The crystal structure of turneaureite, ideally Ca 5 (AsO 4 ) 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 > 4σ( F o ) in the P 6 3 / m space group, with unit-cell parameters a = 9.9218(3), c = 6.8638(2) A, V = 585.16(4) A 3 . The chemical composition of the sample, determined by electron-microprobe analysis, is (in wt%; average of 10 spot analyses): SO 3 0.22, P 2 O 5 0.20, V 2 O 5 0.01, As 2 O 5 51.76, SiO 2 0.06, CaO 41.39, MnO 1.89, SrO 0.12, BaO 0.52, PbO 0.10, Na 2 O 0.02, F 0.32, Cl 2.56, H 2 O calc 0.58, O(≡F+Cl) −0.71, total 99.04. On the basis of 13 anions per formula unit, the empirical formula corresponds to (Ca 4.82 Mn 0.17 Ba 0.02 Sr 0.01 ) ∑ 5.02 (As 2.94 P 0.02 S 0.02 Si 0.01 ) ∑ 2.99 O 12 [Cl 0.47 (OH) 0.42 F 0.11 ] ∑ 1.00 . Turneaureite is topologically similar to the other members of the apatite supergroup: columns of face-sharing M 1 polyhedra running along c are connected through T O 4 tetrahedra with channels hosting M 2 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.