Calcium fluorapatite unit cell

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... 

Apatite Calcium phosphate compound, stoichiometric chemical formula Ca5(P04)3 X, where X is OH (hydroxyapatite), F (fluorapatite), CU (chlorapatite), etc. There are two molecules in the basic crystal unit cell. 

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... 

Figure 28.4 The relative positions of the ions in part of a crystal of fluorapatite as they would appear when viewed along the c axis. The ions do not all lie in the same plane. The small hexagonal outlines have been formed by joining the projection of ions in the calcium triangles . The parallelogram is the boundary of a unit cell. N.B. It is not possible in a simple two-dimensional representation to show the relationship Caio(P04) F2 quantitatively because fractions of ions are shared between different unit cells...

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