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Abstract: 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 [(hkl), dobs (Å), I/Imax (%)]: [(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 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.

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Abstract: Laser heated diamond-anvil-cell experiments performed on a “tetragonal almandine pyrope phase” (TAPP) bulk composition constrain high-temperature phase relations between 6 and 48 GPa. Phase assemblages determined by synchrotron X-ray diffraction consist of TAPP + garnet ± pseudobrookite enstatite from 6 to 10 GPa, garnet from 13 to 22 GPa, Mg-perovskite + garnet from 23 to 30 GPa, and Mg-perovskite above 30 GPa. TAPP has the same structure at 1 atm and at high pressure based on in situ X-ray diffraction data, and is stable at a maximum pressure of 10–13 GPa at 1300–1700 K. These results rule out direct incorporation of TAPP in diamond at transition-zone or lower-mantle pressures. Possible origins of TAPP are (1) entrapment by diamonds in the upper mantle, and (2) retrograde formation from a high-pressure garnet or Mg-perovskite precursor. We suggest that single-phase TAPP inclusions and possibly composite TAPP-bearing inclusions originated as Mg-perovskite in the lower mantle in mafic protoliths, and that TAPP forms upon retrograde conversion at pressures below 13 GPa upon exhumation of the diamonds from the lower mantle to shallower depths of less than 400 km.