CaAl2Si4O12 crystallizes in the monoclinic C2/c space group. Ca(1) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(6) atoms to form CaO6 octahedra that share corners with six equivalent Si(1)O4 tetrahedra and corners with six equivalent Si(2)O4 tetrahedra. Al(1) is bonded to two equivalent O(1), two equivalent O(4), and two equivalent O(5) atoms to form distorted AlO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra, corners with two equivalent Si(2)O4 tetrahedra, and edges with three equivalent Al(1)O6 octahedra. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(2), one O(3), one O(4), 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 Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 50-68°. In the second Si site, Si(2) is bonded to one O(2), one O(3), 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 Si(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 51-68°. There are six inequivalent O sites. In the first O site, O(2) is bonded in a distorted trigonal planar geometry to one Ca(1), one Si(1), and one Si(2) atom. In the second O site, O(3) is bonded in a distorted bent 120 degrees geometry to one Ca(1), one Si(1), and one Si(2) atom. In the third O site, O(4) is bonded in a distorted trigonal non-coplanar geometry to two equivalent Al(1) and one Si(1) atom. In the fourth O site, O(5) is bonded in a distorted trigonal non-coplanar geometry to two equivalent Al(1) and one Si(2) atom. In the fifth O site, O(6) is bonded in a distorted trigonal non-coplanar geometry to one Ca(1), one Si(1), and one Si(2) atom. In the sixth O site, O(1) is bonded in an L-shaped geometry to two equivalent Al(1) atoms.
CaAl2Si4O12 crystallizes in the monoclinic C2/c space group. Ca(1) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(6) atoms to form CaO6 octahedra that share corners with six equivalent Si(1)O4 tetrahedra and corners with six equivalent Si(2)O4 tetrahedra. Both Ca(1)-O(2) bond lengths are 2.57 Å. Both Ca(1)-O(3) bond lengths are 2.66 Å. Both Ca(1)-O(6) bond lengths are 2.59 Å. Al(1) is bonded to two equivalent O(1), two equivalent O(4), and two equivalent O(5) atoms to form distorted AlO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra, corners with two equivalent Si(2)O4 tetrahedra, and edges with three equivalent Al(1)O6 octahedra. Both Al(1)-O(1) bond lengths are 1.85 Å. There is one shorter (1.95 Å) and one longer (2.00 Å) Al(1)-O(4) bond length. There is one shorter (1.95 Å) and one longer (1.99 Å) Al(1)-O(5) bond length. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(2), one O(3), one O(4), 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 Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 50-68°. The Si(1)-O(2) bond length is 1.67 Å. The Si(1)-O(3) bond length is 1.68 Å. The Si(1)-O(4) bond length is 1.58 Å. The Si(1)-O(6) bond length is 1.70 Å. In the second Si site, Si(2) is bonded to one O(2), one O(3), 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 Si(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 51-68°. The Si(2)-O(2) bond length is 1.67 Å. The Si(2)-O(3) bond length is 1.70 Å. The Si(2)-O(5) bond length is 1.58 Å. The Si(2)-O(6) bond length is 1.67 Å. There are six inequivalent O sites. In the first O site, O(2) is bonded in a distorted trigonal planar geometry to one Ca(1), one Si(1), and one Si(2) atom. In the second O site, O(3) is bonded in a distorted bent 120 degrees geometry to one Ca(1), one Si(1), and one Si(2) atom. In the third O site, O(4) is bonded in a distorted trigonal non-coplanar geometry to two equivalent Al(1) and one Si(1) atom. In the fourth O site, O(5) is bonded in a distorted trigonal non-coplanar geometry to two equivalent Al(1) and one Si(2) atom. In the fifth O site, O(6) is bonded in a distorted trigonal non-coplanar geometry to one Ca(1), one Si(1), and one Si(2) atom. In the sixth O site, O(1) is bonded in an L-shaped geometry to two equivalent Al(1) atoms.