Abstract

Rickturnerite, which has the ideal formula Pb7O4[Mg(OH)4](OH)Cl3, is a new mineral from Torr Works (Merehead) quarry, near the village of Cranmore in Somerset, United Kingdom. It occurs as pale emerald green to grey porous aggregates of disordered interwoven minute fibrous crystals with mereheadite, cerussite, calcite, aragonite, mimetite, hydrocerussite, ‘plumbonacrite’ and an uncharacterized lead oxychloride, in cavities inside a manganite and pyrolusite pod. The crystals are typically less than 5 μm wide and 200 μm long, but they can reach 40×100 μm in cross-section and over 1 mm in length. The mineral is translucent with a vitreous lustre and each needle is brittle with an indistinct cleavage, breaking with a splintery fracture. The streak is white, the Mohs hardness ~3 and the density calculated using the empirical formula 6.886 g cm−3. Electron microprobe analyses yielded PbO 87.7, MgO 1.79, CuO 0.14, Cl 6.62 wt.%; H2O was calculated on the basis of structural considerations as 2.27 wt.% totalling 97.02 wt.%. A charge-balanced formula, based on 12 anions, is Pb7.16Mg0.81Cu0.03Cl3.40H4.60O8.60. Rickturnerite is orthorhombic Pnma, with a = 5.8024(6), b = 22.717(2), c = 25.879(3) Å, V = 3411.2(6) Å3 and Z = 8. The diffraction pattern contains strong reflections that define a subcell with a = 5.8034(5), b = 11.3574(9), c = 12.939(2) Å, V = 852.9(6) Å3 (space group Pmm2 which is related to the real unit cell by the transformation matrix [100/020/002]), and weak reflections that correspond to doubled b and c parameters. Since the difference between the large and small cells is only in a number of split and low-occupancy positions in the disordered region of the structure we provide the description of the subcell structure. The five strongest lines in the X-ray powder diffraction pattern [listed as dobs (Å), Iobs, (hkl)] are as follows: 6.474, 100, (400); 3.233, 73, (107); 2.867, 57, (705); 5.636, 44, (011); 3.112, 31, (802). The crystal structure was solved by direct methods and refined using 1318 unique reflections to R1 = 0.063. The structure is composed of a fully ordered part consisting of double [O2Pb3]2+ chains of oxocentred [OPb4] tetrahedra extended along the b-axis, which together with Cl ions form 2-dimensional blocks parallel to (001). In between these blocks, there is a disordered region containing ordered [Mg(OH)6]4− octahedra and low-occupancy Pb and OH sites with a slight degree of ordering; these produce the weak supercell reflections.

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