Thirty-three samples from natural apatite (Ap) solid solutions, ideal structural formula
[9]Ca1
2[7]Ca2
3(
[4]PO
4)
3[3](F,OH,Cl), (Z = 2) were examined with electron-probe microanalysis, synchrotron high-resolution powder X-ray diffraction (HRPXRD), and Rietveld refinements. Apatite
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Thirty-three samples from natural apatite (Ap) solid solutions, ideal structural formula
[9]Ca1
2[7]Ca2
3(
[4]PO
4)
3[3](F,OH,Cl), (Z = 2) were examined with electron-probe microanalysis, synchrotron high-resolution powder X-ray diffraction (HRPXRD), and Rietveld refinements. Apatite has space group
P6
3/
m for the general chemical formula above. In Ap, the two different Ca sites are generally occupied by Ca, Mn, Sr, Na, or REE
3+ cations; the P site is occupied by P, Si, or S, and the X is occupied by F, OH, Cl, O
2−, or (CO
3)
2− anions. However, it may be possible for CO
32− + F
− anions to partially replace PO
43− groups. In this study, the unit-cell parameters
a,
c, and
c/
a ratio, vary smoothly and non-linearly with the unit-cell volume,
V. The data falls on two distinct trend lines. The average <P-O>[4] distance is nearly constant across the Ap series, whereas the average <O-P-O>[6] angle decreases linearly. The coordination numbers for the atoms are given in square brackets in the general chemical formula above. The average <Ca1-O>[9], <Ca2-O>[6], <Ca2-O,X>[7], and Ca2-X distances change non-linearly with increasing
V. Although Cl
− anion is larger than OH
− and F
− anions, the
c unit-cell parameter in F-Ap is larger than that in Cl-Ap. In Cl-Ap, the Ca2 polyhedra are larger than in F-Ap, so the O and Cl anions are under-bonded, which cause the Ca1 polyhedra to contract and charge balance the anions. Alternatively, the Ca1 polyhedra are smaller in Cl-Ap than in F-Ap, so the Ca1 polyhedra in Cl-Ap cause the
c axis to contract compared to that in F-Ap.
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