Mineralization apatite crystal substitutions

By far the most abundant phosphate mineral is apatite, which accounts for more than 95% of all P in the Earth s crust. The basic composition of apatite is listed in Table 14-2. Apatite exhibits a hexagonal crystal structure with long open channels parallel to the c-axis. In its pure form, F , OH , or Cl occupies sites along this axis to form fluorapatite, hydroxyapatite, or chlor-apatite, respectively. However, because of the "open" nature of the apatite crystal lattice, many minor substitutions are possible and "pure" forms of apatite as depicted by the general formula in Table 14-2 are rarely found. 

Enamel mineral has many large hydroxyapatite crystals, whereas bone has many small ones with numerous vacancies and substitutions. These differences increase the elasticity of bone compared with enamel and promote its interactions with the surrounding collagen. Recently, a tightly bound hydration shell that fills a porous collagen-apatite junction was discovered around normal bone crystals. The water-filled pores are normally immobile, but repeated stresses cause the water to leak out from between the mineral and collagen. The drying increases mineralization and crystal formation, which may explain the decreased elasticity of bones with age. 

Mineralized tissues contain a significant amount of Mg (Table 3). Although Mg seems to be important in mineralized tissues, there has always been doubt about whether it was all adsorbed on the apatites crystal surfaces, or whether a significant amount replaced Ca ions in the apatite lattice (Glimcher 1998). Lattice substitution is doubted because the radius of the Mg ion (0.72 A) is much less than that of the Ca ion (1.00 A) and because definitive proof of the possibility of such a substitution was lacking. However, Bigi et al. (1996) have used Rietveld analysis of XRD data to show that a limited replacement (at most 10 atom %) appeared to be possible. It seemed that Mg " had replaced some Ca ions in Ca2 sites. 

These varieties of carbonated apatite whose formulae may be represented as Cajo ,(P04)6 (C03) j (F,0H)2, where jc = 1, are often designated as Francolite (F OH) or Dahllite (OH F). Up to 25% replacement of PO4 by CO3 is, however, sometimes found, and replacement of up to 10% Ca by Mg can occur. A wide variety of other metals, including uranium are often incorporated in trace amounts. Common major impurities found with phosphorites are iron, alumina, quartz, montmorillonite and organic matter. Almost every element has been found, at least in trace amounts, in phosphorite minerals. Much of this arises from the remarkable nature of the Apatite crystal structure which allows substitution of the Ca ", and F by alternative cations and anions (Chapter 5.3). 

Other phases of Ca-P than ACP reveal a crystalline structure with characteristic peaks on XRD analysis. There is a broad range in crystal morphology depending on composition and preparation characteristics such as temperature, pH, impurity, and the presence of macromolecules. Impurities, as commonly occur in bone mineral, greatly influence crystallinity (reflecting crystal size and crystal strain) but depend on the type of substitution. For example, type B carbonated apatite (CO3 for PO4 substitution) has a lower crystallinity and increased solubility, whereas F substitution (F for OH) give the opposite effects due to a better fit of the F ion in the apatite crystal structure. 

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 principal mineral species is an apatite which is best defined as an isomorphous mixture consisting of two endmembers hydroxyapatite and carbonate apatite, i.e. Ca10(P04)6(OH)2 and (Na, Ca)io(P04, C03)6(0H)2. Because of the isomorphous nature of the material the proportions of C03 that substitute for P04 most certainly vary from one unit cell of bone mineral to another and no uniform composition should actually exist. The extent to which carbonate groups proxy for phosphate groups in the crystal lattice is in the order of a few to a maximal of 10% by weight. 

Crystallinity is a metric related to mineral maturity and is a measure of mineral crystallite size, mineral maturity, and the amount of substitution into the apatitic lattice. Crystallinity increases when crystals are larger and more perfect (i.e. less substitution). It is directly proportional to the inverse width of the 002 reflection (c-axis reflection) in the powder x-ray diffraction pattern of bone mineral. Several features in the infrared spectra of bone correlate with mineral crystallinity, most of which are components of the phosphate Vi,V3 envelope [8]. Any of these correlations should be usable in the Raman spectrum provided there are no other overlapping Raman peaks. However, there has been less emphasis on crystallinity in the bone Raman literature and only the inverse width of the phosphate Vi band has been used as a measure of crystallinity [9-12]. 

The minerals in bone change as an animal ages. Juvenile or immature bone contains amorphous calcium phosphate or the mineral brushite (CaHPO 2H20). Mature bone contains the mineral hydroxyapatite (Ca5(P04)3(0H)), but with some modifications. There tends to be some internal crystal disorder within bone apatite, and the carbonate radical may substitute for phosphate. Calcium deficiency and other health problems may affect the composition and properties of bone. 

