Pattersonite, PbFe3(PO4)2(OH)4[(H2O)0.5(OH)0.5]2, a new supergene phosphate mineral: description and crystal structure

Abstract

Pattersonite, PbFe 3 (PO 4 ) 2 (OH) 4 [(H 2 O) 0 5 (OH) 0 5 ] 2 , is a new supergene lead iron(III) phosphate mineral. It was discovered in a single boulder on the dumps of the Grube Vereinigung, near Eisenbach, Taunus, Hesse, Germany. The mineral forms aggregates of dark yellow, subparallel, pinacoidal plates up to 0.5 mm in size, and is accompanied by goethite, kintoreite and (rare) pyromorphite. Pattersonite is translucent, with a very pale yellow streak and adamantine lustre. It is brittle and shows one poor cleavage of unknown orientation, conchoidal fracture, a Mohs hardness of 4½ (VHN 25 = 530(80) kg/mm 2 ), and has D (meas.) >4.04, D (calc.) 4.17 g/cm 3 (from crystal-structure solution). Optically, it is biaxial negative, with α=1.86(1), β=1.917 (calc.), γ =1.93(1), 2 V α = 50(5)°. Dispersion is very strong ( r > v ) and the pleochroism is weak, X =nearly colourless to very pale yellow, Y = pale yellow to yellow, Z = yellow to dark yellow (depending on grain size), with absorption Z > Y > X . Orientation (polar coordinates in terms of ϕ and ρbased on (010) = 0°/90°) is X (−113°/85°); Y (155°/70°); Z (−10°/21°). Pattersonite is non-fluorescent. The mineral is triclinic, space group P 1 (No. 2), with a = 5.309(1), b = 7.211(1), c = 7.349(1) A, α = 87.74(3), β = 86.38(3), γ = 71.40(3)°, V = 266.06(7) A 3 (single-crystal data), and Z = 1. Strongest lines in the X-ray powder diffraction pattern are [ d in A ( I calc ) ( hkl )]: 4.848 (100) (110), 6.839 (64) (0–10), 3.547 (57) (1–10), 3.417 (52) (0–20), 3.022 (51) (112), 3.667 (47) (00–2), 2.8339 (45) (11–2). The crystal structure was determined from single-crystal X-ray diffraction data (Mo- K α, CCD area detector, R ( F ) = 5.6 %). The chemical formula obtained, PbFe 3 (PO 4 ) 2 (OH) 4 [(H 2 O) 0 5 (OH) 0 5 ] 2 , is in good agreement with quantitative electron microprobe analysis which also indicated negligible amounts of impurity elements. The unique crystal structure consists of an interrupted three-dimensional framework based on intersecting zig-zag chains of corner-sharing FeO 6 octahedra, linked by PO 4 tetrahedra. The [8]-coordinated Pb atom is located within elliptical [100] channels. The asymmetric unit contains one unique Pb, three Fe, one P, seven O (two of which represent OH groups and one a mixed [(H 2 O) 0 5 (OH) 0 5 ] ligand with a calculated bond-valence sum of 0.75 valence units) and 3.5 H atoms. The coordination environment of the Pb 2+ cation can be described as a monoclinically distorted cuboid; the lone electron pair on the cation is not stereochemically active. Average Pb-O, Fe-O and P-O bond lengths are 2.65, 2.02 and 1.53 A, respectively. Single-crystal laser-Raman spectroscopy is used to describe the vibrational properties. The new mineral may be considered a dimorph of kintoreite, ideally PbFe 3 (PO 4 ) 2 (OH,H 2 O) 6 , a member of the alunite super-group. The structure is, however, only remotely similar to that of kintoreite. The apparent rarity of the new mineral suggests it may be metastable with respect to kintoreite. The name honours Arthur Lindo Patterson (1902–1966), innovative crystallographer who devised the well-known Patterson method.