energy. The term is used both for fluorescent or phosphorescent substances which glow on exposure to ultraviolet or ...
s 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 + (3+x) + (1−x) + (2x) → 2 + (3+x) + (3+x) + (2x)
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 obsolescence of these early phosphor types were advantageous to health.
Since about 1990 the third generation tri-phosphors, three separate red, blue and green phosphors activated with rare earth ions and mixed in proportions to produce acceptable whites, have largely replaced halophosphors.
Fluorapatite can be used as a precursor for the production of phosphorus
Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...
. It can be reduced by carbon
Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...
in the presence of quartz
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...
:
:4 + 21 + 30 C → 20 + 30 CO + + 6
Upon cooling, white phosphorus
White phosphorus, yellow phosphorus, or simply tetraphosphorus (P4) is an allotrope of phosphorus. It is a translucent waxy solid that quickly yellows in light (due to its photochemical conversion into red phosphorus), and impure white phospho ...
(P4) is generated:
:2 →
Fluorapatite is also used as a gemstone.[Gemstones of the World By Walter Schumann, p. 18, 23, 29, 34, 56, 83]
References
{{Phosphate minerals
Phosphate minerals
Gemstones
Hexagonal minerals
Minerals in space group 176
Fluorine minerals