Abstract:
Studtite, UO4·4H2O, and metastudtite, UO4·2H2O, are the only minerals thought to contain peroxide. Determination of the structure of studtite has shown it to contain peroxide, with the structural formula [(UO2)(O2)(H2O)2](H2O)2. The structure is monoclinic, space group C2/c, a = 14.068(6), b = 6.721(3), c = 8.428(4) Å, β = 123.356(6)°, V = 665.6(3) Å3, Z = 4. It was refined on the basis of F2 for 1398 unique reflections collected using MoKα X-radiation and a CCD-based detector to R1 = 3.66%, calculated for the 716 unique observed reflections (|Fo| ≥ 4σF). The structure of studtite contains one symmetrically distinct U6+ cation and four O atoms, two of which occur as H2O groups. The O-O bond-length in the peroxide group is 1.46(1) Å. The U6+ cation occurs as part of a linear (UO2)2+ uranyl ion, and each U6+ cation is bonded to six additional O atoms, two of which are H2O groups, and four of which are O atoms of peroxide groups. The O-O bonds of two peroxide groups constitute two equatorial edges of each distorted uranyl hexagonal bipyramid. Uranyl polyhedra are polymerized into chains extending along [001] by sharing peroxide groups. Chains are linked by H bonds extending to and from an interstitial H2O group. It is proposed that studtite forms by incorporating peroxide created by alpha-radiolysis of water, and that radiation is necessary for its formation in nature.
