Siidraite, Pb₂Cu(OH)₂I₃, from Broken Hill, New South Wales, Australia: the third halocuprate(I) mineral

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) ų and Z = 32. The empirical formula derived from a combination of electron-microprobe analysis and structure determination is Pb_2.06Cu_0.89(OH)₂I_2.97, 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 $[01‾1]$ and the structure was refined taking this into account to R₁ = 0.037, wR₂ = 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.