Apatites

In their publication, LeGeros and LeGeros (1993) oudine the different apatites, ranging from natural apatite (minerals) to biological (human dentin, enamel, and bone) and synthetic (chemically synthesized) apatite. This publication clearly shows that apatite is a group of crystalline compounds. The most important of these compounds is calcium hydroxyapatite. All the related crystal structures, such as fluoroapatite, chloroapatite, and carbonate apatite are derived from it. 

The crystal structure of hydroxyapatite, Ca Q(P04)g(0H)2, has been studied by Beevers and McIntyre (1956), Kay et al. (1992), Elliott (1994), and Young et al. (1966). Hydroxyapatite has the hexagonal PG Im space group. This designation refers to the sixfold c-axis perpendicular to three equivalent -axes ( j, 3) at angles of 120° to each other. Ten Ca ions are located 

Fluoroapatite (F-apatite), chloroapatite (Cl-apatite) and carbonate apatite (CO -apatite) are derived from hydroxyapatite. In F-apatite and Cl-apatite, the F and Cl ions assume the position of the OH ions. When F and Cl ions are inserted in the Ca triangle, their position in relation to the OH in hydroxyapatite changes (Fig. 1-23). As a result, the lattice parameters change compared with those of hydroxyapatite (Young and Elliot, 1966). When F ions are inserted in place of OH ions, the -axis is reduced and the c-axis remains constant (F = apatite a = 9.382 A, c = 6.880 A). The insertion of Cl ions enlarges the unit cell (Cl apatite a = 9.515 A, c = 6.858 A). The crystal structure of fluoroapatite is shown in Appendix 19. 

The following ions are generally known to be suitable for insertion into the apatite structure  

Scientifically speaking, biological apatites, such as apatite crystals of human dentin, enamel, and bone must be classified as carbonate apatites. These substances also demonstrate different levels of solubility because of their various degrees of crystallinity. Therefore, enarnel carbonate apatite is 

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Never found free in nature, it is widely distributed in combination with minerals. Phosphate rock, which contains the mineral apatite, an impure tri-calcium phosphate, is an important source of the element. Large deposits are found in Russia, in Morocco, and in Florida, Tennessee, Utah, Idaho, and elsewhere. 

Calcium is a metallic element, fifth in abundance in the earth s crust, of which if forms more than 3%. It is an essential constituent of leaves, bones, teeth, and shells. Never found in nature uncombined, it occurs abundantly as limestone, gypsum, and fluorite. Apatite is the fluorophosphate or chlorophosphate of calcium. 

Manne Marine algae Marine apatites Marine applications Marine coatings Marine equipment Marine oil (menhaden)... 

Direct Application Rock. Finely ground phosphate rock has had limited use as a direct-appHcation fertilizer for many years. There have been widely varying results. Direct appHcation of phosphate rock worldwide amounts to about 8% of total fertilizer phosphate used, primarily in the former Soviet Union, France, Brazil, Sri Lanka, Malaysia, and Indonesia. The agronomic effectiveness of an apatitic rock depends not only on the fineness of the grind but also strongly on the innate reactivity of the rock and the acidity of the sod performance is better on more acid sods. Probably more than half of the potentially productive tropical sods are acidic, some with pH as low as 3.5—4.5. Certain phosphate rocks may thus become increasingly important as fertilizer in those areas. The International Fertilizer Development Center at Muscle Shoals, Alabama is active in researching this field (30). 

Igneous apatite Marine phosphorite Igneous apatite Marine phosphorite... 

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). 

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). 

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). 

Apatite and other phosphorites constitute a substantial resource of rare earths. The REO content is highly variable and ranges from trace amounts to over 1%. Apatite- [1306-05-4] rich tailings of the iron ore at Mineville, New York, have been considered a potential source of yttrium and lanthanides. Rare-earth-rich apatites are found at the Kola Peninsula, Russia, and the Phalaborwa complex in South Africa. In spite of low REO content apatites could become an important source of rare earths because these are processed in large quantities for the manufacturing of fertilisers (qv). 

