The crystal structure of turneaureite, Ca₅(AsO₄)₃Cl, the arsenate analog of chlorapatite, and its relationships with the arsenate apatites johnbaumite and svabite

Abstract

The crystal structure of turneaureite, ideally Ca5(AsO4)3Cl, was studied using a specimen from the Brattfors mine, Nordmark, Värmland, Sweden, by means of single-crystal X-ray diffraction data. The structure was refined to R1= 0.017 on the basis of 716 unique reflections with Fo> 4Ï\u83(Fo) in the P63/m space group, with unit-cell parameters a = 9.9218(3), c = 6.8638(2) Ã\u85, V = 585.16(4) Ã\u853. The chemical composition of the sample, determined by electron-microprobe analysis, is (in wt%; average of 10 spot analyses): SO30.22, P2O50.20, V2O50.01, As2O551.76, SiO20.06, CaO 41.39, MnO 1.89, SrO 0.12, BaO 0.52, PbO 0.10, Na2O 0.02, F 0.32, Cl 2.56, H2Ocalc0.58, O(=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)Σ5.02(As2.94P0.02S0.02Si0.01)Σ2.99O12[Cl0.47(OH)0.42F0.11]Σ1.00. Turneaureite is topologically similar to the other members of the apatite supergroup: columns of face-sharing M1 polyhedra running along c are connected through TO4tetrahedra 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

Fluoralforsite, Ba5(PO4)3F – a new apatite-group mineral from the Hatrurim Basin, Negev Desert, Israel

“…Previous studies show that complete disordering along anion columns is unrealistic due to contacts being too short with neighbouring anions (Hughes et al , 1989, 1990; Biagioni et al , 2017; Pieczka et al , 2018). The anion position in hexagonal ternary apatites depends on the adjacent anionic sites in the channel (Hughes and Rakovan, 2002).…”

“…The anion position in hexagonal ternary apatites depends on the adjacent anionic sites in the channel (Hughes and Rakovan, 2002). Therefore, the ordering arrangement of anions is required within individual columns, but the disordering of columns throughout the apatite structure allows the hexagonal symmetry to be maintained (Hughes et al , 1989; Hughes and Rakovan, 2002; Biagioni et al , 2017). In fluoralforsite, the F sites, located above and below the mirror plane and Cl site cannot be occupied simultaneously (Table 7, Fig.…”

Parafiniukite, Ca2Mn3(PO4)3Cl, a New Member of the Apatite Supergroup from the Szklary Pegmatite, Lower Silesia, Poland: Description and Crystal Structure

“…Since parafiniukite is a ternary apatite, containing also minor F, the situation is more complicated. Indeed, in order to avoid short anion-anion contacts, only some configurations are possible, confirming that local order within individual [001] anion columns should be present (e.g., [18,26]). In parafiniukite, the anion columns are occupied by (Cl 0.50 OH 0.40 F 0.10 ).…”

“…However, any trial to refine the structure in that space group symmetry was not successful and the space group P6 3 /m was eventually chosen. The crystal structure was refined starting from the atomic coordinates of turneaureite [18] using Shelxl-2014 [19]. Scattering curves for neutral atoms were taken from the International Tables for Crystallography [20].…”

“…However, the anisotropic displacement parameter of O(2) was negatively defined. The displacement parameter of the X anion was refined isotropically, as discussed in [18]. A residual of~2.5 e/Å 3 at (0, 0, 0.1667) was found.…”