Keywords : Crystal chemistry
The Chemical Formula of Francolite in the Phosphorite of Akashat Formation from X-Ray Diffraction Analysis
Iraqi National Journal of Earth Science,
2018, Volume 18, Issue 2, Pages 61-88
Francolite (one of apatite group) found in marine formation sedimentary rocks. It is called carbonate-fluorapatite; owing to the substitution of CO3 for PO4 in its unit cell; Ca10(PO4CO3)6F2. Francolite unit cell composed of 4 Ca occupy the sites on ternary axis which are represent the Ca(I) sites, 6 Ca occupy the sites on hexad axis which are represent the Ca(II) sites. 6 P are joined with 24 oxygen to form 6 phosphate tetrahedrons and 2 sites occupied by F in the intermediate channel of the francolite crystal.
There is no differences will noticed along the three a-axis due to the slight effect of the substitution in the 4 Ca(I), this is may be due to the homogenous effects along the three a-axis in these sites. On the other hand, the substitution of planner carbonates for the tetrahedron phosphates cause decreasing in a-axis and slight increasing in the c-axis. These changes are reflected on the x-ray diffraction patterns; the angular distance (Δ2θ distance) between the two faces 410 and 004 will decrease, shifting in some peak positions and increasing in the width of reflecting peaks. The present study deals with the idea of the substitution of (CO32-+F-) for (PO43-) and suggests the Ca(I) sites are candidate to the substitution for large cations e.g. Sr2+, Ba2+and K+ and the Ca(II) sites for smaller cations e.g. Mg2+ .