Abstract

Symesite, Pb10 (SO4) O7 Cl4 (H2O), 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, Dmeas = 7.3(2) g/cm3 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, SO3 3.15, Cl 5.83 (O = Cl 1.32), sum 98.32, giving the anhydrous formula Pb10.31 S1.00 O11.22 Cl4.18; solution of the crystal structure gave the ideal formula Pb10 (SO4) O7 Cl4 (H2O). 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)(71̅1, 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) Å3, Z = 4. The structural unit of symesite is a [Pb10 (SO4) O7]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 SO4 tetrahedron and a compensating O vacancy within the PbO sheet. The distribution of Pb and SO4 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 H2O groups, and one site is vacant. Incident bond-valence sums at O atoms indicate that hydrogen bonds occur between the H2O group and the apical oxygen of the SO4 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[O4 Cl4] square-antiprism that is characteristic of these minerals.

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