Fluoroapatite

In the geochemistry of fluorine, the close match in the ionic radii of fluoride (0.136 nm), hydroxide (0.140 nm), and oxide ion (0.140 nm) allows a sequential replacement of oxygen by fluorine in a wide variety of minerals. This accounts for the wide dissemination of the element in nature. The ready formation of volatile silicon tetrafluoride, the pyrohydrolysis of fluorides to hydrogen fluoride, and the low solubility of calcium fluoride and of calcium fluorophosphates, have provided a geochemical cycle in which fluorine may be stripped from solution by limestone and by apatite to form the deposits of fluorspar and of phosphate rock (fluoroapatite [1306-01 -0]) approximately CaF2 3Ca2(P0 2 which ate the world s main resources of fluorine (1). 

The majority of the fluorine ia the earth s cmst is present in the form of the phosphoms fluoride fluoroapatite [1306-05 ] Ca (P0 2F- Phosphate rock deposits contain an average concentration of 3.5 wt % fluorine. During phosphate processing these fluorine values are partially recovered as by-product fluorosihcic acid. The amount of fluorosiUcic acid recovered has grown steadily, in part because of environmental requirements (see Phosphoric acid and THE phosphates). 

Sodium monofluorophosphate is used ia most dentifrices at a concentration of 0.76 wt % which produces the desired fluoride level of 1000 ppm although one extra strength dentifrice has 1.14 wt % and 1500 ppm F. Although the mechanism of its efficacy ia reducing dental decay is not completely understood (75), it almost certainly reacts with the apatite of the tooth converting it to fluoroapatite which is less soluble ia mouth acids (see Dentifrices). 

The crystal stmcture of the calcium fluoroapatite has two different crystallographic sites for the Ca " ion. The Ca(I) site has a threefold axis of symmetry and is coordinated to six oxygen ions at the vertices of a distorted trigonal prism. The Ca(Il) ions are located at the corners of equilateral... 

Although tetrafluorosilane can be readily produced by the action of hydrogen fluoride on sihca, its production is a by-product of HF production by the reaction of fluorospar and sulfuric acid and as a by-product from phosphate fertilizer production by the treatment of fluoroapatite with sulfuric acid (171). The most significant U.S. production is by IMC-Agrico at Uncle Sam, Louisiana. 

Phosphorus is the eleventh element in order of abundance in crustal rocks of the earth and it occurs there to the extent of 1120 ppm (cf. H 1520 ppm, Mn 1060 ppm). All its known terrestrial minerals are orthophosphates though the reduced phosphide mineral schrieber-site (Fe,Ni)3P occurs in most iron meteorites. Some 200 crystalline phosphate minerals have been described, but by far the major amount of P occurs in a single mineral family, the apatites, and these are the only ones of industrial importance, the others being rare curiosities. Apatites (p. 523) have the idealized general formula 3Ca3(P04)2.CaX2, that is Caio(P04)6X2, and common members are fluorapatite Ca5(P04)3p, chloroapatite Ca5(P04)3Cl, and hydroxyapatite Ca5(P04)3(0H). In addition, there are vast deposits of amorphous phosphate rock, phosphorite, which approximates in composition to fluoroapatite. " These deposits are widely... 

Despite its relatively late discovery, phosphorus is the eleventh most abundant element in Earth s crustal rock. It has been estimated that world reserves of phosphate rock are sufficient to last for several hundred years. Virtually all phosphorus deposits contain apatite, whose general formula is Caj (P04)3 X, where X — OH, or Cl. Fluoroapatite is the least soluble, hence most abundant, of the three apatite minerals. Phosphorus Is found in aqueous systems as HPOq and H2 PO4 ions. In biological organisms, phosphorus is a component of nucleic acids and energy-shuttling molecules such as ATP. 

C21-0082. Calcium dihydrogen phosphate is a common phosphoras fertilizer that is made by treating fluoroapatite with phosphoric acid. Hydrogen fluoride is a by-product of the synthesis. Write a balanced equation for the production of this fertilizer and calculate the mass percent of phosphorus in the fertilizer. 

It is probable that the prevalent oxidation states of phosphorus on the young Earth were lower than they are today, so calcium salts with a much better solubility than that of apatite could have been formed. As Glindemann et al. (1999) were able to show in model experiments, up to 11 % of the starting material could be converted to phosphite in CH4/N2 atmospheres (10% CH4) using Na2HP04, hydroxyapatite or fluoroapatite sources. Similar processes cannot be excluded for the primeval Earth, for example, under the influence of electrical discharges. 

