Hydroxylhedyphane, Ca2Pb3(AsO4)3(OH), a new member of the apatite supergroup from Långban, Sweden

Abstract

The new mineral species hydroxylhedyphane, ideally Ca2Pb3(AsO4)(3)(OH), has been discovered in the Langban Fe-Mn-(Ba-As-Pb-Sb) deposit, Filipstad district, Varmland, 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), H(2)O(calc )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)(Sigma 2.01) (M(2)) (Pb2.80Ba0.24Ca0.09)(Sigma 3.13) (T)(AS(2.64)P(0.) 16V0.01Si0.18)(Sigma 2.99)O-12 (X)[(OH)(0.61)Cl0.35F0.04]. Main diffraction lines are [d(angstrom) (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) over bar, with a= 10.0414(3), c = 7.2752(2) angstrom, v = 635.28(4) angstrom(3), Z= 2. The crystal structure has been refined to R 1 = 0.034 on the basis of 1356 unique reflections with F-o > 4 sigma (F-o) and 67 refined parameters. It agrees with the topology of the other apatite-supergroup minerals, with a symmetry reduction from P6(3)/m to P (3 ) over bar and the splitting of the 4f M(1) site in the space group P6(3)/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.