Apatite, crystal symmetry

In a study of ternary apatites sensu stricto, Hughes et al. (1990) revealed a monoclinic variant of ternary apatite that illustrates the unaddressed complexities of apatite crystal chemistry. In the P2 lb variety, the positions of the non-column atoms are similar to those in P6j,/m ternary apatite, but the reduction in symmetry results from ordering in the anion columns, similar to the cause of monoclinicity in the chlorapatite and hydroxylapatite end-members. The positions of the column anions in hexagonal and monoclinic ternary apatite are depicted in Figure 5. The petrologic implications of monoclinic ternary apatite remain unexplored, and it is unknown how common the phase is. 

Fluorapatite (FA) corresponds to the chemical formula Caio(P04)eF2 and crystallises in the hexagonal space group PGs/m, with Z = 1 and unit-cell parameters a = b = 9.367 A and c = 6.884 A [1] (Fig. 2). From a structural viewpoint, fluorapatite is often considered as a crystalline model for other apatites and is seen as a reference apatitic array [2]. It is one of the very first apatite structures to have been solved. It has been thoroughly studied since the 1930s [3] and is well documented in the literature. In particular, Sudarsanan et al. [1] reported the single crystal refinement of X-ray diffraction (XRD) data, and the detailed description of atomic positions and local symmetry is fully available [4,5],... 

Correction equations similar to (4) but valid for cylinders, hexagonal prisms, and cubes have been published (Farley et al. 1996). These equations are appropriate for some phosphates (especially apatite), but may not apply to others, e g., to commonly low-symmetry crystals of monazite. An approximation that may be useful for low symmetry minerals relates Ft to surface to volume ratio (P), specifically ... 

Ternary Compounds REiMi(Si0i) 02. In order to follow the correlation between cation substitution and variation of cell dimensions in 2 8 mixed-cation apatites, we bring forward crystal data for 4 8 hydroxy apatites from 44). It is especially atractive to introduce the 4 6 apatite compounds into the discussion of crystal chemistry because their mixed-cation ratio exactly corresponds to the ratio of the symmetry-... 

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]. 

An Over-all view of the crystal structure of belovite-(Ce) (fig, 25) (Klevtsova and Borisov 1964) shows a similarity to that of apatite (fig. 24). However, cerium atoms occupy the nine-coordinated sites in contrast to the rare earths in apatite, and the symmetry of belovite-(Ce), P5, is lower than that of apatite, Pbj/m,... 

Thus different luminescence takes place after activation in air and in vacuum. As it was already mentioned, in the apatite structure Ca5(P04)3F there are two types of Ca site Ca(l) with C3 symmetry and Ca(II) with Cs symmetry. The Dq- Fq transition has been reported to exist in cases where the site symmetry allows an electric dipole process Cs, C , Cnv This is consistent with the conclusion that the centers with the main line at 574 nm belongs to Ca(II) site with Cs symmetry, while the asymmetry of the crystal field is lower for the center with the main line at 618 nm. The ratio between Dq- Fo and Dq- F2 which are forced electric dipoles to Dq- Fi which is magnetic dipole tells us about the synunetry of the site in which Eu + is situated (Blasse and Grabmaier 1994 Reisfeld 1973). In fluor-apatite activated in air the ratio is higher meaning a lower synunetry Ca(II) site, while in namral fluorapatite or synthetic one activated in vacuum the ratio is lower... 

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