Ca2Al4Si4O17 crystallizes in the monoclinic C2/c space group. Ca(1) is bonded in a 6-coordinate geometry to one O(3), one O(5), one O(6), one O(9), and two equivalent O(7) atoms. There are two inequivalent Al sites. In the first Al site, Al(1) is bonded to one O(1), one O(2), one O(4), and one O(9) atom to form AlO4 tetrahedra that share  a cornercorner with one Si(2)O4 tetrahedra and corners with two equivalent Si(1)O4 tetrahedra. In the second Al site, Al(2) is bonded to one O(3), one O(5), one O(6), and one O(7) atom to form AlO4 tetrahedra that share  a cornercorner with one Si(1)O4 tetrahedra and corners with three equivalent Si(2)O4 tetrahedra. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(1), one O(2), one O(3), and one O(8) atom to form SiO4 tetrahedra that share  a cornercorner with one Al(2)O4 tetrahedra,  a cornercorner with one Si(1)O4 tetrahedra, and corners with two equivalent Al(1)O4 tetrahedra. In the second Si site, Si(2) is bonded to one O(4), one O(5), one O(6), and one O(7) atom to form SiO4 tetrahedra that share  a cornercorner with one Al(1)O4 tetrahedra and corners with three equivalent Al(2)O4 tetrahedra. There are nine inequivalent O sites. In the first O site, O(7) is bonded to two equivalent Ca(1), one Al(2), and one Si(2) atom to form distorted edge-sharing OCa2AlSi trigonal pyramids. In the second O site, O(8) is bonded in a bent 120 degrees geometry to two equivalent Si(1) atoms. In the third O site, O(9) is bonded in a distorted bent 120 degrees geometry to one Ca(1) and one Al(1) atom. In the fourth O site, O(1) is bonded in a linear geometry to one Al(1) and one Si(1) atom. In the fifth O site, O(2) is bonded in a bent 150 degrees geometry to one Al(1) and one Si(1) atom. In the sixth O site, O(6) is bonded in a distorted trigonal planar geometry to one Ca(1), one Al(2), and one Si(2) atom. In the seventh O site, O(3) is bonded in a distorted trigonal planar geometry to one Ca(1), one Al(2), and one Si(1) atom. In the eighth O site, O(4) is bonded in a linear geometry to one Al(1) and one Si(2) atom. In the ninth O site, O(5) is bonded in a distorted bent 120 degrees geometry to one Ca(1), one Al(2), and one Si(2) atom.
Ca2Al4Si4O17 crystallizes in the monoclinic C2/c space group. Ca(1) is bonded in a 6-coordinate geometry to one O(3), one O(5), one O(6), one O(9), and two equivalent O(7) atoms. The Ca(1)-O(3) bond length is 2.43 Å. The Ca(1)-O(5) bond length is 2.45 Å. The Ca(1)-O(6) bond length is 2.44 Å. The Ca(1)-O(9) bond length is 2.22 Å. Both Ca(1)-O(7) bond lengths are 2.56 Å. There are two inequivalent Al sites. In the first Al site, Al(1) is bonded to one O(1), one O(2), one O(4), and one O(9) atom to form AlO4 tetrahedra that share  a cornercorner with one Si(2)O4 tetrahedra and corners with two equivalent Si(1)O4 tetrahedra. The Al(1)-O(1) bond length is 1.73 Å. The Al(1)-O(2) bond length is 1.74 Å. The Al(1)-O(4) bond length is 1.76 Å. The Al(1)-O(9) bond length is 1.78 Å. In the second Al site, Al(2) is bonded to one O(3), one O(5), one O(6), and one O(7) atom to form AlO4 tetrahedra that share  a cornercorner with one Si(1)O4 tetrahedra and corners with three equivalent Si(2)O4 tetrahedra. The Al(2)-O(3) bond length is 1.77 Å. The Al(2)-O(5) bond length is 1.77 Å. The Al(2)-O(6) bond length is 1.78 Å. The Al(2)-O(7) bond length is 1.81 Å. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(1), one O(2), one O(3), and one O(8) atom to form SiO4 tetrahedra that share  a cornercorner with one Al(2)O4 tetrahedra,  a cornercorner with one Si(1)O4 tetrahedra, and corners with two equivalent Al(1)O4 tetrahedra. The Si(1)-O(1) bond length is 1.62 Å. The Si(1)-O(2) bond length is 1.62 Å. The Si(1)-O(3) bond length is 1.68 Å. The Si(1)-O(8) bond length is 1.65 Å. In the second Si site, Si(2) is bonded to one O(4), one O(5), one O(6), and one O(7) atom to form SiO4 tetrahedra that share  a cornercorner with one Al(1)O4 tetrahedra and corners with three equivalent Al(2)O4 tetrahedra. The Si(2)-O(4) bond length is 1.59 Å. The Si(2)-O(5) bond length is 1.66 Å. The Si(2)-O(6) bond length is 1.65 Å. The Si(2)-O(7) bond length is 1.66 Å. There are nine inequivalent O sites. In the first O site, O(7) is bonded to two equivalent Ca(1), one Al(2), and one Si(2) atom to form distorted edge-sharing OCa2AlSi trigonal pyramids. In the second O site, O(8) is bonded in a bent 120 degrees geometry to two equivalent Si(1) atoms. In the third O site, O(9) is bonded in a distorted bent 120 degrees geometry to one Ca(1) and one Al(1) atom. In the fourth O site, O(1) is bonded in a linear geometry to one Al(1) and one Si(1) atom. In the fifth O site, O(2) is bonded in a bent 150 degrees geometry to one Al(1) and one Si(1) atom. In the sixth O site, O(6) is bonded in a distorted trigonal planar geometry to one Ca(1), one Al(2), and one Si(2) atom. In the seventh O site, O(3) is bonded in a distorted trigonal planar geometry to one Ca(1), one Al(2), and one Si(1) atom. In the eighth O site, O(4) is bonded in a linear geometry to one Al(1) and one Si(2) atom. In the ninth O site, O(5) is bonded in a distorted bent 120 degrees geometry to one Ca(1), one Al(2), and one Si(2) atom.