Crystal structures and crystal chemistry of the uranyl oxide hydrates becquerelite, billietite, and protasite

Abstract

The related crystal structures of three uranyl oxide hydrate minerals, becquerelite, billietite, and protasite, have been determined by single-crystalX-ray diffraction. The chemical formulae have been determined by electron-microprobe analyses. Becquerelite, Ca[(U02)604(OH)6].8H20,is orthorhombic, Pn21a,a = 13.8378(8), b = 12.3781(12),c = 14.9238(9) A, Z = 4, R(Fobs)= 0.083 (2853 reflections). Billietite, Ba[(U02)604(OH)6]. 4H20, is orthorhombic, Pbn21,a = 12.0720(22), b = 30.167(4), c = 7.1455(5) A, Z = 4, R(Fobs) = 0.139 (4104 reflections). Protasite, Ba[(U02)30l0H)2]. 3H20, is monoclinic, Pn, a = 12.2949(16),b = 7.2206(10), c = 6.9558(8) A, (j = 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 aU308 in projection. The sheets are bonded together by large interlayer cations and water molecules.