Rey C, Collins B, Goehl T, Dickson IR, Glimcher MJ (1989) The carbonate enviromnent in bone mineral A resolution-enhanced Fourier transform infrared spectroscopy study. Calcif Tissue Inti 45 157-164 Roeder PL, MacArthur D, Ma XP, Palmer GR (1987) Cathodoluminescence and microprobe study of rare-earth elements in apatites. Am Mineral 72 801-811 Ronsbo JG (1989) Coupled substitution involving REEs and Na and Si in apatites in alkaline rocks from the Illimaussaq intmsions. South Greenland, and the petrological implications. Am Mineral 74 896-901 Rouse RC, Dunn PJ (1982) A contribution to the crystal chemistry of ellestadite and the sihcate srrlfate apatites. Am Mineral 67 90-96... 

Hughes JM, Drexler JW (1991) Cation substitution in the apatite tetrahedral site crystal stractrrres of type hydroxylellestadtite and type fermorite. N Jahrb Mineral Monatsh 327-336 Hughes JM, Cameron M, Crowley KD (1990) Crystal stmctures of natrrral ternary apatites solid solution in the Cas(P04)3X (X= F, OH, Cl) system. Am Mineral 75 295-304 Hughes JM, Foord EE, Hubbard MA, Ni YX (1995) The crystal stractrrre of cheralite-(Ce), (LREE, Ca, Th, U)(P, Si)04, a monazite-group mineral. N Jahrb Mineral Monatsh 344-350 Huminicki DMC, Hawthorne FC (2000) Refinement of the crystal stractrrre of vayrynenite. Can Mineral 38 1425-1432... 

The cations Sr and Ba concentrate in the vertebrate skeleton, and the amounts of these elements vary as a function of mineral stmcture. In vivo, strontium has been found to accumulate in bone by exchange onto crystal surfaces, and is rapidly washed out after exogenous strontium is withdrawn (Dahl et al. 2001). Incorporation of strontium into the crystal lattice as a substitute of calcium occurs at a low level in vivo, in contrast to the extensive lattice substitution of strontium for calcium in fossil bone. Strontium is not easily washed out of subfossil bone (Tuross et al. 1989), and the uptake of strontium into biological apatite was once proposed as a potentially useful chronometer analogous to fluorine uptake (Turekian and Kulp 1956). The combined uptake of strontium and fluorine into vertebrate calcified tissue may in no small part account for the existence of a fossil record. Both of these elements stabilize biological apatite, and add substantially to the crystal stability of apatite under acidic conditions (Curzon 1988). 

Calcium phosphate can be crystallized into salts such as hydroxyapatite and 8-whitlockite depending on the Ca P ratio, presence of water, impurities, and temperature. In a wet environment and at lower temperatures (<900°C), it is more likely that hydroxyl- or hydroxyapatite will form, while in a dry atmosphere and at a higher temperature, /3-whitlockite will be formed [ Park and Lakes 1992]. Both forms are very tissue compatible and are used as bone substitutes in a granular form or a solid block. The apatite form of calcium phosphate is considered to be closely related to the mineral phase of bone and teeth. 

Substitutions in the HA structure are possible. Substitutions for Ca, PO4, and OH groups result in changes in the lattice parameter as well as changes in some of the properties of the crystal, such as solubility. If the OH" groups in HA are replaced by F" the anions are closer to the neighboring Ca " ions. This substitution helps to further stabilize the structure and is proposed as one of the reasons that fluoridation helps reduce tooth decay as shown by the study of the incorporation of F into HA and its effect on solubility. Biological apatites, which are the mineral phases of bone, enamel, and dentin, are usually referred to as HA. Actually, they differ from pme HA in stoichiometry, composition, and crystallinity, as well as in other physical and mechanical properties, as shown in Table 35.7. Biological apatites are usually Ca deficient and are always carbonate substituted (COs) " for (P04). For... 

The mineral has a plate-like habit, the crystals being extremely small, about 4 nm by 50 nm by 50 nm. The mineral is a variant of hydroxyapatite, itself a variant of calcium phosphate Cajo(P04)6(OH)2 [7]. The crystals are impure. In particular there is about 4- % of carbonate replacing the phosphate groups, making the mineral technically a carbonate apatite, dahllite, and various other substitutions take place [8]. 

Some substitutions which can occur, wholly or in part, in natural or synthetic apatites are listed in Table 5.19. Many minerals and many synthetic orthophosphates adopt an apatite-type crystal structure which usually has hexagonal symmetry or a closely related structure. Only very recently has the successful synthesis of the iodide Ca,o(P04)6l2 been reported, moreover it is believed that introduction of the radioactive isotope T may give a product with important medical uses [87]. 

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