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Symesite, Pb₁₀ (SO₄) O₇ Cl₄ (H₂O), a new PbO-related sheet mineral: Description and crystal structure

¹ Department of Mineralogy, The Natural History Museum, Cromwell Road, London, SW7 5BD, U.K.
² Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, R3T 2N2, Canada

Correspondence: ^* E-mail: mdw{at}nhm.ac.uk

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, D_meas = 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 Pb_10.31 S_1.00 O_11.22 Cl_4.18; 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, 33), 3.286 (9)(004), 2.955 (9)(41), 2.793 (8)(71, 131), 6.573 (4)(002), 3.768 (4)(412, 31). The structure of symesite was solved by direct methods and refined to an R index of 4.0%. Symesite is triclinic, space group B, a = 19.727(2), b = 8.796(1), c = 13.631(2) Å, = 82.21(1), ß = 78.08(1), = 100.04(1)°, V = 2242.4(5) ų, 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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