Calcium fluorapatite

Calcium fluorapatite (Ca,0(PO4)6F2, FM 1 008.6) laser crystals were doped with chromium to improve their efficiency. It was suspected that the chromium could be in the +4 oxidation state. 

Lapraz D, Gaume F, Barland M (1985) On the thermoluminescent mechanism of a calcium fluorapatite single crystal doped with Mn. Phys Stat Solidi A 89 249-253 Leckebusch R (1979) Comments on the luminescence of apatites from Panasqueria (Portugal). N Jahib Mineral Monat 17-21... 

Suitch PR, LaCout JL, Hewat A, Young RA (1985) The stractural location and role of Mn partially substituted for Ca in fluorapatite. Acta Crystallogr 641 173-179 Tachihante M, Zambon D, Cousseins JC (1996) optical study of the Tb to Eu energy transfer in calcium fluorapatite. Eur J Solid State Inorg Chem 33 713-725 Taraschan A. (1978) Luminescence of Minerals. Naukova Dumka, Kiev (in Russian)... 

Zhang XX, Loutts GB, Bass M, Chai BHT (1994) Growth of laser-quahty single crystals of Nd -doped calcium fluorapatite and their efficient lasing performance. Appl Phys Lett 64 10-12... 

Heling I, Heindel R, Merin B. Calcium-fluorapatite. A new material for bone implants. J Oral Implantol 1981 9(4) 548—55. 

The crystalline mineral in bones and teeth is generally regarded as an imperfect calcium hydroxyapatite. Apatite minerals, principally calcium fluorapatite, are both abundant and ubiquitous and are the principal source of phosphate for fertilizers. Their abundance is probably an expression of the very high affinity which calcium and phosphate ions have for each other so that it is perhaps not surprising that, on account of its stability, calcium hydroxyapatite has been selected to play an important part, both structurally and physiologically, in many living things. Ions other than calcium, phosphate and hydroxyl are present in the crystallites in which the atomic ratio of calcium to phosphorus departs considerably from the theoretical value of 1-67 (Table 35.1). 

The accurate determination of the spatial relationships of ions in a crystal lattice requires that large well-formed single crystals be studied using a variety of techniques including X-ray diffraction, spectroscopic and optical methods. The best apatite crystals for such studies are those of artificially prepared calcium fluorapatite and, the next best, articifically prepared calcium hydroxyapatite. The description which follows is therefore principally of calcium fluorapatite, but the structure of calcium hydroxyapatite will also be mentioned. 

Figure 28.2 Basic structure of the calcium fluorapatite lattice. The large hexagonal cylinders have been formed by joining the centres of Ca " ions in the crystal lattice. The projection of a phosphate tetrahedron is a triangle...

In order to simplify description of the ionic interrelationships in a crystal lattice, an entity known as the unit cell is used. The unit cell is a portion of a crystal which contains the least number of ions necessary to establish all of the ionic relationships which occur in the lattice. Even though a unit cell does not exist by itself, a crystal can be thought of as being built up of hundreds or even thousands of unit cells. The unit cell of calcium fluorapatite contains ten calcium ions, six phosphate ions and two fluoride ions, whilst the unit cell of calcium hydroxyapatite contains two hydroxyl ions instead of the fluoride ions. The ions within a unit ceU of calcium fluorapatite and calcium hydroxyapatite can, thus, be written Caio(P04)6X2 where X is F in fluorapatite and OH in hydroxyapatite. However, since a unit cell has no separate existence, the shorthand expression above is not analogous to the formula for a chemical compound which exists as... 

One way to help prevent tooth decay is by using fluoride. Fluoride reacts with hydroxyapatite in a doubledisplacement reaction. It displaces the 0H group in hydroxyapatite to produce fluorapatite, Ca5(P04)3F. Studies show that calcium fluorapatite is about 20% less soluble than hydroxyapatite in acid. Therefore, fluoride lowers the incidence of tooth decay. 

Phosphate fertilizers have been known for more than 150 years. Phosphate rock contains minerals such as calcium fluorapatite, Ca5(P04)3F, which are generally too insoluble to be of much use for plant uptake. (See pp. 367 and 554 for several viewpoints on the role of the insoluble calcium apatites in bone and teeth structure.)... 

Phospha.tes. Many phosphates cl aim unique material advantages over siUcates that make them worth the higher material costs for certain apphcations. Glass-ceramics containing the calcium orthophosphate apatite, for example, have demonstrated good biocompatibiUty and, in some cases even bioactivity (the abiUty to bond with bone) (25). Recent combinations of fluorapatite with phlogopite mica provide bioactivity as well as machinability and show promise as surgical implants (26). 

Phosphates. The primary constituent of phosphate rock is fluorapatite, Ca3FP2022- Industrial phosphates including phosphate fertilizers (qv), phosphoric acid, and calcium phosphates (11) (see Phosphoric acid and the phosphates) are obtained from the large deposits of fluorapatite found in Florida in the United States, and in Morocco. Because phosphate rock is too insoluble to be useful as a fertilizer, it is converted to superphosphate [12431 -88-8] Ca(H2P0 2 CaSO, by H2SO and to triple superphosphate [7758-23-8] by H PO (l )- Phosphoric acid may also be... 

