The crystal structure of turneaureite, Ca (sub 5) (AsO (sub 4) ) (sub 3) Cl, the arsenate analog of chlorapatite, and its relationships with the arsenate apatites johnbaumite and svabite

The crystal structure of turneaureite, ideally Ca (sub 5) (AsO (sub 4) ) (sub 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 (sub 1) = 0.017 on the basis of 716 unique reflections with F (sub o) > 4sigma (F (sub o) ) in the P6 (sub 3) /m space group, with unit-cell parameters a = 9.9218(3), c = 6.8638(2) A, V = 585.16(4) A (super 3) The chemical composition of the sample, determined by electron-microprobe analysis, is (in wt%; average of 10 spot analyses): SO (sub 3) 0.22, P (sub 2) O (sub 5) 0.20, V (sub 2) O (sub 5) 0.01, As (sub 2) O (sub 5) 51.76, SiO (sub 2) 0.06, CaO 41.39, MnO 1.89, SrO 0.12, BaO 0.52, PbO 0.10, Na (sub 2) O 0.02, F 0.32, Cl 2.56, H (sub 2) O (sub calc) 0.58, O( identical with F+Cl) -0.71, total 99.04. On the basis of 13 anions per formula unit, the empirical formula corresponds to (Ca (sub 4.82) Mn (sub 0.17) Ba (sub 0.02) Sr (sub 0.01) ) (sub Sigma ) (sub 5.02) (As (sub 2.94) P (sub 0.02) S (sub 0.02) Si (sub 0.01) ) (sub Sigma ) (sub 2.99) O (sub 12) [Cl (sub 0.47) (OH) (sub 0.42) F (sub 0.11) ] (sub Sigma ) (sub 1.00) . Turneaureite is topologically similar to the other members of the apatite supergroup: columns of face-sharing M1 polyhedra running along c are connected through TO (sub 4) tetrahedra with channels hosting M2 cations and X anions. Owing to its particular chemical composition, the studied turneaureite can be considered as a ternary calcium arsenate apatite; consequently it has several partially filled anion sites within the anion columns. Polarized single-crystal FTIR spectra of the studied sample indicate stronger hydrogen bonding and less diverse short-range atom arrangements around (OH) groups in turneaureite as compared to the related minerals johnbaumite and svabite. An accurate knowledge of the atomic arrangement of this apatite-remediation mineral represents an improvement in our understanding of minerals able to sequester and stabilize heavy metals such as arsenic in polluted areas.