Al2GeO4(OH)2 : Rietveld refinement and stereochemical discussion

The structure of Al 2 GeO 4 (OH) 2 [C2/c; a = 5.4197(3)A, b = 8.2464(3)A, c = 9.3187(3)A, β = 110.933(3)°; Z = 4] was refined by X-ray Rietveld methods. It contains GeO 4 tetrahedra having all four O atoms in common with an AlO 2 OH framework built from AlO 4 (OH) 2 octahedra sharing three edges. The H atom could be located only approximately. The coordinations of the atoms are mostly as in topaz, Al 2 SiO 4 (F, OH) 2 , and its synthetic OH endmember, but the arrangement of the oxygen atoms is that of a distorted cubic close-packing, while in topaz it is based on a close-packing of the type ABAC.