CaCuAlSi2O9 crystallizes in the monoclinic C2/m space group. Ca(1) is bonded in a 8-coordinate geometry to two equivalent O(2), two equivalent O(3), two equivalent O(4), and two equivalent O(6) atoms. Cu(1) is bonded to one O(6), two equivalent O(1), and two equivalent O(2) atoms to form distorted CuO5 trigonal bipyramids that share corners with two equivalent Si(1)O4 tetrahedra and edges with two equivalent Al(1)O6 octahedra. Al(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form AlO6 octahedra that share corners with four equivalent Si(1)O4 tetrahedra and edges with two equivalent Cu(1)O5 trigonal bipyramids. Si(1) is bonded to one O(1), one O(3), one O(4), and one O(5) atom to form SiO4 tetrahedra that share corners with two equivalent Al(1)O6 octahedra, corners with two equivalent Si(1)O4 tetrahedra, and  a cornercorner with one Cu(1)O5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 37-49°. There are six inequivalent O sites. In the first O site, O(4) is bonded in a distorted bent 150 degrees geometry to two equivalent Ca(1) and two equivalent Si(1) atoms. In the second O site, O(5) is bonded in a bent 150 degrees geometry to two equivalent Si(1) atoms. In the third O site, O(6) is bonded in a trigonal non-coplanar geometry to two equivalent Ca(1) and one Cu(1) atom. In the fourth O site, O(1) is bonded in a distorted trigonal planar geometry to one Cu(1), one Al(1), and one Si(1) atom. In the fifth O site, O(2) is bonded in a distorted trigonal non-coplanar geometry to one Ca(1), one Cu(1), and one Al(1) atom. In the sixth O site, O(3) is bonded in a 3-coordinate geometry to one Ca(1), one Al(1), and one Si(1) atom.
CaCuAlSi2O9 crystallizes in the monoclinic C2/m space group. Ca(1) is bonded in a 8-coordinate geometry to two equivalent O(2), two equivalent O(3), two equivalent O(4), and two equivalent O(6) atoms. Both Ca(1)-O(2) bond lengths are 2.47 Å. Both Ca(1)-O(3) bond lengths are 2.35 Å. Both Ca(1)-O(4) bond lengths are 2.95 Å. Both Ca(1)-O(6) bond lengths are 2.31 Å. Cu(1) is bonded to one O(6), two equivalent O(1), and two equivalent O(2) atoms to form distorted CuO5 trigonal bipyramids that share corners with two equivalent Si(1)O4 tetrahedra and edges with two equivalent Al(1)O6 octahedra. The Cu(1)-O(6) bond length is 2.10 Å. Both Cu(1)-O(1) bond lengths are 1.93 Å. Both Cu(1)-O(2) bond lengths are 1.85 Å. Al(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form AlO6 octahedra that share corners with four equivalent Si(1)O4 tetrahedra and edges with two equivalent Cu(1)O5 trigonal bipyramids. Both Al(1)-O(1) bond lengths are 1.94 Å. Both Al(1)-O(2) bond lengths are 1.94 Å. Both Al(1)-O(3) bond lengths are 1.88 Å. Si(1) is bonded to one O(1), one O(3), one O(4), and one O(5) atom to form SiO4 tetrahedra that share corners with two equivalent Al(1)O6 octahedra, corners with two equivalent Si(1)O4 tetrahedra, and  a cornercorner with one Cu(1)O5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 37-49°. The Si(1)-O(1) bond length is 1.64 Å. The Si(1)-O(3) bond length is 1.60 Å. The Si(1)-O(4) bond length is 1.67 Å. The Si(1)-O(5) bond length is 1.64 Å. There are six inequivalent O sites. In the first O site, O(4) is bonded in a distorted bent 150 degrees geometry to two equivalent Ca(1) and two equivalent Si(1) atoms. In the second O site, O(5) is bonded in a bent 150 degrees geometry to two equivalent Si(1) atoms. In the third O site, O(6) is bonded in a trigonal non-coplanar geometry to two equivalent Ca(1) and one Cu(1) atom. In the fourth O site, O(1) is bonded in a distorted trigonal planar geometry to one Cu(1), one Al(1), and one Si(1) atom. In the fifth O site, O(2) is bonded in a distorted trigonal non-coplanar geometry to one Ca(1), one Cu(1), and one Al(1) atom. In the sixth O site, O(3) is bonded in a 3-coordinate geometry to one Ca(1), one Al(1), and one Si(1) atom.