Since there are no volatile components this halophosphate phosphor is prepared with close to the stoichiometric amounts of SrHPO, SrCO, CaCO, BaCO, SrCl2, or NH Cl and EU2O2. The blend is fired under an atmosphere containing 1—2% hydrogen at 1100°C. A small excess of chloride provides some fluxing action and gives weU-formed crystals of apatite. The chlorapatites are dimorphous one modification is hexagonal and the other monoclinic. 

Zaire, Norway, and the United States (14). It also occurs with calcite, dolomite, apatite, magnetite, and some siUcates. The density of pyrochlore is ca 4.0—4.4 g/cm. The tantalum content usually is low, ca 0.1—0.3% on a metal basis. 

Phosphorite Deposits. Sedimentary phosphorites contain low concentrations of uranium in fine-grained apatite. Uranium of this type is considered an unconventional resource. Significant examples of these uranium ore types include the U.S. deposits in Elorida, where uranium is recovered as a by-product, and the large deposits in North African and Middle Eastern countries (16). 

Phosphorites and Glauconite. Phosphorites, or marine apatites, Ca (F,Cl, OH,l/2C02)(P0 2 commonly, though not predominantiy,... 

Calcium Phosphates. The alkaline-earth phosphates are generally much less soluble than those of the alkaH metals. Calcium phosphates include the most abundant natural form of phosphoms, ie, apatites, Ca2Q(P0 3X2, where X = OH, F, Cl, etc. Apatite ores are the predominant basic raw material for the production of phosphoms and its derivatives. Calcium phosphates are the main component of bones and teeth. After sodium phosphates, the calcium salts are the next largest volume technical- and food-grade phosphates. Many commercial appHcations of the calcium phosphates depend on thek low solubiHties. 

Hydroxyapatite, Ca2Q(PO (OH)2, may be regarded as the parent member of a whole series of stmcturaHy related calcium phosphates that can be represented by the formula M2q(ZO X2, where M is a metal or H O" Z is P, As, Si, Ga, S, or Cr and X is OH, F, Cl, Br, 1/2 CO, etc. The apatite compounds all exhibit the same type of hexagonal crystal stmcture. Included are a series of naturally occurring minerals, synthetic salts, and precipitated hydroxyapatites. Highly substituted apatites such as FrancoHte, Ca2Q(PO (C02) (F,0H)2, are the principal component of phosphate rock used for the production of both wet-process and furnace-process phosphoric acid. 

The Phalaborwa complex ia the northeastern Transvaal is a complex volcanic orebody. Different sections are mined to recover magnetite, apatite, a copper concentrate, vermicuhte, and baddeleyite, Hsted in order of aimual quantities mined. The baddeleyite is contained in the foskorite ore zone at a zirconium oxide concentration of 0.2%, and at a lesser concentration in the carbonatite orebody. Although baddeleyite is recovered from the process tailings to meet market demand, the maximum output could be limited by the requirements for the magnetite and apatite. The baddeleyite concentrate contains ca 96% zirconium oxide with a hafnium content of 2% Hf/Zr + Hf. A comminuted, chemically beneficiated concentrate containing ca 99% zirconium oxide is produced also. 

Phosphate rock, mined widely throughout the world for its fertilizer value (see Fertilizers), in certain regions contains a few percent of lanthanides. For example, the apatite deposits in the Kola peninsula on the Russian/Finnish border. The Ln content is recoverable from the various processing residues, and because other Ln-containing minerals, such as loparite [12173-83-0], are also found there, the location suppHes a significant part of the demand in Eastern Europe. 

Another desirable property for a ceramic color is a high refractive index. For example, valuable pigments are based on spinels [1302-67-6] ( 2jj = 1.8) and on zircon ( 2j = 1.9), but no valuable pigments are based on apatite ( 2j = 1.6), even though the lattice of apatite is as versatile for making ionic substitutions as that of spinel. 

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