Crystal structure of synthetic “magnesiorietveldite”, Raman spectroscopy data and bond-valence approach to the svornostite group of minerals

The new synthetic counterpart of a potentially new member of the svornostite group of minerals, "magnesiorietveldite", ideally Mg(UO 2 )(SO 4 ) 2 (H 2 O) 5 , is orthorhombic, space group Pmn2 1 , a = 12.7950( 9), b = 8.3288(4), c = 11.2962(4), V = 1203.80(11) 3 and Z = 2.The crystal structure (R 1 = 0.0168 for 3113 I > 3I reflections) contains uranyl-sulfate chains that are linked into sheets by M1O 2 (H 2 O) 4 and M2O 2 (H 2 O) 4 octahedra, where Mg is a dominant cation over Zn at both sites.The structure is well known and has already been described for other members of the svornostite group; the fundamental building unit -an infinite [(UO 2 )(SO 4 ) 2 (H 2 O)] 2-chain -is common in several uranyl minerals and synthetic compounds.Synthetic "magnesiorietveldite" was obtained after treatment of a specimen containing pitchblende with sulfuric acid.Electron microprobe analyses yielded the empirical formula (Mg 0.63 Zn 0.22 Cu 0.09 Fe 0.05 ) 0.99 (UO 2 )(SO 4 ) 2 5H 2 O, confirming the predominance of Mg over the other divalent cations.Infrared and Raman spectroscopy confirmed the presence of structurally non-equivalent molecular H 2 O and sulfate tetrahedra.We append an extensive discussion on the role of molecular H 2 O in the svornostite-group of minerals based on the bond-valence approach.