A New Cu, Al Fluoride Disilicate CuAl2F2(Si2O7) and its Relations to Topaz

CuAl₂F₂(Si₂O₇) has been prepared by hydrothermal synthesis and its crystal structure was determined by single crystal X‐ray diffraction: space group Pnma, a = 8.8697(9), b = 14.084(2), c = 4.7553(5) Å, wR₂ = 0.056, R = 0.022. Cu²⁺ shows elongated square pyramidal coordination. Edge‐ and corner‐sharing AlO₄F₂] octahedra with fluorine atoms in cis position form layers parallel to the ac plane. Along b these layers are linked by Si₂O₇ groups to form a three‐dimensional framework Al₂F₂(Si₂O₇]^2–. In addition, the CuO₅] pyramides connect two Al octahedra of neighbouring layers. The crystal structure is discussed as a derivative from topaz structure. The modular (or polysomatic) approach is used for this purpose, and for modelling hypothetical related compounds.