Symesite, Pb 10 (SO 4 ) O 7 Cl 4 (H 2 O), a new PbO-related sheet mineral: Description and crystal structure

Symesite, Pb 10 (SO 4 )O 7 Cl 4 (H 2 O), is a Pb sheet mineral found in the oxidized zone of a Carboniferous Mn-Pb-Cu deposit at Merehead Quarry, Somerset. It occurs as pink crystal blebs up to 2 mm long and as pink crystalline aggregates up to 1 cm in diameter, and is associated with cerussite, hydrocerussite, paralaurionite, blixite, chloroxiphite, pyrolusite, coronadite, hematite, parkinsonite, and mereheadite. Crystals of symesite are blocky, translucent pink with a vitreous luster and a white streak. Mohs hardness is 4, D meas = 7.3(2) g/cm 3 and there is a perfect cleavage parallel to {001}; the refractive indices exceed 2. Electron-microprobe analysis gave the following composition (wt%): PbO 90.66, SO 3 3.15, Cl 5.83 (O = Cl 1.32), sum 98.32, giving the anhydrous formula Pb 10.31 S 1.00 O 11.22 Cl 4.18 ; solution of the crystal structure gave the ideal formula Pb 10 (SO 4 )O 7 Cl 4 (H 2 O). The six strongest peaks in the X-ray powder-diffraction pattern [d in Å, (I), (hkl)] are: 2.911 (10)(414, 32̅3), 3.286 (9)(004), 2.955 (9)(412̅), 2.793 (8)(711̅, 131), 6.573 (4)(002), 3.768 (4)(412, 32̅1). The structure of symesite was solved by direct methods and refined to an R index of 4.0%. Symesite is triclinic, space group B1̅, a = 19.727(2), b = 8.796(1), c = 13.631(2) Å, α = 82.21(1), β = 78.08(1), γ = 100.04(1)°, V = 2242.4(5) Å 3 , Z = 4. The structural unit of symesite is a [Pb 10 (SO 4 )O 7 ] 4+ single sheet; adjacent sheets are linked by layers of Cl. One-eleventh of the Pb atoms are replaced by S, with the addition of an apical oxygen to form an SO 4 tetrahedron and a compensating O vacancy within the PbO sheet. The distribution of Pb and SO 4 groups is highly ordered and defines a 22 cation-site superstructure motif within the PbO sheet. Eight of eleven interlayer anion sites are occupied by Cl, two are occupied by O of H 2 O groups, and one site is vacant. Incident bond-valence sums at O atoms indicate that hydrogen bonds occur between the H 2 O group and the apical oxygen of the SO 4 group, providing additional linkage between adjacent PbO sheets. The structure of symesite is closely related to those of tetragonal PbO and the family of PbO-related sheet minerals that includes nadorite, thorikosite, mereheadite, parkinsonite, and kombatite. There are ten non-equivalent Pb sites with coordination numbers of five, seven, or eight; these polyhedra are variants of the Pb[O 4 Cl 4 ] square-antiprism that is characteristic of these minerals.