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Steverustite, Pb52+(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

Published online by Cambridge University Press:  05 July 2018

M. A. Cooper ,
F. C. Hawthorne  and
E. Moffatt
M. A. Cooper
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
F. C. Hawthorne*
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
E. Moffatt
Affiliation:
Canadian Conservation Institute, 1030 Innes Road, Ottawa, Ontario K1A 0M5, Canada
*
* E-mail: frank_hawthorne@umanitoba.ca
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Abstract

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 <S–O> and S–S distances are distinct for these two types of structures.

Keywords

steverustitenew mineralthiosulphateWalesUK.
Type
Research Article
Information
Mineralogical Magazine , Volume 73 , Issue 2 , April 2009 , pp. 235 - 250
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2009

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