Phosphorus compounds occur widely in nature, with some of the most common forms being phosphate rocks and minerals, bones, and teeth. Phosphate minerals include calcium phosphate, Ca3(P04)2 apatite, Ca5(P04)30H fluoroapatite, Ca5(P04)3F and chloroapatite, Ca5(P04)3Cl. Elemental phosphorus was first obtained by H. Brand, and its name is derived from two Greek words meaning "light" and "I bear" because of the phosphorescence of white phosphorus due to slow oxidation. 

Fluoroapatite, Ca5(P04)3F, is sometimes found with Ca3(P04)2, and it also reacts with sulfuric acid,... 

There are numerous minerals that contain halogens. Minerals that contain fluorine include fluorite, CaF2, cryolite, Na3AlFs, and fluoroapatite, Ca5(P04)3F. As was discussed in Chapter 14, fluoroapatite is found with calcium phosphate, which is very important in the production of fertilizers. Fluorite is found in Southeastern Illinois and Northwestern Kentucky, and cryolite is found in Greenland, although it is also produced synthetically because of its use in the electrochemical production of aluminum. 

Hydroxyapatite and fluoroapatite surfaces differ from oxide surfaces in as far as they are expected to carry two different classes of surface groups (Wu et al., 1991). From a simple pictorial presentation... 

Wu et al. (1991) propose for fluoroapatite Ca-OHg and =P-0 as dominating surface groups, characterized by the following equilibrium constants (0.1 M NaCI, 25°... 

According to these equilibrium constants a pHpzc of 8.15 would be expected for hydrous fluoroapatite. 

Occurrence. Widely distributed in many minerals (more than 200 minerals are known). Commercial sources of phosphorus are apatites (for instance fluoroapatite [3Ca3(P04)2 Ca(F,Cl)2]). 

The fluoride ion is a much weaker base than the hydroxide ion, and fluoroapatite is therefore less reactive towards acids. 

Major structural components of hard parts Ca, C, SI, 0, P, F, Sr, S Calcite, aragonite, opaline silica, celestite, apatite, fluoroapatite Components of frustules and tests, bone, teeth... 

Phosphate is remineralized during the oxidation of organic matter and dissolution of hard parts, such as bones and teeth, that are composed of the minerals hydroxyapatite and fluoroapatite. Unlike the other products of remineralization, pore-water phosphate concentrations are regulated only by mineral solubility, such as through vivianite (iron phosphate) and francolite (carbonate fluoroapatite). Redox reactions are not significant because phosphorus exists nearly entirely in the h-5 oxidation state. 

SrS04>. Carbonate-rich fluoroapatite minerals, such as francolite, are the major component of bones. As noted in Chapter 11, hard and soft parts can contain significant amoimts of trace metals. 

The cycling of phosphorus in marine sediments is quite different from that of nitrogen in that its remineralized form, HPO , is readily sequestered in several mineral phases, namely carbonate fluoroapatite (CFA) and iron-rich clays and oxyhydroxides. 

Apatites are complexes of cationic Ca " matched by HP04, COa ", OH , or F as anions. Depending on the counter-ion, apatite can occur in the forms carbonate apatite Caio(P04)6C03, as hydroxyapatite Caio(P04)e (0H)2, or fluoroapatite Caio(P04)6F2. In addition, alkaline earth carbonates also occur in bone. In adults, more than 1 kg calcium is stored in bone. 

The most important form of protection against caries involves avoiding sweet substances (foods containing saccharose, glucose, and fructose). Small children in particular should not have very sweet drinks freely available to them. Regular removal of plaque by cleaning the teeth and hardening of the dental enamel by fluoridization are also important. Fluoride has a protective effect because fluoroapatite (see A) is particularly resistant to acids. 

Fig. 6. 3F1 -D MAS-NMR spectra of type B carbonated fluoroapatite weakly hydroxylated (a) F projection of C F HetCor MAS-NMR spectra, (b) F projection of 2-D P F HetCor MAS NM, (c) F SPE-MAS NMR spectrum.

The synthesis of B-type carbonate-fluoroapatites was reported by Montel et al. with fluoride ions present in two kinds of sites in the apatitic structure [126]. Moreover, the correlation between the respective amounts of fluoride and... 

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