Apatite element substitution

Cook (1972) has recognized two different types of phosphorite in northwest Queensland pelletal and non-pelletal. The lanthanide distribution in the former is normal for marine phosphorites, whereas it is depleted in the latter type with the exception of the heavy lanthanides which are relatively more abundant. Pelletal types contain greater concentrations of elements which are known to substitute in the structure of apatite, whereas the other types contain components which probably are of detrital origin or are derived from weathering. 

Available data from natural occurrences and synthetic materials have shown that apatites are capable of accommodating a large number of elements and molecules because of the remarkable tolerance of these phases to structural distortion and chemical substitution. The chemistry of apatites is further complicated by nonstoichiometry, order-disorder in all of the c-axis anion channel, tetrahedral and Ca sites, and the presence of elements with multiple valences (e.g., Cr, Eu, Mn, and S). The example on the uptake of REEs in FAp, OHAp, and ClAp showed that the complex compositional variation in apatites is controlled by both crystal-chemical and external factors. 

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... 

Belousova EA, Walters S, Griffin WL, O Reilly SY (2001) Trace-element signatures of apatites in granitoids from the Mt. Isa Inlier, northwestern Queensland. Austral J Earth Sci 48 603-619 Binder G, Troll G (1989) Coupled anion substitution in natural caibon-bearing apatites. Contrib Mineral Petrol 101 394-401... 

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). 

Mineral structure. The apatite structure is a complex association of a large Ca-site and PO4 tetrahedra around a channel containing a big anion such as F, Cl, OH. In detail, the unit-cell has two types of Ca bearing sites a seven-fold site, and a nine-fold site (see Boatner, this volume). The two sites are not equivalent, so large ions such as U and Pb are preferentially incorporated in the nine-fold site. Natural apatite contains mainly uranium as actinide element and almost no Th. The exact substitution mechanism is not fully understood. The replacement of Ca by would require two charge compensation mechanisms. This can be achieved by complex substitutions such as 2 Ca + P =... 

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). 

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