Minerals and Mineralization
The composition CaO·2UO3·2SiO2 ·5H2O exists in two polymorphic forms, uranophane and beta-uranophane. The crystal structure of beta-uranophane has been solved using data collected on a Picker FACS-1 diffractometer. The cell is monoclinic with a = 13.898 ± 0.011 Å, b = 15.394 ± 0.014 Å, c = 6.609 ± 0.005 Å, and β = 91°25’ ± 1’. The space group is P21/a. The a-axis is parallel to the direction of fibrous growth.
The structure consists of chains of edge-shared (UO2)O5 polyhedra and SiO4 tetrahedra. These chains lie parallel to the fiber axis and account for the common acicular habit. The chains are cross linked by corner-sharing to form sheets parallel to (010). The Ca2+ ions and water molecules lie between these sheets. The structural formula for beta-uranophane is probably best written as Ca(UO2) (UOOH) (SiO4) (SiO3OH) ·4H2O.