Original paper
Hydroxylhedyphane, Ca2Pb3(AsO4)3(OH), a new member of the apatite supergroup from Långban, Sweden
Biagioni, Cristian; Hålenius, Ulf; Pasero, Marco; Karlsson, Andreas; Bosi, Ferdinando
European Journal of Mineralogy Volume 31 Number 5-6 (2019), p. 1015 - 1024
44 references
published: Dec 20, 2019
published online: Jul 27, 2019
manuscript accepted: May 30, 2019
manuscript revision received: May 29, 2019
manuscript received: Apr 17, 2019
Abstract
The new mineral species hydroxylhedyphane, ideally Ca2Pb3(AsO4)3(OH), has been discovered in the Långban Fe–Mn–(Ba–As–Pb–Sb) deposit, Filipstad district, Värmland, Sweden. It occurs as colourless prismatic crystals, up to 2.5 cm in length, forming an oriented intergrowth with a serpentine-subgroup mineral, as fracture-fillings cutting braunite and hausmannite ore. Electron-microprobe analysis yielded (mean of 16 spot analyses): P2O5 0.96(9), V2O5 0.07(4), As2O5 25.36(19), SiO2 0.91(2), CaO 7.74(11), MnO 0.03(2), BaO 2.95(10), PbO 59.81(50), Na2O 0.09(2), F 0.06(7), Cl 1.03(6), H2Ocalc 0.46, O = −(F + Cl) = –0.26, total 99.21. On the basis of 13 anions per formula unit, taking into account the crystal-structure data and the general formula of apatite-group minerals, the empirical formula of hydroxylhedyphane is M(1)(Ca1.56Pb0.41Mn0.01Na0.03)Σ2.01M(2)(Pb2.80Ba0.24Ca0.09)Σ3.13T(As2.64P0.16V0.01Si0.18)Σ2.99O12X[(OH)0.61Cl0.35F0.04]. Main diffraction lines are [d(Å) (relative intensity) hkl]: 4.354 (21) 200; 4.138 (24) 111; 3.643 (33) 002; 3.291(31) 210; 2.999 (100) 211; 2.949 (41) 112; 2.903 (86) 300; and 2.177 (23) 400. Hydroxylhedyphane is trigonal, space group P 3 ¯ , with a = 10.0414(3), c = 7.2752(2) Å, V = 635.28(4) Å3, Z = 2. The crystal structure has been refined to R1 = 0.034 on the basis of 1356 unique reflections with Fo > 4σ (Fo) and 67 refined parameters. It agrees with the topology of the other apatite-supergroup minerals, with a symmetry reduction from P63/m to P 3 ¯ and the splitting of the 4f M(1) site in the space group P63/m into two distinct 2d sites M(1) and M(1)’. This lowering of symmetry is likely related to the preferential partitioning of Pb at the M(1) site. Finally, infrared spectroscopy suggests the possible occurrence of minor CO2 in hydroxylhedyphane, but the crystal-structure refinement did not permit locating CO3 groups.
Keywords
hydroxylhedyphane • apatite supergroup • calcium • lead • arsenic • arsenate • crystal structure • Långban • Sweden