The related crystal structures of three uranyl oxide hydrate minerals, becquerelite, billietite, and protasite, have been determined by single-crystal X-ray diffraction. The chemical formulae have been determined by electron-microprobe analyses. Becquerelite, Ca[(UO2)6O4(OH)6]⋅8H2O, is orthorhombic, Pn21a, a = 13.8378(8), b = 12.3781(12), c = 14.9238(9) Å, Z = 4, R(Fobs) = 0.083 (2853 reflections). Billietite, Ba[(UO2)6O4(OH)6]⋅4H2O, is orthorhombic, Pbn21, a = 12.0720(22), b = 30.167(4), c = 7.1455(5) Å, Z = 4, R(Fobs = 0.139 (4104 reflections). Protasite, Ba[(UO2)3O3(OH)2]⋅ 3H2O, is monoclinic, Pn, a = 12.2949(16), b = 7.2206(10), c = 6.9558(8) Å, β = 90.401(15)°, Z = 2, R(Fobs = 0.073 (2505 reflections). Each uranyl ion is coordinated to five other oxygen atoms in a plane nearly perpendicular to the uranyl axis forming infinite sheets that resemble those of α-U3O8 in projection. The sheets are bonded together by large interlayer cations and water molecules.

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