The crystal structure of magnesiozippeite, Mg[(UO2)2O2(SO4)](H2O)3.5, from East Saddle Mine, San Juan County, Utah (U.S.A.)

The crystal structure of natural magnesiozippeite, Mg[(UO2)2O2(SO4)](H2O)3.5, from East Saddle mine, San Juan County, U.S.A., has been determined by single-crystal X-ray diffraction analysis and refined to Robs = 0.0348. The mineral is monoclinic, crystallizing in the space group C2/m, with unit cell parameters a = 8.7005(5), b = 14.2541(6), c = 8.8433(5)Å, β = 104.408(5)°, V = 1062.24(9) Å3 and Z = 4. The structure consists of the structural sheets of zippeite topology, represented by a structure unit [(UO2)2O2(SO4)]2−, and an interlayer where two Mg2+ Φ6 octahedra are located. The crystal structure of natural magnesiozippeite shows considerable differences to the structure of previously reported synthetic analogue, namely in coordination of Mg2+ in the interlayer.