CRYSTAL-CHEMISTRY OF BERYLLOPHOSPHATES - THE CRYSTAL-STRUCTURE OF MORAESITE, BE2(PO4)(OH).4H2O

Moraesite, Be2(PO4)(OH).4H2O, is a hydrated beryllophosphate mineral. The crystal structure of moraesite from Humaita, Brazil has been solved and refined in the space group C2/c, a = 8.553(6), b = 12.319(6), c = 7.155(8) angstrom, beta = 97.93(9)-degrees, up to R = 0.062 for 262 reflections. The structure is characterized by infinite double chains, running along c, formed by [PO4] and [BeO2(OH)H2O] tetrahedra. Within such chains three- and four-membered rings of tetrahedra occur. Additional water molecules, not linked to any cation, fill the large channels of the structure. Within each channel, the hydrogen bonds assume either of two schemes. Evidences were obtained that the true unit cell of moraesite is three times larger, with a b parameter of 36.96 angstrom and space group symmetry Cc. It is suggested 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.