Superstructure of moraesite: a synchrotron study

Moraesite (Lindberg et al., 1953) is a hydrated berillophosphate with ideal chemical formula Be2(PO4)(OH)·4H2O. Its structure has been known since 1992 (Merlino & Pasero, 1992) and refined in the space group C2/c, a = 8.553(6) Å, b = 12.319(6) Å, c = 7.155(8) Å, β = 97.93(9)°. The main structural feature of moraesite is the presence of large structural cavities occupied by water molecules. The latter are implicated in a complex system of hydrogen bonds where two possible hydrogen bond schemes are equally possible. Few very weak superstructure reflections were observed (Merlino & Pasero, 1992), indicating that the true unit cell of moraesite was probably three times larger, with a b parameter of 36.96 Å and space group symmetry Cc. It was suggested by the authors that the ordering of the hydrogen bond system, concerted with minor adjustments of the structure, could be responsible for the triplication of the b axis. X-ray diffraction data, more recently obtained through synchrotron radiation, confirmed the occurrence of superstructure reflections which were indexed on the basis of an unit cell with tripled b axis. The superstructure of moraesite (a = 8.572(3) Å, b = 36.971(8) Å = 3 x bsub, c = 7.153(2) Å, β = 97.72(1)°, space group Cc) was refined up to R = 0.081 for 1789 unique reflections with Fo > 4sigma(Fo) and 0.0906 for all 2088 data. The structural refinement confirmed that the triplication of the parameter b is due to the ordering of the two possible hydrogen bonding schemes. 1) Lindberg M.L., Pecora W.T. & Barbosa A.L.M. (1953) - Moraesite, a new hydrous beryllium phosphate from Minas Gerais, Brazil. American Mineralogist, 38, 1126-1133. 2) Merlino S. & Pasero M. (1992) - Crystal chemistry of beryllophosphates: The crystal structure of moraesite, Be2(PO4)(OH)·4H2O, Zeitschrift für Kristallographie, 201, 253-262.