Abstract

Steverustite, ideally

\(\mathrm{Pb}_{5}^{2+}\)
(OH)5 [Cu+(S6+O3S2-)3](H2O)2, is a new supergene mineral from the Frongoch mine dump, Devils Bridge, Ceredigion, Wales. It generally forms fibrous fan-like bundles that occur in small cavities in quartz veins with other supergene species. Crystals are fibrous to acicular, elongated along [010], and are bounded by (h0l) faces too small to index reliably. It is transparent, colourless to white with a white streak, has a vitreous lustre, does not fluoresce under ultraviolet light and is brittle with a splintery fracture. The calculated density is 5.150 g/cm3, and the calculated mean refractive index is 1.94. The mineral is monoclinic, P21/n, a 12.5631(7), b 8.8963(5), c 18.0132(11) Å, β 96.459(1)°, V 2000.5(3) Å3, Z=4, a:b:c = 1.41217:1:2.02480. The seven strongest lines in the X-ray powder diffraction pattern are as follows: d (Å), I,(h k l): 3.934, 10, (⁠
\({\bar{1}}\)
14); 3.934, 8, (⁠
\({\bar{1}}\)
11); 3.348, 7, (⁠
\({\bar{3}}\)
13); 6.211, 6, (200); 4.797, 6, (211); 3.026, 6, (⁠
\({\bar{3}}\)
14); 2.837, 5, (016). Chemical analysis by electron microprobe gave PbO 72.59, SO3 15.78, Cu2O 4.47, S2- 6.32, H2O 4.83, O=S2- -3.15, total 100.84 wt.% where the amount of H2O was determined by crystal-structure analysis. The resulting empirical formula is
\(\mathrm{Pb}_{4.99}^{2+}\)
\(\mathrm{Cu}_{0.96}^{+}\)
(S6+O3S2-)3.03(OH)4.88(H2O)1.67, based on O + OH + H2O + S2- = 18.67 a.p.f.u. (atoms per formula unit) with H2O = 1.67 a.p.f.u. (from crystal-structure solution and refinement).

The crystal structure of steverustite was solved by direct methods and refined to R1 = 2.7% for 3366 unique (Fo > 4σF) reflections. There are five distinct Pb2+ cations with coordination numbers from [8] to [11], all of which show stereoactive lone-pair behaviour and which form a strongly bonded cluster of composition [Pb5(OH)5]. There is one Cu+ cation triangularly coordinated by three S2- atoms that belong to three thiosulphate groups, forming a Cu+(S6+O3S2-)3 group. The [Pb5(OH)5] units and [Cu(S2O3)3] groups occur at the vertices of interpenetrating 36 nets to form layers of composition [Pb5(OH)5Cu(S2O3)3] parallel to (010) which are linked by weaker bonds. Examination of the stereochemistry of thiosulphate and thionate structures shows that the combination of <S-O> and S-S distances are distinct for these two types of structures.

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