A new Cu, Al fluoride disilicate CuAl2F2(Si2O7) and its relations to topaz

CuAl2F2(Si2O7) has been prepared by hydrothermal synthesis and its cn:stal structure was determined by single crystal X-ray diffraction: space group P nma, a = 8.8697(9), b = 14.084(2), c = 4.7553(5) Angstrom, wR(2) = 0.056, R = 0.022. Cu2+ shows elongated square pyramidal coordination. Edge- and cor ner-sharing [AlO4F2] octahedra with fluorine atoms in cis position form lay ers parallel to the ne plane. Along b these layers are linked by Si2O7 grou ps to form a three-dimensional framework [Al2F2(Si2O7](2-). In addition, th e [CuO5] pyramides connect two Al octahedra of neighbouring layers. The cry stal structure is discussed as a derivative from topaz structure. The modul ar (or polysomatic) approach is used for this purpose, and for modelling hy pothetical related compounds.