Fluorinated apatite

Lack of carbonated or fluorinated hydroxyapatite. The HAP formation by the reaction in Eq. 13.13 still does not produce a composition exactly the same as that of the bone. Bone contains carbonated and fluorinated apatite, or dahUite, and it is difficult to mimic such a composition by chemical reactions. 

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). 

Bones and teeth consist of hydroxyl-apatite (Ca phosphate). Tiny amounts of fluorine improves their resistance dramatically. Fluorine compounds in toothpaste prevent cavities. 

Inorganic salts that contain halogens are usually soluble. They commonly occur as simple, single, negatively charged anions in soil. There are two common exceptions to this generalization. First, fluorine is commonly found bonded to phosphate in insoluble minerals called apatites, which are calcium phosphate fluorides. 

Reiche, I., Yignaud, C., Favre-Quattropani, L., and Menu, M. (2002b). Fluorine analysis in biogenic and geological apatite by analytical transmission electron microscopy and nuclear reaction analysis. Journal of Trace and Microprobe Techniques 20 211-231. 

L.M. Rodriguez-Lorenzo, J.N. Hart, K.A. Gross, Influence of fluorine in the synthesis of apatites. Synthesis of solid solutions of hydroxy-fluorapatite. Biomaterials 24 (2003) 3777-3785. 

The incorporation of fluoride in place of hydroxyl groups is chemically straightforward [59,60] and, as we have seen, results in a substance of greater resistance to acid attack. This is partly due to the greater electronegativity of fluorine, which means that the electrostatic attraction between Ca + and F is greater than that between Ca + and OH. As a result, the fluoridated apatite lattice is more stable than hydroxyapatite [61-63]. It is also more crystalline [64]. 

APATITE. The mineral apatite is a phosphate of calcium with either fluorine or chlorine or sometimes both, hence the distinction between fluorapatite and chlorapatite. Sometimes both fluorine and chlorine are present. Most apatite is. however, fluorapatite. Uari PO )) 1F. 

As soon as chemists become interested in natural apatites, the problem with fluorine appears. When phosphate ores are treated the fluorine must be removed. On the other hand it is sometimes necessary to fluorinate calcium phosphates in the mineral part of calcified tissues. Obviously, in industry and medecine it is neces sary to know the mechanism of phosphate fluorination as well as the structure and the properties of products obtained. 

The world s major source of phosphorus is apatite, a class of phosphate minerals. Commercially, the most important is fluoroapatite, a calcium phosphate that contains fluorine. This fluorine must be removed for the manufacture of phosphoric acid, but it also can be used to produce hydrofluoric acid and fluorinated compounds. 

In humid environments, hydroxyapatite (Ca5(P04)30H), the main component of the inorganic bone and tooth matrix, is transformed into the more stable fluor-apatite (Ca5(P04)3F). In an idealized sample, fluorine uptake from the environment leads to a U-shaped concentration profile, which slowly develops into the bulk from the outer surface and from the marrow cavity inwards according to Fick s second law... 

Fluorine enrichment measured on individual apatite crystals by TEM-... 

The F content in recent bone or dentine apatite is normally less than 0.1 wt.%. For ancient specimen, F is known to diffuse during burial into bone material. Its enrichment is generally a part of many complex diagenetic changes of bone and tooth, which remains after their deposit. Fluorine can react with the bone and dentine mineral phase to form calcium fluoride compounds. It usually substitutes for hydroxyl ions in hydroxyapatite, leading to the less soluble fluorapatite compound (Ca10(PO4)6(F)2, FAP). 

Fluorine occurs in nature in minerals such as fluorite, Cap2 fluor-apatite, Ca.(PO )3F, which is a constituent of bones and teeth and cryolite, Na3AlF<. and in small quantities in sea water. Its name fluorine, from Latin finere, to flow, refers to the use of fluorite as a flux (a material which forms a melt with metal oxides). 

Fluorine fluorite fluor-apatite cryolite hydrogen fluoride hydrofluoric acid silicon tetrafluoride sodium fluoride oxygen fluoride. 

In an extensive review of the geochemistry of volatile-bearing minerals in mantle xenoliths, Ionov et al (1997) have pointed out that although minerals such as mica, amphibole, and apatite are often referred to as hydrous, in many cases they have very low H2O contents (Boettcher and O Neill, 1980). In such cases, these minerals may have significant amounts of fluorine, chlorine and CO2. Mica, amphibole, and apatite, together with the oxide phases, are important hosts for titanium, potassium, rubidium, strontium, barium, and niobium (Table 9). 

Most apatites are chlorine-rich (2-4.3%) with low F/Cl (0.1 -0.3). High fluorine ( 5%) and low chlorine (—0.25%) have been reported for apatite in spinel Iherzolites from Pacific OIB (Hauri et al., 1993). Extremely high strontium contents, commonly >2X10 ppm and up to 7 wt.% (Ionov et al., 1997 Table 9) are common in mantle apatites meaning that this phase is a major repository for strontium when present in peridotites at abundances of 0.1% or above. Rb/Sr is very low. Apatites have high levels of REE and are LREE-enriched (Table 9). Lanthanum and cerium concentrations can reach >1 wt.% and neodymium concentrations can be above 1,000 ppm. Sm/Nd is below PUM. HFSE are low and so the presence of this phase does not affect bulk rock HFSE chemistry. 

Mantle micas are also chlorine-poor Smith et al. (1981) report ranges of 0.04-0.11 wt.% for chlorine and 0.15-2.2 wt.% for fluorine in primary-textured micas. Matson et al. (1986) report 0.35-0.53 wt.% F and 0.04-0.07 wt.% Cl in primary micas from peridotites. They found a similar range in chlorine in secondary micas in peridotites but a wider range in fluorine, from 0.02 wt.% to 0.54 wt.%. Subsequent analyses (e.g., Wagner et al., 1996) have not found micas richer in either halogen. In contrast, mantle apatites are often halogen-rich. Smith et al. 

The presence of apatite in subcontinental mantle samples raises the question of the thermal stability of apatite, and whether it would be stable in mantle other than cool lithospheric roots. A limited amount of experimental work has been done that addresses this question Vukadinovic and Edgar (1993) determined the solidus for phlogopite-apatite mixtures at 2 GPa in the KaO-CaO-MgO-AljOa-SiOa-PaOs-HjO-F system. They looked at two bulk compositions, one with the hydroxy end-members and the other where F/OH = 1. The sohdus was 1,225 °C for the fluorine-free system, and 1,260 °C for the fluorinebearing system. It seems likely that adding iron to the system will decrease solidus temperatures. Given that the average current mantle adiabat is... 

Vukadinovic D. and Edgar A. D. (1993) Phase relations in the phlogopite-apatite system at 20kbar implications for the role of fluorine in mantle melting. Contrib. Mineral. Petrol. 114, 247 -254. 

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