A new copper aluminum silicate CuAl 2 Si 2 O 7 (F,OH) 2 (cualsite) was synthesized hydrothermally from SiO 2 and Al 2 O 3 in a mineralizing solution 1
3 Na 3 AlF 6 ุ 4 9 Al(NO 3 ) 3 ุ 1 9 Cu(NO 3 ) 2 ุ 27.8H 2 O at 600ยฐC and 150 MPa. Cualsite grew epitaxially on the pinacoidal plane of the topaz single-crystal seed. Blue crystals of the bipyramidal habitus exhibit orthorhombic symmetry Pbnm with a โซุโฌ 4.757(1) A s , b โซุโฌ 8.877(3) A s , c โซุโฌ 14.075(4) A s , and Z โซุโฌ 4. Structural analysis led to R 1 โซุโฌ 0.018 and R w โซุโฌ 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 the c axis) as a sandwich of topaz layered fragments and (CuSiO 3 ) layers. Copper has a (4ุ1) squarepyramidal coordination, with the vertex oxygen being a bridging oxygen of an isolated [Si 2 O 7 ] diortho group. Inside one layer, [CuO 4 ] squares separated by [Si 2 O 7 ] diortho groups form a zigzag chain with an angle of 103ยฐ. The ESR spectrum of interacting Cu 2ุ ions in cualsite coincides well with this arrangement. Full identity between the cualsite and topaz structures in the ab plane explains well the epitaxial growth of cualsite on the topaz singlecrystal seeds. A comparison is made of the cualsite structure and other known copper silicates.