Apatite, Ca10 (PO4)6 (OH,F,Cl)2: Structural Variations, Natural Solid Solutions, Intergrowths, and Zoning

Abstract

Thirty-three samples from natural apatite (Ap) solid solutions, ideal structural formula[9] Ca12[7] Ca23 ([4] PO4)3[3] (F,OH,Cl), (Z = 2) were examined with electron-probe microanalysis, syn-chrotron high-resolution powder X-ray diffraction (HRPXRD), and Rietveld refinements. Apatite has space group P63 /m for the general chemical formula above. In Ap, the two different Ca sites are generally occupied by Ca, Mn, Sr, Na, or REE3+ cations; the P site is occupied by P, Si, or S, and the X is occupied by F, OH, Cl, O2−, or (CO3)2− anions. However, it may be possible for CO32− + F− anions to partially replace PO43− 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 [4] distance is nearly constant across the Ap series, whereas the average [6] angle decreases linearly. The coordination numbers for the atoms are given in square brackets in the general chemical formula above. The average [9], [6], [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.