Hydrothermal synthesis, crystal structure, and properties of CuAl2Si2O7(F,OH)(2)

A new copper aluminum silicate CuAl2Si2O7(F,OH)(2) (cualsite) was synthesiz ed hydrothermally from SiO2 and Al2O3 in a mineralizing solution 1/3 Na3AlF 6 + 4/9 Al(NO3)(3) + 1/9 Cu(NO3)(2) + 27.8H(2)O at 600 degrees C and 150 MP a, Cualsite grew epitaxially on the pinacoidal plane of the topaz single-cr ystal seed. Blue crystals of the bipyramidal habitus exhibit orthorhombic s ymmetry Pbnm with a = 4.757(1) Angstrom, b = 8.877(3) Angstrom, c = 14.075( 4)Angstrom, and Z= 4, Structural analysis led to R-1 = 0.018 and R-w = 0.04 6 for 899 unique reflections. The cualsite structure was shown to be in clo se relation with that of topaz and may be represented (perpendicular to the c axis) as a sandwich of topaz layered fragments and (CuSiO3) layers. Copp er has a (4+1) square-pyramidal coordination, with the vertex oxygen being a bridging oxygen of an isolated [Si2O7] diortho group. Inside one layer, [ CuO4] squares separated by [Si2O7] diortho groups form a zigzag chain with an angle of 103 degrees, The ESR spectrum of interacting Cu2+ ions in cuals ite coincides well with this arrangement. Full identity between the cualsit e and topaz structures in the ab plane explains well the epitaxial growth o f cualsite on the topaz single-crystal seeds. A comparison is made of the c ualsite structure and other known copper silicates. (C) 1998 Academic Press .