The Crystal Chemistry of the Zippeite Group

Abstract

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.

An integrated study of uranyl mineral dissolution processes: etch pit formation, effects of cations in solution, and secondary precipitation

“…All mineral samples are originally from the Shinkolobwe Mine, Democratic Republic of Congo. Additionally, zippeite, uranophane, becquerelite and Na-substituted metaschoepite were synthesized according to the methods of Burns et al [17], Klingensmith and Burns [18], Burns and Li [19] and Klingensmith et al [20], respectively. Phase identity and purity were verified using diffraction patterns collected with a Scintag theta-theta diffractometer with Cu K α radiation.…”

“…The latter morphology suggests that adsorbed Na is arranged differently than Mg, Ca and Pb on the surface of zippeite. Similarly, interstitial Na in Na-zippeite is arranged differently than interstitial Mg in Mg-zippeite [17].…”

“…For example, metaschoepite, becquerelite and uranophane are common uranyl minerals in the vadose zone of uranium ore deposits [16] and minerals of the zippeite-group are common in sulfate-rich mine tailings of uranium mines. Crystals of these minerals with relatively large basal surfaces (> 50 × 50 μm) became available through synthesis experiments by Burns and coworkers [17][18][19][20]. Furthermore, their relative high solubility in comparison to e.g.…”

“…The structure of zippeite consists of sheets of polymerized uranyl-and sulfate polyhedra with interstitial K + and H 2 O (Fig. 7a,b [17,48]). The stability of an edge along the basal surface of zippeite can be estimated by calculating the bond-valence deficiency along polyhedron chains parallel to the edge (for details see [7] …”

“…Fig. 7e shows the arrangement of interstitial Mg between the layers of Mg-zippeite [17]. The cations are arranged in rows parallel to [201], indicating that similar arrangements of adsorbed cation species on the surface of zippeite promote the stability of the latter edge.…”