Publication Date:
2009-04-01
Description:
Steverustite, ideally Pb52+(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)º, 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, (Ī14); 3.934, 8, (Ī11); 3.348, 7, (313); 6.211, 6, (200); 4.797, 6, (211); 3.026, 6, (314); 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 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 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 and S–S distances are distinct for these two types of structures.
Print ISSN:
0026-461X
Electronic ISSN:
1471-8022
Topics:
Geosciences
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