Revision of the crystal structure and chemical formula of haiweeite, Ca(UO2)2(Si5O12)(OH)2{middle dot}6H2O

Abstract

The previously published crystal structure study suggested that haiweeite is orthorhombic, Cmcm , with a = 7.125(1), b = 17.937(2), c = 18.342(2) A, and V = 2344.3(7) A 3 , and an ideal chemical formula Ca[(UO 2 ) 2 Si 5 O 12 (OH) 2 ]·3H 2 O, with Z = 4. Using single-crystal X-ray diffraction and electron microprobe analysis we re-examined haiweeite from the Teofilo Otoni, Minas Gerais, Brazil. Our diffraction experiment provided weak reflections responsible for doubling of the b cell parameter (for the current space-group settings), leading finally to the choice of a different space group. Haiweeite is thus orthorhombic, the space group Pbcn , with the unit-cell parameters a = 18.3000(5), b = 14.2331(3), c = 17.9192(5) A, V = 4667.3(2) A 3 , and an ideal formula Ca[(UO 2 ) 2 (SiO 3 OH) 2 (Si 3 O 6 )]·6H 2 O (6.25 H 2 O inferred from the thermal analysis; 7.50 H 2 O from the structure model), with Z = 8. The structure refinement yielded R 1 = 0.0512 for 2498 observed reflections [ I obs > 3σ( I )] and wR 2 = 0.1286 for all 6117 unique reflections. Structure solution confirmed by subsequent refinement provided a structure model with full occupancies for U, Si, and Ca atoms, contrasting to previous average structure model. Although the general topology of our structure resembles that reported previously, all Si and O sites in our structure are fully occupied, in contrast to the previous structure determination.