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Siidraite, Pb (sub 2) Cu(OH) (sub 2) I (sub 3) , from Broken Hill, New South Wales, Australia; the third halocuprate(I) mineral

Michael S. Rumsey, Mark D. Welch, Annette K. Kleppe and John Spratt
Siidraite, Pb (sub 2) Cu(OH) (sub 2) I (sub 3) , from Broken Hill, New South Wales, Australia; the third halocuprate(I) mineral
European Journal of Mineralogy (June 2017) 29 (6): 1027-1030

Abstract

Siidraite, Pb (sub 2) Cu(OH) (sub 2) I (sub 3) , 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 (super -3) . Siidraite is orthorhombic, space group Fddd, a = 16.7082(9) A, b = 20.846(1) A, c = 21.016(1) A, V = 7320.0(8) A (super 3) and Z = 32. The empirical formula derived from a combination of electron-microprobe analysis and structure determination is Pb (sub 2.06) Cu (sub 0.89) (OH) (sub 2) I (sub 2.97) , the ideal formula has (in wt%) 8.01 Cu (sub 2) O, 50.01 PbO, 42.65 I and 2.02 H (sub 2) O. The five strongest lines in the calculated X-ray powder diffraction pattern are [(hkl), d (sub obs) (A), I/I (sub max) (%)]: [(246), 2.746, 100], [(404), 3.270, 81], [(264), 2.738, 77], [(315), 3.312, 76], [(351), 3.296, 69]. The crystal used for structure determination had minor pseudomerohedral twinning on [011] and the structure was refined taking this into account to R (sub 1) = 0.037, wR (sub 2) = 0.052, GooF = 1.016, based upon 1368 unique reflections having I > 2sigma (I). The structure of siidraite is a framework comprising an alternation of two structural elements, a cubane-like [Pb (sub 4) (OH) (sub 4) ] (super 4+) group and a [Cu (sub 2) I (sub 6) ] (super 4) dimer of edge-sharing CuI (sub 4) tetrahedra with non-equivalent Cu. Six halocuprate groups surround each [Pb (sub 4) (OH) (sub 4) ] (super 4+) nucleus, and each halocuprate group is shared between six adjacent [Pb (sub 4) (OH) (sub 4) ] (super 4+) groups, five long Pb-I bonds are required to complete the co-ordination of each Pb atom. The resulting Pb(OH) (sub 3) I (sub 5) polyhedra are centred on a tetrahedron of O atoms to form a Pb (sub 4) (OH) (sub 4) I (sub 16) 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 Pb (sub 2) Cu (sub 2) (OH) (sub 2) I (sub 2) Br has been described that has cubane and CuI (sub 4) groups, but a very different structural topology from that of siidraite. Bideauxite, Pb (sub 2) Ag(OH)FCl (sub 3) , which has the [Pb (sub 4) (OH) (sub 4) ] (super 4+) group, shares some topological features with siidraite.


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 29
Serial Issue: 6
Title: Siidraite, Pb (sub 2) Cu(OH) (sub 2) I (sub 3) , from Broken Hill, New South Wales, Australia; the third halocuprate(I) mineral
Affiliation: Natural History Museum, Department of Earth Sciences, London, United Kingdom
Pages: 1027-1030
Published: 20170628
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Germany
References: 11
Accession Number: 2017-082611
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table
S31°57'38" - S31°57'38", E141°27'57" - E141°27'57"
Secondary Affiliation: Diamond Light Source, GBR, United Kingdom
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute. Abstract, copyright, Schweizerbart'sche Verlagsbuchhandlung. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201743
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