Ca6Si2O13 crystallizes in the trigonal P-3 space group. There are two inequivalent Ca sites. In the first Ca site, Ca(1) is bonded to one O(1), one O(3), two equivalent O(2), and two equivalent O(4) atoms to form CaO6 octahedra that share corners with two equivalent Ca(1)O6 octahedra,  a cornercorner with one Si(2)O4 tetrahedra, corners with two equivalent Si(1)O4 tetrahedra, and  an edgeedge with one Ca(1)O6 octahedra. The corner-sharing octahedral tilt angles are 61°. In the second Ca site, Ca(2) is bonded in a 7-coordinate geometry to one O(2), one O(4), one O(5), two equivalent O(1), and two equivalent O(3) atoms. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(5) and three equivalent O(2) atoms to form SiO4 tetrahedra that share corners with six equivalent Ca(1)O6 octahedra and  a cornercorner with one Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 48-53°. In the second Si site, Si(2) is bonded to one O(5) and three equivalent O(1) atoms to form SiO4 tetrahedra that share corners with three equivalent Ca(1)O6 octahedra and  a cornercorner with one Si(1)O4 tetrahedra. The corner-sharing octahedral tilt angles are 53°. There are five inequivalent O sites. In the first O site, O(5) is bonded in a linear geometry to three equivalent Ca(2), one Si(1), and one Si(2) atom. In the second O site, O(1) is bonded in a distorted rectangular see-saw-like geometry to one Ca(1), two equivalent Ca(2), and one Si(2) atom. In the third O site, O(2) is bonded to one Ca(2), two equivalent Ca(1), and one Si(1) atom to form a mixture of distorted corner and edge-sharing OCa3Si trigonal pyramids. In the fourth O site, O(3) is bonded in a trigonal planar geometry to one Ca(1) and two equivalent Ca(2) atoms. In the fifth O site, O(4) is bonded in a trigonal planar geometry to one Ca(2) and two equivalent Ca(1) atoms.
Ca6Si2O13 crystallizes in the trigonal P-3 space group. There are two inequivalent Ca sites. In the first Ca site, Ca(1) is bonded to one O(1), one O(3), two equivalent O(2), and two equivalent O(4) atoms to form CaO6 octahedra that share corners with two equivalent Ca(1)O6 octahedra,  a cornercorner with one Si(2)O4 tetrahedra, corners with two equivalent Si(1)O4 tetrahedra, and  an edgeedge with one Ca(1)O6 octahedra. The corner-sharing octahedral tilt angles are 61°. The Ca(1)-O(1) bond length is 2.41 Å. The Ca(1)-O(3) bond length is 2.40 Å. There is one shorter (2.35 Å) and one longer (2.38 Å) Ca(1)-O(2) bond length. Both Ca(1)-O(4) bond lengths are 2.37 Å. In the second Ca site, Ca(2) is bonded in a 7-coordinate geometry to one O(2), one O(4), one O(5), two equivalent O(1), and two equivalent O(3) atoms. The Ca(2)-O(2) bond length is 2.60 Å. The Ca(2)-O(4) bond length is 2.30 Å. The Ca(2)-O(5) bond length is 2.90 Å. There is one shorter (2.36 Å) and one longer (2.65 Å) Ca(2)-O(1) bond length. There is one shorter (2.31 Å) and one longer (2.36 Å) Ca(2)-O(3) bond length. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(5) and three equivalent O(2) atoms to form SiO4 tetrahedra that share corners with six equivalent Ca(1)O6 octahedra and  a cornercorner with one Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 48-53°. The Si(1)-O(5) bond length is 1.66 Å. All Si(1)-O(2) bond lengths are 1.64 Å. In the second Si site, Si(2) is bonded to one O(5) and three equivalent O(1) atoms to form SiO4 tetrahedra that share corners with three equivalent Ca(1)O6 octahedra and  a cornercorner with one Si(1)O4 tetrahedra. The corner-sharing octahedral tilt angles are 53°. The Si(2)-O(5) bond length is 1.66 Å. All Si(2)-O(1) bond lengths are 1.64 Å. There are five inequivalent O sites. In the first O site, O(5) is bonded in a linear geometry to three equivalent Ca(2), one Si(1), and one Si(2) atom. In the second O site, O(1) is bonded in a distorted rectangular see-saw-like geometry to one Ca(1), two equivalent Ca(2), and one Si(2) atom. In the third O site, O(2) is bonded to one Ca(2), two equivalent Ca(1), and one Si(1) atom to form a mixture of distorted corner and edge-sharing OCa3Si trigonal pyramids. In the fourth O site, O(3) is bonded in a trigonal planar geometry to one Ca(1) and two equivalent Ca(2) atoms. In the fifth O site, O(4) is bonded in a trigonal planar geometry to one Ca(2) and two equivalent Ca(1) atoms.