CCP in milk is mentioned in connection with casein above (Section VI.C). Fluorapatite is a major constituent of phosphate rocks, and a constituent, probably important, of human tooth enamel for those whose drinking water contains significant amounts of naturally occurring or added fluoride. Fluorapatite is significantly less soluble than hydroxyapatite - the relationship between the solubilities of fluorapatite and hydroxyapatite parallels (but is much less extreme than) that between calcium fluoride (Ksp — 3.9 x 10 11 mol3 dm-9) and calcium hydroxide (Ksp = 7.9 x 10 6 mol3 dm 9). Calcium diphosphate, Ca2P207, is believed to be the least soluble of the calcium phosphates. 

Wu, L., W. Forsling, and P. W. Schindler (1991), "Surface Complexation of Calcium Minerals in Aqueous Solution. 1. Surface Protonation at Fluorapatite-Water Interfaces," J. Colloid Interface Sci. 147/1, 178-185. 

Fluorine occurs widely in nature as insoluble fluorides. Calcium fluoride occurs as jluospar or fluorite, for example in Derbyshire where it is coloured blue and called bluejohn . Other important minerals are cryolite Na3AlF6 (p. 141) and fluorapatite CaF23Ca3 (P04)2. Bones and teeth contain fluorides and some natural water contains traces. 

Fig. 2. Structure of fluorapatite. Projection on the (001) cristallographic plane, perpendicular to the c axis of the hexagonal structure. (Reproduced by permission of lUCrfrom Ref. [2]). Purple Calcium green Fluorine red Oxygen yellow Phosphorus. (See Colour Plate Section at the end of this book.)...

Substituting the original ions of calcium phosphate fluorapatite often leads to changes in the unit-cell parameters. Frequently, the variations of the unit-cell dimensions are proportional to the substitution ratio and follow Vegard s law (i.e. the unit-cell parameter varies linearly with the substitution ratio). This is the case, for example, with chlor-fluorapatite. 

Fluorapatite is a highly insoluble calcium phosphate phase. The solubility product of stoichiometric fluorapatite at 37°C is 3.19 0.14x10 " mol 1 (for Cas(P04)3F as reported by Moreno et al. [53]) and appears significantly lower than that of HA in the same conditions (7.36 0.93 x 10 ° mol for Ca5(P04)30H). Asuggested explanation for this very low solubility product is that cohesive forces are stronger in fluorapatite than in other apatites due to smaller unit-cell dimensions. The complete solid solution Ca-,o(P04)6(OH)2-xFx can be obtained. Initial solubility determinations have shown a solubility minimum for x close to 1 [54], related to the formation of hydrogen bonding between F and OH ions. These results were subsequently... 

When HA is in contact with a fluoride-containing solution at low concentration, this F /OH substitution mostly occurs at the crystal surface [60] and can be accompanied by the formation of CaF2 as a secondary phase in acidic media (Fig. 8). The fluoride ions fixed on the HA crystal surface after exposure to a fluoride-containing solution were found to be coordinated by three calcium ions, as with the regular fluorapatite structure [61]. The fluoride uptake was shown to... 

In addition to end-member phases, such as fluorapatite and HA, several studies have reported thermodynamic data related to solid solutions of apatite with various cations involving substitutions like Ca-Mg, Ca-Cd, Ca-Pb and Ca-Sr [72-74]. The related enthalpies of mixing were obtained, and their variation versus composition was generally indicative of a non-statistical occupancy of the cationic sites of the apatitic structure. In some instances, the limits of cationic substitution for calcium were estimated (e.g. in the range 0.073-0.101 for Ca-Mg fluorapatites according to Ben Abdelkader et al. [74]). 

Fluorapatite can be prepared by heating a mixture of calcium pyrophosphate (Ca2P207) and Cap2 at 750°C [114,115]. POF3 gas is formed according to the following chemical reaction equation ... 

Fluorhydroxyapatite can be synthesised by the traditional double-decomposition method generally used for apatite precipitation. An ammonium phosphate and fluoride solution (solution B) is added, dropwise, into a hot (generally at boiling temperature) calcium solution (solution A) at a basic pH level as previously published [122,123]. Fluorapatites close to stoichiometry are obtained (a = 2, see the following reaction equation) however, a very small residual amount of OH always seems to be present. Filtration and several washing operations are necessary to remove the counter-ions. The reaction is almost total due to the very low solubility of fluorhydroxyapatites. 

K. A. Smith, D.P. Burum, Application of Fluorine-19 CRAMPS to the analysis of calcium fluoride/fluorapatite mixtures, J. Magn. Reson. 84 (1989) 85-94. 

D. E. Sandstrom, O.N. Antzutkin, W. Forsling, A spectroscopic study of calcium surface sites and adsorbed iron species at aqueous fluorapatite by means of 1H and 31P MAS NMR, Langmuir 22 (2006) 11060-11064. 

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