Abstract
A new copper aluminum silicate CuAl2Si2O7(F,OH)2(cualsite) was synthesized hydrothermally from SiO2and Al2O3in a mineralizing solution 13Na3AlF6+49Al(NO3)3+19Cu(NO3)2+27.8H2O at 600°C and 150 MPa. Cualsite grew epitaxially on the pinacoidal plane of the topaz single-crystal seed. Blue crystals of the bipyramidal habitus exhibit orthorhombic symmetryPbnmwitha=4.757(1) Å,b=8.877(3) Å,c=14.075(4) Å, andZ=4. Structural analysis led toR1=0.018 andRw=0.046 for 899 unique reflections. The cualsite structure was shown to be in close relation with that of topaz and may be represented (perpendicular to thecaxis) as a sandwich of topaz layered fragments and (CuSiO3) layers. Copper 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°. The ESR spectrum of interacting Cu2+ions in cualsite coincides well with this arrangement. Full identity between the cualsite and topaz structures in theabplane explains well the epitaxial growth of cualsite on the topaz single-crystal seeds. A comparison is made of the cualsite structure and other known copper silicates.
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