The solid solution series of the sulfate apatite system Na6.45Ca3.55(SO4)6(FxCl1-x)1.55

The crystal structures of I I synthetic Na-Ca-sulfate-apatites, Na6.45Ca3.55(SO4)(6)(FxCl1-x)(1.55) with x = 1 to 0, were refined using X-ray diffraction data yielding residuals between R1 = 0.0409 and 0.0629 in space group P6(3)/m (Z = 1). Lattice constants vary between 9.436(2) and 9.5423(1) Angstrom (for a) and 6.919(2) to 6.8429(1) Angstrom (for c). The sulfate tetrahedra and the two symmetrically independent cation polyhedra about M1 and M2 (occupied by Na and Ca, respectively) are generally very similar to the analogous polyhedra in phosphate apatites. A common structural feature of all members of the solid solution series is a deficiency in the total Cl- and F-content compared with the phosphate apatites. The mean value of (Cl + F) for the solid solution equals 1.55(6) atoms per unit cell compared with the ideal value of 2 atoms per unit cell observed in phosphate apatites. The solid solution series Na6.45Ca3.55(SO4)(6)Cl-1.55-Na-6.45 Ca-3.55(SO4)(6)F-1.55 shows a gap toward the side of fluoride rich compounds. Under ambient pressure the gap exists between 0 < n(Cl)/n(Cl) + n(F) < 0.33, where n(Cl) and n(F) represent the numbers of Cl- and F-atoms per unit cell. (C) 2002 Elsevier Science.