Apatite packing

Apatite, a natural calcium fluoride phosphate, can adsorb low to moderate levels of dissolved metals from soils, groundwater, and waste streams. Metals naturally chemically bind to the apatite, forming extremely stable phosphate phases of metal-substituted apatite minerals. This natural process is used by UFA Ventures, Inc., and is called phosphate-induced metals stabilization (PIMS). The PIMS material can by used in a packed bed, mixed with the contaminated media, or used as a permeable barrier. The material may be left in place, disposed of, or reused. It requires no further treatment or stabilization. Research is currently being conducted on using apatite to remediate soil and groundwater contaminated with heavy metals, and the technology may also be applicable to radionuclides. The technology is not yet commercially available. 

Thus, the difference between these structures and the apatites is simply in the choice of tetrahedral interstices in the alloy which are used to accommodate the anions. This determines the type of cation-centred polyhedron for the minority cation. (We ignore the trivial variations in anions inserted into the [OOz] tunnels.) This suggests that the substitution of C03 for P04 in apatites may well be a less radical change than is usually assumed (when anion packing is accepted as being dominant). 

The different functions of the gla proteins and phosphoproteins must be related to the packing in hydroxyapatite and apatite [Ca2(OH)(P04)J,but probably depend too on the relative disposition of the oxygen donors in the gla and phosphoryl groups. 

Several comments are warranted about the apatite structure before its description is proffered. As noted in Table 1, the atoms lie on or near four (00/) planes in the atomic arrangement. Ca2, P, 01, 02, and the X anion (where X= F, OH, Cl) lie on (or are disordered about) special positions on the mirror planes at z = V4 and /4. Intercalated approximately halfway between these planes are Cal (in a special position at z = 0, -1/2) and 03, in the general position with z values of -0.07 and -0.57. On the basis of the layer structure of the atomic arrangement, O Keeffe and Hyde (1985) noted the similarity between the cation positions in the apatite structure and the MnsSis intermetallic phase, and offered a description of apatite as a ca//on-closest-packed atomic arrangement. Dai et al. (1991) and Dai and Harlow (1991) elucidated that intriguing cation-closest packing relationship in an examination of arsenate apatites, and their work is worthy of detailed review. 

The polyhedral components of the apatite atomic arrangement combine to form the atomic arrangement pictured in Figure 3. That [001] projection shows the packing of the three polyhedra described above to yield the atomic arrangement of apatite sensu stricto. 

The majority of new materials for SOFCs are perovskite stmctured oxides of general form ABO3.5 [23]. The ideal perovskite structure is a cubic close-packed ABO3 structure where the B-site cation sits within the octahedral interstices. Fig. 3.5. This stmcture is very flexible toward cation composition and tolerates large substitution fractions on either cation site. The Goldschmidt factor, a ratio of A, B, and O ionic radii, is often utilized to predict if a metal oxide will crystallize into the perovskite structure [24]. The A site of the commonly utilized perovskites is typically occupied by La, Ca, Sr, or Ba. The B site is typically a transition metal. Other stmctures investigated include double perovskites, apatites, and fluorites. 

The most characteristic features of the Early Devonian protegula are their ornament. Surfaces of all well-preserved protegula feature pits or tubercles. Of the 22 specimens with a preserved primary layer, thirteen feature pits and the remaining nine display tubercles. The pits are rather deep, more or less circular in outline, hemispherical in profile, and measure usually 1-3 pm in diameter (Figs 10.2C,E and 10.3A). Pits are rather loosely packed, bounded by gentle and wide rims and have walls composed of spherular apatite (Fig. 10.2H). Some of the pits are compound features consisting of 2-3 smaller pits that are separated from one another by low ridges (Fig. 10.2H). 

Analysis of synthetic amorphous calcium phosphate indicates that it is composed of 1 nm diameter Ca9(P04)6 clusters which close-pack randomly to form 30-80 nm spheres. These take up calcium and hydroxyl ions to form crystalline apatite and the following stepwise sequence of reactions can be envisaged ... 

HA displays ionic character, and its crystalline structure can be describe like a compact hexagonal packing of oxygen atom with metals occupying the tetrahedral and octahedral holes of the periodic network. The basic apatite structure is hexagonal with space group Pbs/m and approximate lattice parameters a = 9.4 and c = 6.9 A, being the fluorapatite... 

Similar trends in variation of the intensity of ion currents with sputtering depths were observed for Al-doped AILS (Fig. 45). In this case, the increase of La ion current is more pronoimeed than in the case of Sr-doped apatite. It apparently correlates with admixture of perovskite-like LaAIOs phase whieh can be located in the surface layer of apatite partieles. Indeed, in the densely-packed perovskite-like phase known for its high lattice stability, sputtering is expected to proeeed less easier than in the case of framework apatite structure. A higher bonding strength of eations in the surface layer revealed by SIMS and explained by preferential location of admixed phases in it is expected to be reflected in a lower grain boimdary eonductivity. 

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