Siidraite, Pb 2 Cu(OH) 2 I 3 , from Broken Hill, New South Wales, Australia: the third halocuprate(I) mineral
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Issue date
01/12/2017Submitted date
2018-04-27Subject Terms
siidraitenew mineral
halocuprate(I)
[Pb4(OH)4]4+ group
iodine
Broken Hill
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Siidraite, Pb2Cu(OH)2I3, is a new mineral from the Broken Hill deposit in New South Wales, Australia. It occurs as an extremely rare secondary phase alongside marshite, other lead and copper secondaries and supergene cuprite on a single specimen, BM 84642 preserved in the collection of the Natural History Museum, London. Siidraite is yellow and occurs in crystalline grainy aggregates up to 0.3 mm around relict galena. The mineral is translucent with a vitreous lustre and yellow streak, no cleavages or forms have yet been observed. It is non-fluorescent in mixed-wavelength UV light. The calculated density is 6.505 g cm−3. Siidraite is orthorhombic, space group Fddd, a = 16.7082(9) Å, b = 20.846(1) Å, c = 21.016(1) Å, V = 7320.0(8) Å3 and Z = 32. The empirical formula derived from a combination of electron-microprobe analysis and structure determination is Pb2.06Cu0.89(OH)2I2.97, the ideal formula has (in wt%) 8.01 Cu2O, 50.01 PbO, 42.65 I and 2.02 H2O. The five strongest lines in the calculated X-ray powder diffraction pattern are [(h k l), d obs (Å), I/I max (%)]: [(2 4 6), 2.746, 100], [(4 0 4), 3.270, 81], [(2 6 4), 2.738, 77], [(3 1 5), 3.312, 76], [(3 5 1), 3.296, 69]. The crystal used for structure determination had minor pseudomerohedral twinning on [ 0 1 ‾ 1 ] and the structure was refined taking this into account to R 1 = 0.037, wR 2 = 0.052, GooF = 1.016, based upon 1368 unique reflections having I > 2σ(I). The structure of siidraite is a framework comprising an alternation of two structural elements, a cubane-like [Pb4(OH)4]4+ group and a [Cu2I6]4− dimer of edge-sharing CuI4 tetrahedra with non-equivalent Cu. Six halocuprate groups surround each [Pb4(OH)4]4+ nucleus, and each halocuprate group is shared between six adjacent [Pb4(OH)4]4+ groups, five long Pb–I bonds are required to complete the co-ordination of each Pb atom. The resulting Pb(OH)3I5 polyhedra are centred on a tetrahedron of O atoms to form a Pb4(OH)4I16 cluster. Siidraite has a unique composition and structure. It is the third naturally occurring halocuprate(I) after marshite and nantokite. A compositionally similar synthetic compound Pb2Cu2(OH)2I2Br has been described that has cubane and CuI4 groups, but a very different structural topology from that of siidraite. Bideauxite, Pb2Ag(OH)FCl3, which has the [Pb4(OH)4]4+ group, shares some topological features with siidraite.Citation
Siidraite, Pb2Cu(OH)2I3, from Broken Hill, New South Wales, Australia: the third halocuprate(I) mineral. Rumsey, Michael S.; Welch, Mark D.; Kleppe, Annette K.; Spratt, John. European Journal of Mineralogy Volume 29 Number 6 (2017), p. 1027 - 1030. DOI: 10.1127/ejm/2017/0029-2676Journal
European Journal of MineralogyType
Journal ArticleItem Description
This article is available on open access repositories published under a Creative Commons BY-NC-ND license: https://creativecommons.org/licenses/by-nc-nd/3.0/. The attached file is the published version of the article.NHM Repository
ISSN
0935-1221ae974a485f413a2113503eed53cd6c53
10.1127/ejm/2017/0029-2676
Scopus Count