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Single crystals of eight zippeite-group compounds have obtained using mild hydrothermal synthesis techniques. The structure of each has been determined with single-crystal diffraction data collected using MoK␣ X-radiation and an APEX CCD-based detector, and refined on the basis of F 2 for all unique data. The structure of zippeite, K 3 (H 2 O) 3 [(UO 2) 4 (SO 4) 2 O 3 (OH)], is monoclinic, C2, a 8.7524(4), b 13.9197(7), c 17.6972(8) Å, ␤ 104.178(1)°, V 2090.39(17) Å 3 , R1 3.30%, D c 4.794 g/cm 3. The structure of sodium-zippeite, Na 5 (H 2 O) 12 [(UO 2) 8 (SO 4) 4 O 5 (OH) 3 ], is monoclinic, P2 1 /n, a 17.6425(11), b 14.6272(9), c 17.6922(11) Å, ␤ 104.461(1)°, V 4421.0(5) Å 3 , R1 6.88%, D c 4.517 g/cm 3. The structure of magnesium-zippeite, Mg(H 2 O) 3.5 [(UO 2) 2 (SO 4)O 2 ], is monoclinic, C2/m, a 8.6514(4), b 14.1938(7), c 17.7211(9) Å, ␤ 104.131(1)°, V 2110.24(18) Å 3 , R1 2.39%, D c 4.756 g/cm 3. The structure of zinc-zippeite, Zn(H 2 O) 3.5 [(UO 2) 2 (SO 4)O 2 ], is monoclinic, C2/m, a 8.6437(10), b 14.1664(17), c 17.701(2) Å, ␤ 104.041(3)°, V 2102.7(4) Å 3 , R1 4.57%, D c 5.032 g/cm 3. The structure of cobalt-zippeite, Co(H 2 O) 3.5 [(UO 2) 2 (SO 4)O 2 ], is monoclinic, C2/m, a 8.650(4), b 14.252(9), c 17.742(10) Å, ␤ 104.092(19)°, V 2122(2) Å 3 , R1 5.55%, D c 4.948 g/cm 3. The structure of (NH 4) 4 (H 2 O)[(UO 2) 2 (SO 4)O 2 ] 2 is monoclinic, C2/m, a 8.6987(15), b 14.166(2), c 17.847(3) Å, ␤ 104.117(4)°, V 2132.9(3) Å 3 , R1 4.31%, D c 4.442 g/cm 3. The structure of (NH 4) 2 [(UO 2) 2 (SO 4)O 2 ] is orthorhombic, Cmca, a 14.2520(9), b 8.7748(5), c 17.1863(10) Å, V 2149.3(2) Å 3 , R1 5.11%, D c 4.353 g/cm 3. The structure of Mg 2 (H 2 O) 11 [(UO 2) 2 (SO 4)O 2 ] 2 is monoclinic, P2 1 /c, a 8.6457(4), b 17.2004(8), c 18.4642(9) Å, ␤ 102.119(1)°, V 2684.6(2) Å 3 , R1 4.73%, D c 3.917 g/cm 3. Each structure contains the zippeite-type sheet consisting of chains of edge-sharing uranyl pentagonal bipyramids that are cross-linked by vertex sharing with sulfate tetrahedra, although the compositional details of the sheet are varied. The interlayer configurations are diverse, and are related to the bonding requirements of the sheets.

Understanding the geochemical behaviour of the siderophile elements--those tending to form alloys with iron in natural environments--is important in the search for a deep-mantle chemical 'fingerprint' in upper mantle rocks, and also in the evaluation of models of large-scale differentiation of the Earth and terrestrial planets. These elements are highly concentrated in the core relative to the silicate mantle, but their concentrations in upper mantle rocks are higher than predicted by most core-formation models. It has been suggested that mixing of outer-core material back into the mantle following core formation may be responsible for the siderophile element ratios observed in upper mantle rocks. Such re-mixing has been attributed to an unspecified metal-silicate interaction in the reactive D'' layer just above the core-mantle boundary. The siderophile elements are excellent candidates as indicators of an outer-core contribution to the mantle, but the nature and existence of possible core-mantle interactions is controversial. In light of the recent findings that grain-boundary diffusion of oxygen through a dry intergranular medium may be effective over geologically significant length scales and that grain boundaries can be primary storage sites for incompatible lithophile elements, the question arises as to whether siderophile elements might exhibit similar (or greater) grain-boundary mobility. Here we report experimental results from a study of grain-boundary diffusion of siderophile elements through polycrystalline MgO that were obtained by quantifying the extent of alloy formation between initially pure metals separated by approximately 1 mm of polycrystalline MgO. Grain-boundary diffusion resulted in significant alloying of sink and source particles, enabling calculation of grain-boundary fluxes. Our computed diffusivities were high enough to allow transport of a number of siderophile elements over geologically significant length scales (tens of kilometres) over the age of the Earth. This finding establishes grain-boundary diffusion as a potential fast pathway for chemical communication between the core and mantle.

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