CaAl4(SiO6)2 crystallizes in the monoclinic C2/c space group. Ca(1) is bonded to two equivalent O(1), two equivalent O(4), and two equivalent O(5) atoms to form CaO6 octahedra that share corners with six equivalent Al(2)O4 tetrahedra and corners with six equivalent Si(1)O4 tetrahedra. There are two inequivalent Al sites. In the first Al site, Al(1) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(6) atoms to form distorted AlO6 octahedra that share corners with two equivalent Al(2)O4 tetrahedra, corners with two equivalent Si(1)O4 tetrahedra, and edges with three equivalent Al(1)O6 octahedra. In the second Al site, Al(2) is bonded to one O(1), one O(3), one O(4), and one O(5) atom to form AlO4 tetrahedra that share corners with two equivalent Al(1)O6 octahedra, corners with three equivalent Ca(1)O6 octahedra, and corners with three equivalent Si(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 48-66°. Si(1) is bonded to one O(1), one O(4), one O(5), and one O(6) atom to form SiO4 tetrahedra that share corners with two equivalent Al(1)O6 octahedra, corners with three equivalent Ca(1)O6 octahedra, and corners with three equivalent Al(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 50-61°. There are six inequivalent O sites. In the first O site, O(2) is bonded in a 3-coordinate geometry to two equivalent Al(1) and one O(2) atom. In the second O site, O(3) is bonded in a distorted trigonal planar geometry to one Al(2) and two equivalent Al(1) atoms. In the third O site, O(4) is bonded in a trigonal planar geometry to one Ca(1), one Al(2), and one Si(1) atom. In the fourth O site, O(5) is bonded in a trigonal planar geometry to one Ca(1), one Al(2), and one Si(1) atom. In the fifth O site, O(6) is bonded in a distorted trigonal planar geometry to two equivalent Al(1) and one Si(1) atom. In the sixth O site, O(1) is bonded in a distorted trigonal planar geometry to one Ca(1), one Al(2), and one Si(1) atom.
CaAl4(SiO6)2 crystallizes in the monoclinic C2/c space group. Ca(1) is bonded to two equivalent O(1), two equivalent O(4), and two equivalent O(5) atoms to form CaO6 octahedra that share corners with six equivalent Al(2)O4 tetrahedra and corners with six equivalent Si(1)O4 tetrahedra. Both Ca(1)-O(1) bond lengths are 2.57 Å. Both Ca(1)-O(4) bond lengths are 2.43 Å. Both Ca(1)-O(5) bond lengths are 2.49 Å. There are two inequivalent Al sites. In the first Al site, Al(1) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(6) atoms to form distorted AlO6 octahedra that share corners with two equivalent Al(2)O4 tetrahedra, corners with two equivalent Si(1)O4 tetrahedra, and edges with three equivalent Al(1)O6 octahedra. There is one shorter (1.86 Å) and one longer (1.87 Å) Al(1)-O(2) bond length. There is one shorter (1.81 Å) and one longer (1.88 Å) Al(1)-O(3) bond length. There is one shorter (1.87 Å) and one longer (2.00 Å) Al(1)-O(6) bond length. In the second Al site, Al(2) is bonded to one O(1), one O(3), one O(4), and one O(5) atom to form AlO4 tetrahedra that share corners with two equivalent Al(1)O6 octahedra, corners with three equivalent Ca(1)O6 octahedra, and corners with three equivalent Si(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 48-66°. The Al(2)-O(1) bond length is 1.78 Å. The Al(2)-O(3) bond length is 1.71 Å. The Al(2)-O(4) bond length is 1.80 Å. The Al(2)-O(5) bond length is 1.80 Å. Si(1) is bonded to one O(1), one O(4), one O(5), and one O(6) atom to form SiO4 tetrahedra that share corners with two equivalent Al(1)O6 octahedra, corners with three equivalent Ca(1)O6 octahedra, and corners with three equivalent Al(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 50-61°. The Si(1)-O(1) bond length is 1.66 Å. The Si(1)-O(4) bond length is 1.66 Å. The Si(1)-O(5) bond length is 1.67 Å. The Si(1)-O(6) bond length is 1.64 Å. There are six inequivalent O sites. In the first O site, O(2) is bonded in a 3-coordinate geometry to two equivalent Al(1) and one O(2) atom. The O(2)-O(2) bond length is 1.59 Å. In the second O site, O(3) is bonded in a distorted trigonal planar geometry to one Al(2) and two equivalent Al(1) atoms. In the third O site, O(4) is bonded in a trigonal planar geometry to one Ca(1), one Al(2), and one Si(1) atom. In the fourth O site, O(5) is bonded in a trigonal planar geometry to one Ca(1), one Al(2), and one Si(1) atom. In the fifth O site, O(6) is bonded in a distorted trigonal planar geometry to two equivalent Al(1) and one Si(1) atom. In the sixth O site, O(1) is bonded in a distorted trigonal planar geometry to one Ca(1), one Al(2), and one Si(1) atom.