Steverustite, Pb-5(2+)(OH)(5) Cu+(S6+O3S2-)(3) (H2O)(2), a new thiosulphate mineral from the Frongoch Mine Dump, Devils Bridge, Ceredigion, Wales: description and crystal structure
Cooper, MA; Hawthorne, FC; Moffatt, E
Steverustite, ideally 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/cm(3), and the calculated mean refractive index is 1.94. The mineral is monoclinic, P2(1)/n, a 12.5631(7), b 8.8963(5), c 18.0132(11) angstrom, beta 96.459(1)degrees, V 2000.5(3) angstrom(3), 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 (angstrom), (h k l): 3.934, 10. ((1) over bar 14) 3.934, 8, ((1) over bar 11) 3.348, 7, ((3) over bar 13); 6.211, 6, (200); 4.797, 6, (211); 3.026 6, ((3) over bar 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 Pb4.992+Cu0.96+(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 R-1 = 2.7% for 3366 unique (F-o > 4 sigma F) 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 [Pb-5(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 [Pb-5(OH)(5)] units and [Cu(S2O3)(3)] groups occur at the vertices of interpenetrating 3(6) nets to form layers of composition [Pb-5(OH)(5)Cu(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.