The crystal structure of turneaureite, Ca-5(AsO4)(3)Cl, the arsenate analog of chlorapatite, and its relationships with the arsenate apatites johnbaumite and svabite

摘要

T he crystal structure of turneaureite, ideally Ca-5(AsO4)(3)Cl, was studied using a specimen from the Brattfors mine, Nordmark, Varmland, Sweden, by means of single-crystal X-ray diffraction data. The structure was refined to R-1 = 0.017 on the basis of 716 unique reflections with F-o > 4 sigma(F-o) in the P6(3)/m space group, with unit-cell parameters a = 9.9218(3), c = 6.8638(2) angstrom, V = 585.16(4) angstrom(3). The chemical composition of the sample, determined by electron-microprobe analysis, is (in wt%; average of 10 spot analyses): SO3 0.22, P2O5 0.20, V2O5 0.01, As2O5 51.76, SiO2 0.06, CaO 41.39, MnO 1.89, SrO 0.12, BaO 0.52, PbO 0.10, Na2O 0.02, F 0.32, Cl 2.56, H2Ocalc 0.58, O(equivalent to F+Cl) -0.71, total 99.04. On the basis of 13 anions per formula unit, the empirical formula corresponds to (Ca4.82Mn0.17Ba0.02Sr0.01)(Sigma 5.02) (As2.94P0.02S0.02Si0.01)(Sigma 2.99)O-12[Cl-0.47(OH)(0.42)F-0.11](Sigma 1.00).