The crystal structure of svabite, Ca5(AsO4)3F, an arsenate member of the apatite supergroup

Abstract

The crystal structure of svabite, ideally Ca 5 (AsO 4 ) 3 F, was studied using a specimen from the Jakobsberg mine, Varmland, Sweden, by means of single-crystal X-ray diffraction data. The structure was refined to R 1 = 0.032 on the basis of 928 unique reflections with F o > 4σ( F o ) in the P 6 3 / m space group, with unit-cell parameters a = 9.7268(5), c = 6.9820(4) A, V = 572.07(5) A 3 . The chemical composition of the sample, determined by electron-microprobe analysis, is (in wt%, average of 10 spot analyses): SO 3 0.49, P 2 O 5 0.21, V 2 O 5 0.04, As 2 O 5 51.21, SiO 2 0.19, CaO 39.31, MnO 0.48, SrO 0.03, PbO 5.19, Na 2 O 0.13, F 2.12, Cl 0.08, H 2 O calc 0.33, O (= F+Cl) −0.91, total 98.90. On the basis of 13 anions per formula unit, the empirical formula corresponds to (Ca 4.66 Pb 0.16 Mn 0.04 Na 0.03 ) ∑4.89 (As 2.96 S 0.04 Si 0.02 P 0.02 ) ∑3.04 O 12 [F 0.74 (OH) 0.24 Cl 0.01 ]. Svabite 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. The crystal structure of synthetic Ca 5 (AsO 4 ) 3 F was previously reported as triclinic. On the contrary, the present refinement of the crystal structure of svabite shows no deviations from the hexagonal symmetry. 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.